The series of LiMO2 (M: transition metal) materials are highly relevant as cathode materials of Li-ion batteries. The stability of such systems remains an important factor for their usability in batteries, and depends strongly on the electronic configuration of the transition-metal ions. In particular, the promising class of multi-transition-metal systems exhibits complicated valence states due to intermetallic charge transfer and charge disproportionation. Here we perform a systematic study on the valence of the transition-metal ions using x-ray absorption spectroscopy on the M-L2,3 edges and O-K edges. In Li(Ni0.5Mn0.5)1-xCoxO2 we established that the valence is Co3+ and Ni0.52+Mn0.54+ throughout the whole series. Meanwhile, in LiNi1-xCoxO2 we found that the Ni displays a behavior consistent with a charge disproportionated negative charge transfer system, and that with increased concentration of Co3+, the disproportionation signal decreases. Since the number of O 2p holes also gets reduced, we infer that the material will also become more unstable.

The bulk electronic structure of AV3Sb5 (A=K, Cs) has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). The asymmetric shape of V and Sb core level peaks indicates that the V 3d and Sb 5p electrons are involved in the conduction band. The absence of a satellite structure in the V 2p HAXPES spectra shows a weak electronic correlation in the V 3d states. Splitting of the V 2p peak is not observed in the density wave phase indicating the charge disproportionation between the V sites is undetectably small, consistent with the weakness of the on-site electronic correlation and the possibility of bond order. The Sb 4d5/2 binding energy agrees with that of the Cs-terminated surface, indicating that the electronic structure of the V3Sb5 layer just below the Cs surface is close to the bulk.

We investigated the electronic structure of BaEu2Nb5O15 and Eu3Nb5O15 by means of hard x-ray photoelectron spectroscopy. The Eu 3d core level spectra are primarily dominated by Eu2+ signals. The well-screened feature in the Nb 3d core level spectra of BaEu2Nb5O15 is suppressed in Eu3Nb5O15, consistent with the insulating behavior of Eu3Nb5O15 due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious A2 site. In the valence band spectra, the Nb 4d states at the Fermi level are close to the Eu 4f states located around 2 eV below it. This suggests that the degree of localization of the Nb 4d electrons can be enhanced through the Eu 4f-Nb 4d hybridization with the atomically and magnetically disordered Eu ions at the A2 site.

We have investigated the electronic structure of La3Ni2O7-δ (δ≈0.07) by means of hard x-ray photoemission spectroscopy (HAXPES). Although the nominal Ni valence is close to +2.5, the Ni 2p HAXPES spectra show an absence of Ni2+/Ni3+ charge disproportionation. The Ni 2p spectral shape including the main peak and the charge-transfer satellite indicate that oxygen 2p holes are heavily involved in the transport properties. The spectral weight suppression at the Fermi level indicates that the carriers of O 2p character (mixed with Ni 3d) are affected by electronic correlation which would be associated with the density wave transition and the superconductivity controlled by pressure.

The natural abundance of sodium makes the Na-ion batteries (SIBs) attractive devices in the framework of a global economy change toward net zero CO2 emission. SIBs naturally deliver relatively lower energy density with respect to the Li-ion counterparts (LiBs); however, their lower cost and fast charge/discharge ability make them a promising competitor to LiBs to load level the intermittent power from renewable energy sources for smart grids or renewable power stations. The O3-type NaFeO2 is a promising candidate for SIBs cathodes, even if the irreversible structural transition occurring during Na-ion extraction/insertion seriously hinders its practical application. Partial replacement of Fe by Ni significantly improves its electrochemical properties. The possible reasons of such improvement are here investigated accessing the details on the Fe and Ni local electronic and structural properties by means of x-ray absorption spectroscopy and spin-polarized density functional theory (DFT) calculations. Different Ni concentrations and charge states have been analyzed. The results support the stability of the electronic properties of Fe and Ni as a function of cycling in a partially substituted system. Instead, the local structure is affected by the Fe substitution as well by the charge/discharge cycling. In particular, the decrease of Fe–O covalency and the local disorder by partial substitution of Fe with Ni seem at the origin of the improved performances.

Corundum oxide Ti2O3 shows the metal-insulator transition around 400-600 K accompanying the nearest Ti3+-Ti3+ bond (a1ga1g singlet state) formation along the c axis. In order to clarify the hole-doping effect for the a1ga1g singlet bond in Ti2O3, we investigated Ti 3d orbital anisotropy between corundum-type Ti2O3 and ilmenite-type MgTiO3 using linear dichroism of soft x-ray absorption spectroscopy of the Ti L2,3 edge. From the linear dichroic spectral weight in MgyTi2-yO3, we confirmed that the a1ga1g state is dominant not only in y=0.01 (almost Ti2O3), but also in y=0.29, indicating that the Ti-Ti bond survives against a certain level of hole doping. In y=0.63 corresponding to 46% hole doping per Ti, the 3d orbital symmetry changes from a1g to egπ.

We have investigated the temporal evolution and fluence dependence of band structure in Ta2Ni0.9Co0.1Se5 by means of time- and angle-resolved photoemission spectroscopy (tr-ARPES). With the fluence of 3.00 mJ cm−2 and the delay time of 170 fs, the conduction band crossing the Fermi level (EF) and the valence band reaching EF are observed indicating the photoinduced semimetal phase. However, at the delay time of 70 fs, the valence band top does not reach EF creating a partial gap or a pseudogap, and the transition from the excitonic insulator state to the semimetal state is not completed. The conduction band crossing EF is observed even with the shorter delay time (down to 50–70 fs) and=or the lower fluence (down to 0.32 mJ cm−2). On the other hand, the valence band top stays well below EF up to the fluence of 1.39 mJ cm−2 even at the delay time of 160–170 fs. The different optical responses of the valence and conduction bands suggest a vital role of the lattice fluctuations pointed out by Nakano et al. [Phys. Rev. B 98, 045139 (2018)] on the residual pseudogap at 70 fs after the photoexcitation.

Electronic properties of various insulators can be controlled by chemical substitution. For example, exotic superconducting phases are often obtained by chemical substitution in Mott insulators. Compared to Mott insulators, impact of chemical substitution on excitonic insulators is not well explored yet. In the present work, space-resolved angle-resolved photoemission spectroscopy of the model Ta2Ni(Se1-xSx)5 is reported in which S substitution for Se is used to control the excitonic behavior. The substitution introduces electronic inhomogeneity with the Se 4p/S 3p valence band exhibiting strong position dependence. In contrast, the flat top valence band, which is a signature of the excitonic insulating phase, does not show any appreciable position dependence except the effect of surface corrugation. This indicates that the excitonic coupling in Ta2NiSe5 is robust against the electronic inhomogeneity induced by the S substitution.

We study the electronic structure of electron-doped Pr1.3-xLa0.7CexCuO4 (PLCCO; Tc=27K, x=0.1) and hole-doped Bi2Sr2CaCu2O8+δ (Bi2212; Tc=90K) cuprate superconductors using x-ray absorption spectroscopy and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites, which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be Up = 3.3±0.5eV, and for Bi2212, Up=5.6±0.5eV, while the copper on-site Coulomb correlation energy is Ud = 6.5±0.5eV for Bi2212. The expression for the Heisenberg exchange interaction J in terms of the electronic parameters Ud, Up, charge-transfer energy Δ, and Cu-O hopping tpd obtained from a simple Cu2O cluster model is used to carry out an optimization analysis consistent with J known from scattering experiments. The analysis also provides the effective one-band on-site Coulomb correlation energy U and the effective hopping t. PLCCO and Bi2212 are shown to exhibit very similar values of U/t∼9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both materials.

We have studied the Ti3+/Ti4+ mixed valence state in Mg-doped Ti2O3 using hard x-ray photoemission spectroscopy. The Ti 2p spectrum for the corundum-type Ti2O3 revealed the Ti3+ configuration with strong electronic coupling in the c-axis Ti-Ti pairs whereas the data for the ilmenite-type MgTiO3 confirmed the Mg2+-Ti4+ charge state in the c-axis cation pairs. In Mg0.29Ti1.71O3, the Ti 2p spectrum hardly showed the Ti4+ peak, which was in MgTiO3 indicating that the c-axis pairing of the Mg is with a Ti3+ ion rather than a Ti4+ and that the Ti3+/Ti4+ mixed valence state is materialized within the Ti-Ti c-axis pairs. In Mg0.63Ti1.37O3, we detected the presence of Mg2+-Ti4+ pairs. The results indicate the important role of hybridization within the c-axis pairs not only in the Ti3+-Ti3+ configuration, but also above all in the Ti3+/Ti4+ mixed valence state, superseding the Madelung energy gain of the Mg2+-Ti4+ c-axis pair formation.

We have investigated the local structure of TrZr2 (Tr = transition metal) superconductor with variable configurational entropy using x-ray absorption spectroscopy. Temperature dependent Zr K-edge extended x-ray absorption fine structure (EXAFS) measurements are carried out on three different samples including the pristine CoZr2, medium entropy Co0.33Rh0.34Ir0.33Zr2 and high entropy Co0.2Fe0.2Ni0.2Rh0.2Ir0.2Zr2. We have found that the local bondlengths remain largely temperature independent or tend to show negative thermal expansion, instead the Zr–Zr bondlengths get stiffer with the configurational entropy. The x-ray absorption near edge structure (XANES) spectra at Zr K-edge and Ir L3-edge indicate Zr 4d - Tr nd (n = 2, 3, 4) hybridization to be related with the superconducting transition temperature. The results provide important information on the local structure that can be useful to understand the physical properties of these materials and design new high entropy alloys (HEAs) superconductors.

The photoinduced semimetal phase of Ta2Ni1−xCoxSe5 (x = 0.0 and 0.1) is studied by time-resolved angle-resolved photoemission spectroscopy. The observed band dispersions roughly agree with the generalized gradient approximation (GGA) calculation which fails to reproduce the band gap at the ambient condition. This suggests that the electron–hole interaction, which is not exactly included in GGA, is screened out by the photoexcited carriers. There is no indication of pseudogap. The conduction and valence bands provide quasi one-dimensional Fermi surfaces with similar momentum positions suggesting a quasi one-dimensional line-node semimetal.

Anomalous local structure and magnetic moment of an inhomogeneous FeSe based superconductor indicating importance of the structure and magnetic correlations in these materials.

The local structures of Ti based MXene-type electrode materials have been studied by Ti K-edge X-ray absorption fine structure measurements as a function of temperature to obtain direct information on the local bond lengths and their stiffness. In particular, the parent MAX phases Ti2AlC and Ti3AlC2 and their etched MXene systems are characterized and their properties compared. We find that selective etching has a substantial effect on the local structural properties of the Ti based MXene materials. It leads to an increase in the interatomic distances, i.e. a decrease in the covalency, and corresponding bond stiffness, that is a likely cause of higher achievable performances. The obtained results underline the importance of the local atomic correlations as limiting factors in the diffusion capacity of ion batteries.

Bulk and surface electronic states of MnPSe3 have been investigated by means of x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and theoretical calculations. In Mn 2p XPS, the main peak is accompanied by a charge-transfer satellite which is commonly observed in various Mn chalcogenides. The intensity of the charge-transfer satellite is considerably reduced by oxidization of the surface. The multiplet structure of Mn 2p XAS indicates the high spin Mn2+ configuration. The cluster model analyses of the XPS and XAS spectra indicate that the Se 4p-to-Mn 3d charge-transfer energy A is negative providing the strong p-d hybridization. The magnetic interaction between the third nearest neighboring Mn spins is enhanced by the smallness of A and the unique molecular orbitals of the P2Se6 cluster.

The local structure of the filled tetragonal tungsten bronze (TTB) niobate Ba3Nb5−xTixO15 (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.

We report results of band calculations and photoemission experiments for Ba3Nb5O15 and Ba2SrNb5O15 with tetragonal tungsten bronze structure. In the band calculations, the Nb 4d yz=zx bands dispersive along the c-axis accommodate 0.2 electrons per Nb both in Ba3Nb5O15 and Ba2SrNb5O15, which is consistent with the photoemission results. The anisotropy of the Nb 4d bands at the Fermi level provides the higher conductivity along the c-axis and the higher magnitude of the Seebeck coefficient along the a-axis. The calculated results are roughly consistent with the transport behaviors reported in the literatures. In particular, the magnitude of the Seebeck coefficient along the c-axis of Ba2SrNb5O15 is predicted to be smaller than that of Ba3Nb5O15 below 200K while they are comparable at higher temperatures. However, the magnitude of the Seebeck coefficient along the c-axis is considerably underestimated indicating that effective mass along the c-axis is selectively enhanced due to electronic correlation. The Nb 3d and 4d photoemission spectra suggest that the electronic correlation is enhanced in the surface region.

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

PbCrO3 features an unusual charge distribution Pb0.52+Pb0.54+Cr3+O3 with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb2+Cr4+O3 charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr1-xV
x
O3 are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb2+ and Pb4+ at x = 0.35 to single Pb2+ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb2+Cr4+O3-type phase. With further increase in the V substitution, the PbVO3-type polar tetragonal phase appeared at x = 0.80.

Photoinduced phase transitions have been intensively studied owing to the potential capability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it is currently a central issue as to how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photoinduced phase transitions. Here, we use a recently developed analysis method, which we call frequency-domain angle-resolved photoemission spectroscopy (FDARPES), and reveal mode- and band-selective electron-phonon couplings during the photoinduced insulator-to-metal transition for Ta2NiSe5. We find that the lattice modulation corresponding to the 2 THz phonon mode, where the Ta lattice is sheared along the a axis, is the most relevant for the emergence of photoinduced semimetallic state. Furthermore, we find that the semimetallic and semiconducting bands coexist in the transient state, and demonstrate that FDARPES spectra can selectively detect the phonon-specific couplings to the two coexistent band structures during the photoinduced phase transition by resolving them in the frequency domain.

We have investigated electronic structure evolution by cation substitution in Ba3-xSrxNb5O15 by means of hard x-ray photoemission spectroscopy. Localization of Nb 4d electrons manifests as spectral weight transfer from a coherent component at the Fermi level to an incoherent one at 1-2 eV below it. This behavior is similar to that of electron-doped SrTiO3. On the other hand, Nb 3d and 4p core level spectra exhibit a screening effect with an energy scale of 1-2 eV by the coherent Nb 4d electrons similar to 4d electron systems near Mott transitions. The energy scale indicates that electron correlation is involved in the metal to insulator transition in the present system, although the Nb 4d band is about 1/10 filled. The present results suggest a mechanism of electron localization due to atomic disorder and electron correlation.

<p>The local structure of BiS₂-based compounds shows an asymmetric Bi-environment with a crucial role of the axial sulfur distance in the electronic transport.</p>

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFe
x
O3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+
x
O3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

We have studied temperature-driven effects on photoemission spectra of hollandite-type BaxTi8O16+δ across its metal-insulator transition (MIT) at 220 K by using surface-sensitive photon energies from 27 to 70 eV. The surface Ti 3d spectral weight was suppressed and shifted towards higher binding energy from the bulk if the surface was obtained by cleavage above the MIT temperature. The Ti 3d spectral weight was recovered once the sample was cooled across the MIT, indicating a transfer of Ti 3d electrons from the bulk to the surface across the MIT. The recovered Ti 3d spectral shape is consistent with the bulk-sensitive result and indicates strong localization character of the Ti 3d electron even in the metallic phase. Resonant photoemission spectroscopy with absorption from the Ti 3p core level exhibits resonances of the O 2p-Ti 3d and O 2p-Ti 4s,p hybridized states. The result suggests that the O 2p-Ti 4s,p hybridization plays a role in the interplay between the bulk and the surface in BaxTi8O16+δ.

A spin transition between high-spin (HS) and low-spin (LS) states in a solid can occur when the energies of two spin configurations intersect, which is usually caused by external perturbations such as temperature, pressure, and magnetic fields, with substantial influence to its physical and chemical properties. Here, we discover the electrochemical "redox reaction" as a new driving force to induce reversible HS-LS spin transition. Although reversible solid-state redox reaction has been thoroughly investigated as the fundamental process in battery electrode materials, coupling between redox reactions and spin transitions has not been explored. Using density functional theory calculations, we predicted the existence of redox-driven spin transition occurring exclusively for the Co3+/Co2+ redox couple in layered transition-metal oxides, leading to a colossal potential hysteresis (>1 V) between the cathodic (LS Co3+ to LS Co2+) and anodic (HS Co2+ to HS Co3+) reactions. The predicted potential hysteresis associated with the spin transition of Co was experimentally verified for NxTi0.5Co0.5O2 by monitoring the electrochemical potential, local coordination structure, electronic structure, and magnetic moment.

We review recent advances in strongly correlated oxides as thermoelectric materials in pursuit of energy harvesting. We discuss two topics: one is the enhancement of the ordinary thermoelectric properties by controlling orbital degrees of freedom and orbital fluctuation not only in bulk but also at the interface of correlated oxides. The other topic is the use of new phenomena driven by spin-orbit coupling (SOC) of materials. In 5d electron oxides, we show some SOC- related transport phenomena, which potentially contribute to energy harvesting. We outline the current status and a future perspective of oxides as thermoelectric materials.[GRAPHICS].

© 2018 IOP Publishing Ltd. Isovalent substitution of S by Se in LaOBiS2-xSex has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, we have studied the local structure of LaOBiS2-xSex () using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi-S1 distances separated by ∼0.4 Å instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi-Se distances in the plane being separated by ∼0.2 Å. Temperature dependent study shows that the two Bi-S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi-Se distances in LaOBiSSe (x = 1.0). The out of plane Bi-S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2-xSex is an important factor to describe differing electronic and thermal transport of the two compounds.

© 2018, The Author(s). Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1−xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron–hole coupled systems.

The electronic-structural modulations of Ir1-xPtxTe2 (0x0.12) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructures with wave vectors of Q=(1/50-1/5), (1/80-1/8), and (1/60-1/6) are observed on the same sample of IrTe2 at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around Q=(1/50-1/5) are observed for doped samples (0.02x0.05). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge-density wave (CDW) can coexist with superconductivity. The analysis of the energy-dependent REXS and RIXS line shape reveals the importance of the Te 5p state rather than the Ir 5d states in the formation of the spatial modulation of these systems. The phase diagram reexamined in this work suggests that the CDW incommensurability may correlate with the emergence of superconducting states such as CuxTiSe2 and LixTaS2.

We have studied the surface electronic structure of AV10O15 (A = Ba, Sr) and Ba0.9Sr0.1V13O18 by using x-ray photoemission spectroscopy (XPS). The V 2p XPS shows multiple peaks associated with the charge fluctuation in the system. The relative intensity of each valence component can be extracted by Gaussian and Voigt fittings. The result strongly depends on the number of Gaussians/Voigts. In BaV10O15 and Ba0.9Sr0.1V13O18, the surface state has more V3+ than the bulk. In addition to the presence of surface V3+, the V2.5+ in AV10O15 (A = Ba, Sr) and V2+ in Ba0.9Sr0.1V13O18 systems are observed even in the surface sensitive XPS. The number of Gaussians/Voigts should be as large as four for the V 2p3/2, in order to obtain the results consistent with the bulk sensitive HAXPES study.

We have studied the electronic structure of SnSe and Na-doped SnSe by means of angle-resolved photoemission spectroscopy. The valence-band top reaches the Fermi level by the Na doping, indicating that Na-doped SnSe can be viewed as a degenerate semiconductor. However, in the Na-doped system, the chemical potential shift with temperature is unexpectedly large and is apparently inconsistent with the degenerate semiconductor picture. The large chemical potential shift and anomalous spectral shape are key ingredients for an understanding of the novel metallic state with the large thermoelectric performance in Na-doped SnSe.

Charge distribution changes in Bi- and Pb-3d transition metal perovskite type oxides were examined by comprehensive precise structural analysis, spectroscopy, and theoretical investigations. The change in the depth of the d level of the transition metal caused the intermetallic charge transfer. A temperature-induced charge-transfer transition in chemically modified BiNiO3 results in technologically important negative thermal expansion.

Negative thermal expansion (NTE) induced by intermetallic charge transfer between Bi5+ and Ni2+ was observed in Bi1-xSbxNiO3 (x = 0.05 and 0.10). Bi3+ and Bi5+ had a long-range ordering in the low-temperature phase of Bi0.95Sb0.05NiO3 while the ordering was short-range in Bi0.9Sb0.01NiO3. The latter compound exhibited a continuous NTE with a linear coefficient of thermal expansion (CTE) of αL = %106 ppmK%1 without temperature hysteresis in a wide temperature range of 200-320 K.

We have studied the multi-band electronic structure of ferromagnetic CeRuPO (TC = 15 K) by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES results show that three hole bands exist around the zone center and two of them cross the Fermi level (EF). Around the zone corner, two electron bands are observed and cross EF. These hole and electron bands, which can be assigned to the Ru 4d bands, are basically consistent with the band-structure calculation including their orbital characters. However, one of the electron bands with Ru 4d 3z2 − r2 character is strongly renormalized indicating correlation effect due to hybridization with the Ce 4 f orbitals. The Ru 4d 3z2 − r2 band changes across TC suggesting that the out-of-plane 3z2 − r2 orbital channel plays essential roles in the ferromagnetism.

Recently CeOBiS 2 system without any fluorine doping is found to show superconductivity posing question on its origin. Using space resolved ARPES we have found a metallic phase embedded in the morphological defects and at the sample edges of stoichiometric CeOBiS 2 . While bulk of the sample is semiconducting, the embedded metallic phase is characterized by the usual electron pocket at X point, similar to the Fermi surface of doped BiS 2 -based superconductors. Typical size of the observed metallic domain is larger than the superconducting correlation length of the system suggesting that the observed superconductivity in undoped CeOBiS 2 might be due to this embedded metallic phase at the defects. The results also suggest a possible way to develop new systems by manipulation of the defects in these chalcogenides with structural instability.

Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta2NiSe5 below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence FC=0.30±0.08 mJ/cm2 due to optical absorption saturation. This limits the optical excitation density in Ta2NiSe5 so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta2NiSe5 exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

We have investigated the nanostructuring effects on the local structure of V2O5 cathode material by means of temperature dependent V K-edge X-ray absorption fine structure measurements. We have found that the nanostructuring largely affects V-O and V-V bond characteristics with a general softening of the local V-O and V-V bonds. The obtained bond strengths correlate with the specific capacity shown by the different systems, with higher capacity corresponding to softer atomic pairs. The present study suggests the key role of local atomic displacements in the diffusion and storage of ions in cathodes for batteries, providing important information for designing new functional materials.

We have studied a charge-orbital driven metal-insulator transition (MIT) in hollandite-type BaxTi8O16+δ by means of hard x-ray photoemission spectroscopy (HAXPES). The Ti 2p HAXPES indicates strong Ti3+/Ti4+ charge fluctuation in the metallic phase above the MIT temperature. The metallic phase is characterized by a power-law spectral function near the Fermi level which would be a signature of bad metal with non-Drude polaronic behavior. The power-law spectral shape is associated with the large Seebeck coefficient of the metallic phase in BaxTi8O16+δ.

We have performed scanning photoemission spectromicroscopy of BaV10O15 across the metal-insulator transition at 123 K, which is accompanied by V 3d charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of BaV10O15 single crystals, similar to Cr-doped V2O3 in which electronic configurations of Cr3+ and V3+ are the same as those of V2+ and V3+ in BaV10O15. Typical domain size is similar to 5- 10 mu m at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in BaV10O15 is different from that of Cr-doped V2O3, probably due to the charge degrees of freedom in BaV10O15.

© 2017 American Physical Society.We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estimated to be ∼0.3 per Bi site in Eu3F4Bi2S4, which corresponds to the nominal x in a typical REO1-xFxBiS2 system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the X point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in Eu3F4Bi2S4, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

We have studied the electronic structure of BaV10O15 across the metal-insulator transition with V trimerization by means of hard-x-ray photoemission spectroscopy (HAXPES) and mean-field calculations. The V 2p HAXPES indicates V2.5+-V3+ charge fluctuation in the metallic phase, and V2+-V3+ charge order in the insulating phase. The V2.5+-V3+ charge fluctuation is consistent with the mean-field solution where a V 3d a(1g) electron is shared by two V sites with face-sharing VO6 octahedra. The valence- band HAXPES of the metallic phase exhibits pseudogap opening at the Fermi level associated with the charge fluctuation, and a band gap similar to 200 meV is established in the insulating phase due to the switching of charge correlation.

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta2NiSe5 investigated by time-and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F-C = 0.2 mJ cm(-2), the band gap narrows transiently, while it is enhanced above FC. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta2NiSe5, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta2NiSe5 with light on the femtosecond time scale.

We have studied the local structure of LaO0.5F0.5BiS2-xSex by Bi L-1-edge extended x-ray absorption fine structure (EXAFS). We find a significant effect of Se substitution on the local atomic correlations with a gradual elongation of average in-plane Bi-S bondlength. The associated mean square relative displacement, measuring average local distortions in the BiS2 plane, hardly shows any change for small Se substitution, but decreases significantly for x &gt;= 0.6. The Se substitution appears to suppress the local distortions within the BiS2 plane that may optimize in-plane orbital hybridization and hence the superconductivity. The results suggest that the local structure of the BiS2-layer is one of the key ingredients to control the physical properties of the BiS2-based dichalcogenides.

We have studied the multi-band electronic structure of BaNi2(As1-xPx)(2) (x = 0.00 and 0.092) in which the P substitution suppresses unusual Ni-Ni zigzag bond order and induces strong coupling superconductivity. At x = 0.092, all the Fermi surfaces predicted by the ab-initio band calculations are successfully identified including the small electron pocket around the Z point. Interestingly, whereas the Ni 3d xy and x(2) - y(2) bands crossing E-F agree with the band calculations, the Ni 3d yz/zx bands around 1 eV below the Fermi level (E-F) are moderately renormalized similar to those in the Fe-based superconductors. The orbital-dependent band renormalization in BaNi2(As1-xPx)(2) indicates that the less correlated xy and x(2) - y(2) bands play important roles in the strong coupling superconductivity. Across the Ni-Ni bond order at x = 0.00, the less correlated Ni 3d xy and x(2) - y(2) bands near E-F are selectively reconstructed across the Ni-Ni bond order. The band reconstruction is consistent with the orbital-selective Peierls instability proposed by Streltsov and Khomskii [Phys. Rev. B 89, 161112(R) (2014)], in which the itinerant orbital sector governs the structural instability and fluctuations.

The electronic structure of BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe2X3 exhibit two components, indicating that itinerant and localized Fe 3d sites coexist. Substantial x-ray linear dichroism is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe ladder even above the Neel temperature T-N. Direct exchange interaction along the legs of the Fe ladder stabilizes the orbital and antiferromagnetic orders in BaFe2S3, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe2Se3.

The effect of delithiation in LixCoO2 is studied by high resolution Co K-edge x-ray absorption and x-ray emission spectroscopy. Polarization dependence of the x-ray absorption spectra on single crystal samples is exploited to reveal information on the anisotropic electronic structure. We find that the electronic structure of LixCoO(2) is significantly affected by delithiation in which the Co ions oxidation state tending to change from 3+ to 4+. The Co intersite (intrasite) 4p-3d hybridization suffers a decrease (increase) by delithiation. The unoccupied 3d t(2g) orbitals with a(1g) symmetry, containing substantial O 2p character, hybridize isotropically with Co 4p orbitals and likely to have itinerant character unlike anisotropically hybridized 3d e(g) orbitals. Such a peculiar electronic structure could have significant effect on the mobility of Li in LixCoO2 cathode and hence the battery characteristics.

We report an angle-resolved photoemission spectroscopy (ARPES) study of LixCoO2 single crystals which have a hole-doped CoO2 triangular lattice. Similar to NaxCoO2, the Co 3d a(1g) band crosses the Fermi level with strongly renormalized band dispersion while the Co 3d e '(g) bands are fully occupied in LixCoO2 (x = 0.46 and 0.71). At x = 0.46, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at x = 0.71, the Fermi surface area is larger than the expectation which can be associated with the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co 3d peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at x = 0.71 can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co 3d a(1g) is very robust around x = 0.5 even in the topmost CoO2 layer due to the absence of the polar catastrophe.

We have studied the electronic structure of Ba1-xSrxV13O18 (x = 0,0.2,1) at different temperatures across the trimerization and charge-order transitions using hard x-ray photoemission spectroscopy (HAXPES). The V 2p HAXPES indicates V2+/V3+ charge order and fluctuation in the high-temperature tetramer phase, low-temperature trimer phase, and intermediate-temperature charge-order phase in the series of x = 0,0.2,1. In the valence-band HAXPES, although the spectral weight at the Fermi level tends to be suppressed in all the phases due to strong charge order or fluctuation, it increases in the trimer phase at x = 0.2, in agreement with the decrease of resistivity and the increase of itinerant electrons in the trimer phase. Interestingly, in the most conducting x = 1 without the charge-order phase, the spectral weight at the Fermi level is strongly suppressed even in the trimer phase, indicating that charge fluctuation in the trimer phase is different between x = 0.2 and 1.

We have studied the local structure of superconducting Ca10Pt4As8(Fe2As2)(5) (Pt10418) and Ca10Ir4As8(Fe2As2)(5) (Ir10418) iron arsenides, showing different transition temperatures (T-c = 38 and 16 K, respectively), by polarized Fe K-edge extended x-ray absorption fine-structure measurements. Despite the similar average crystal structures, the local structures of the FeAs4 tetrahedra in the two compounds are found to be very different. The FeAs4 in Pt10418 is close to a regular tetrahedron, while it deviates largely in Ir10418. The Fe-Fe correlations in the two compounds are characterized by similar bond-length characteristics; however, the static disorder in Pt10418 is significantly lower than that in Ir10418. The results suggest that the optimized local structure and reduced disorder are the reasons for higher Tc and well-defined electronic states in Pt10418 unlike Ir10418 showing the coexistence of glassy and normal electrons at the Fermi surface, and hence provide direct evidence of the local-structure-driven optimization of the electronic structure and superconductivity in iron arsenides.

Perovskite PbCoO3 synthesized at 12 GPa was found to have an unusual charge distribution of Pb(2+)Pb(3)(4+)Co(2+)2Co(2)(3+)O(12) with charge orderings in both the A and B sites of perovskite ABO(3). Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb2+Pb34+Co22+Co23+O12 quadruple perovskite structure. It is shown that the average valence distribution of Pb3.5+Co2.5+O3 between Pb3+Cr3+O3 and Pb4+Ni2+O3 can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Highly one-dimensional (1D) Fermi sheets are realized at the surface of a layered Ir telluride IrTe2 which exhibits a stripe-type charge and orbital order below similar to 280 K. The 1D Fermi sheets appear in the low temperature range where the stripe order is well established. The 1D Fermi sheets are truncated by the bulk Fermi surfaces, and the spectral weight suppression at the Fermi level deviates from the typical Tomonaga-Luttinger behavior. The 1D band runs along the stripe and is accompanied by several branches which can be derived from the quantization in the perpendicular direction.

We propose a method for inducting the Boltzmann factor to extract effective classical spin Hamiltonians from mean-field-type electronic structural calculations by means of the Bayesian inference. This method enables us to compare electronic structural calculations with experiments according to the classical model at a finite temperature. Application of this method to the unrestricted Hartree-Fock calculations for NiGa2S4 led to the estimation that the superexchange interaction between the nearest neighbor sites is ferromagnetic at low temperature, which is consistent with magnetic experiment results. This supports the theory that competition between the antiferromagnetic third neighbor interaction and ferromagnetic nearest neighbor interaction may lead to the quantum spin liquid in NiGa2S4.

The valence distribution and local structure of Bi1-xPbxNiO3 (x &lt;= 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi3+ and Bi5+ was observed for all the samples, but it was a long-ranged one with distinct crystallographic sites in the P (1) over bar triclinic structure for x &lt;= 0.15, while the ordering was short-ranged for x = 0.20 and 0.25. An intermetallic charge transfer between Bi5+ and Ni2+, leading to large volume shrinkage, was observed for all the samples upon heating at similar to 500 K.

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (T-c similar to 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L-3 edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L-3 edge XANES is hardly affected by Pb substitution while the Te-L-1 edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi4-xPbxTe6 preferentially goes on Te sites. Similarly, the Cs-L-3 edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L3 edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi4Te6 into a bulk superconductor by Pb-substitution in CsBi4-xPbxTe6.

We have performed Ce L-3-edge x-ray absorption spectroscopy (XAS) and Ce 4d-4f resonant photoemission spectroscopy (PES) on single crystals of CeO1-xFx BiS2 for x = 0.0 and 0.5 in order to investigate the Ce 4f electronic states. In Ce L-3-edge XAS, a mixed valence of Ce was found in the x = 0.0 sample, and F doping suppressed it, which is consistent with the results on polycrystalline samples. As for resonant PES, we found that the Ce 4f electrons in both x = 0.0 and 0.5 systems respectively formed a flat band at 1.0 and 1.4 eV below the Fermi level and there was no contribution to the Fermi surfaces. Interestingly, Ce valence in CeOBiS2 deviates from Ce3+ even though Ce 4f electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce 4f in CeOBiS2 is mixed with unoccupied Bi 6p(z), which is consistent with a previous local structural study. Based on the analysis of the Ce L-3-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller, increasing the number of Ce 4f electrons upon the F substitution for O.

The local structure of IrTe2 has been studied by iridium L-3-edge x-ray absorption spectroscopy (XAS) measurements as a function of pressure, performed at two temperatures (100 and 295 K) across the structural phase transition at similar to 270 K. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra show pressure-dependent anomalies, suggesting phase transitions that are characterized by different local atomic displacements. The high-temperature phase of IrTe2 (trigonal at 295 K) reveals a clear anomaly in the Ir-Te correlations at similar to 4 GPa, while the low-temperature phase (at 100 K) shows a smaller change at similar to 6 GPa, likely to be associated with transitions in lower-symmetry phases. XANES spectra, measuring higher-order atomic correlations, also show nonlinear pressure dependence in the local geometry at the anomalous pressures. These nonlinear changes suggest that IrTe2 goes through lower local symmetry phases with increasing pressure.

We have studied the local structure of a Ba(Fe1-xCox)(2)As-2 superconductor using temperature dependent extended X-ray absorption fine structure (EXAFS) measurements. Polarized EXAFS at the Fe K-edge on an optimally doped (x = 0.06) single crystal has permitted us to determine atomic displacements across the superconducting transition temperature (T-c). The Fe-As bondlength hardly shows any change with temperature; however, the Fe-Fe sublattice reveals a sharp anomaly across T-c, indicated by a significant drop in mean square relative displacements, similar to the one known for cuprates and A15-type superconductors. We have also found a large atomic disorder around the substituted Co, revealed by polarized Co K-edge EXAFS measurements. The Co-Fe/Co bonds are more flexible than the Fe-Fe bonds with the As-height in Co-containing tetrahedra being larger than the one in FeAs4. The results suggest that the local Fe-Fe bondlength fluctuations and the atomic disorder in this sub-lattice should have some important role in the superconductivity of Ba(Fe1-xCox)(2)As-2 pnictides.

We have investigated the local structure of differently charged NaxCoO2, cathode material as a function of temperature by Co K-edge X-ray absorption fine structure (EXAFS) measurements. We have found that the Charge/discharge process has a direct effect on the bond characteristics of the cathode in the Na-ion batteries. The results reveal that the local Co-O bonds get softer, while the Co Co bonds hardly show any change during discharge (sodiation). The present study underlines the key role of local atomic displacements in diffusion and the reversibility of ions in cathodes for batteries and points toward the feasibility of NaxCoO2 to be used as a cathode material. The results are discussed in comparison with the lithiation/delithiation of LixCoO2 battery materials.

We have performed scanning photoelectron spectromicroscopy measurements in the paramagnetic (PM) and the antiferromagnetic (AFM) phases of Fe1+delta Te (delta = 0.09). The spectromicroscopy images reveal stripe modulation of the electronic structure in the AFM monoclinic phase at low temperature, that disappears at high temperature in the PM phase. The stripes, running along the monoclinic crystal axis, are characterized by different density of states around the Gamma point and near the Fermi level (E-F). Since the near E-F electronic states around the Gamma point have strong electron-lattice coupling in Fe1+delta Te, the observed stripes are likely to be related to the strain modulation introduced by the monoclinic lattice distortion.

We evaluate the utilities of fluorescence-yield (FY) modes in soft X-ray absorption spectroscopy (XAS) of several cathode materials for Li-ion batteries. In the case of total-FY (TFY) XAS for LiNi0.5Mn1.5O4, the line shape of the Mn L-3-edge XAS was largely distorted by the self-absorption and saturation effects, while the distortions were less pronounced at the Ni L-3 edge. The distortions were suppressed for the inverse-partial-FY (IPFY) spectra. We found that, in the cathode materials, the IPFY XAS is highly effective for the Cr, Mn, and Fe L edges and the TFY and PFY modes are useful enough for the Ni L edge which is far from the O K edge. (C) 2016 Author(s).

In the hole-doped cuprates, a small number of carriers suppresses antiferromagnetism and induces superconductivity. In the electron-doped cuprates, on the other hand, superconductivity appears only in a narrow window of high-doped Ce concentration after reduction annealing, and strong antiferromagnetic correlation persists in the superconducting phase. Recently, Pr1.3-xLa0.7CexCuO4 (PLCCO) bulk single crystals annealed by a protect annealing method showed a high critical temperature of around 27 K for small Ce content down to 0.05. Here, by angle-resolved photoemission spectroscopy measurements of PLCCO crystals, we observed a sharp quasi-particle peak on the entire Fermi surface without signature of an antiferromagnetic pseudogap unlike all the previous work, indicating a dramatic reduction of antiferromagnetic correlation length and/or of magnetic moments. The superconducting state was found to extend over a wide electron concentration range. The present results fundamentally challenge the long-standing picture on the electronic structure in the electron-doped regime.

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO3 has a valence state of Pb0.52+Pb0.54+Cr3+O3 with Pb2+-Pb4+ correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr charge transfer causes an insulator to metal transition and similar to 10% volume collapse.

We have investigated the electronic structure of BiS2-based CeO0.5F0.5BiS2 superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around the X point, the straight portion parallel to the ky direction is dominated by Bi 6px character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the BiS2 system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

The spin character of the states at the top of the valence band in doped La2-xSrxCuO4 (x = 0.03, 0.07, 0.15, 0.22, and 0.30) has been investigated using spin-polarized resonant photoemission. A clear Zhang-Rice singlet (ZRS) is observed at all doping levels. Its stability and polarization are preserved as a function of doping, suggesting that the concept of the ZRS can be used across a wide doping range and up to the metallic nonsuperconducting overdoped regime. The results are significant for theoretical models that use the ZRS approximation and for the understanding of the peculiar interplay between the ZRS and the remaining localized spins.

We have investigated the electronic structure of BiS2-based CeO0.5F0.5BiS2 superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around the X point, the straight portion parallel to the k(y) direction is dominated by Bi 6p(x) character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the BiS2 system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

We have studied the electronic structure of Li1-x[Mn0.5Ni0.5](1-x)O-2 (x = 0.00 and 0.05), one of the promising cathode materials for Li ion battery, by means of x-ray photoemission and absorption spectroscopy. The results show that the valences of Mn and Ni are basically 4+ and 2+, respectively. However, the Mn3+ component in the x = 0.00 sample gradually increases with the bulk sensitivity of the experiment, indicating that the Jahn-Teller active Mn3+ ions are introduced in the bulk due to the site exchange between Li and Ni. The Mn3+ component gets negligibly small in the x = 0.05 sample, which indicates that the excess Li suppresses the site exchange and removes the Jahn-Teller active Mn3+. (C) 2015 AIP Publishing LLC.

We investigated the electronic and magnetic properties of fully oxidized BaFeO3 thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for Fe4+, which showed good agreement with the experimental results. We also studied SrFeO3 thin films, which have an Fe4+ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that BaFeO3 thin films are insulating with large magnetization (1.7 mu(B)/formula unit) under similar to 1 T, using valence-band HAXPES and Fe 2p XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe 2p core-level HAXPES and Fe 2p XAS spectra of BaFeO3 and SrFeO3 thin films are quite similar, we compared the insulating BaFeO3 to metallic SrFeO3 thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of BaFeO3 is dominated by d(5)(L) under bar ((L) under bar: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O 2p character. We revealed that the differences of the electronic and magnetic properties between BaFeO3 and SrFeO3 arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.

We have studied the effect of RE substitution on the structure and the local atomic disorder in REO0.5F0.5BiS2 (RE = rare-earth) to understand their correlation with the bulk superconductivity in these materials. The mean RE size, affecting the chemical pressure, has been varied in two series namely Ce1-xNdxO0.5F0.5BiS2 and Nd1-ySmyO0.5F0.5BiS2. The lattice parameters evolve anomalously, showing an anisotropic shrinkage (elongation) of the c-axis (a-axis) to an isotropic expansion of the lattice with increasing mean RE size. The Bi L-3-edge extended X-ray absorption fine structure (EXAFS) measurements are performed to investigate local displacements in the BiS2 lattice, revealing a large disorder and a sharp boundary between the Ce-containing and Sm-containing series with a distinct local structure. The results suggest that the bulk superconductivity in REO0.5F0.5BiS2 is correlated with anomalous atomic displacements in the Bi-S1 network, likely to be a combined effect of active Bi 6s electronic states and a possible Jahn-Teller-like instability of the Bi 6p(x)/6p(y) electrons.

We have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2-ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T-c similar to 32 K), phase separation temperature (T-ps similar to 520 K), and iron-vacancy order temperature (T-vo similar to 580 K). In addition to the iron-vacancy ordered tetragonalmagnetic phase and orthorhombicmetallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below similar to 300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in themajority texturewith largemagnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T-ps and T-c is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KxFe2-ySe2.

We report a photoemission study at room temperature on BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are broad and exhibit two components, indicating that itinerant and localized Fe 3d sites coexist similar to KxFe2-ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen p holes in the two-leg ladder structure.

We have investigated the impact of F-doing on CeO1-xFx BiS2 in terms of the electronic-structural parameters of Anderson's impurity-model analysis. It was recently reported using Ce L-3-edge x-ray absorption spectroscopy (XAS) that CeOBiS2 falls in the Ce valence fluctuation regime and the F-doping drives the system into the Kondo regime. The Ce L-3-edge XAS spectra with the various F-doping levels can be reproduced by adjusting the transfer integral in the Anderson's impurity model. The present analysis indicates that the F-doping to the system corresponds to the decrease of the Ce-Bi transfer integral.

We have studied local magnetic moment and electronic phase separation in superconducting KxFe2-ySe2 by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T-c) and the coexisting phases exchange spectral weights with the low-spin state, gaining intensity at the expense of the higher-spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at T-c as well as superconductivity disappear. The results clearly underline that the coexistence of the low-spin and high-spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of the electronically inhomogeneous KxFe2-ySe2 system.

We propose a novel method for extracting effective classical spin Hamiltonians from mean-field type electronic structural calculations by means of the Bayesian inference. We apply the method for a NiS2 triangular lattice in NiGa2S4, where the ground state is a spin disordered state. Starting from unrestricted Hartree-Fock calculations for the spin configurations of 16 Ni sites, we estimated that not only the strongest superexchange interaction between the third nearest neighbor-sites but also those between the nearest and the second nearest neighbor-sites should be considered for the effective classical spin Hamiltonian for NiGa2S4.

We report an electronic structure study on LaO1-xFxBiSe2 (x = 0.18) by means of photoelectron spectromicroscopy. The Fermi surfaces and band dispersions are basically consistent with the band-structure calculations on BiS2-based materials, indicating that the electron correlation effects may be irrelevant to describe physics of the new BiSe2 system. In LaO1-xFxBiSe2 (x = 0.18), the area of the Fermi pockets is estimated to be 0.16 +/- 0.02 per Bi, consistent with the amount of F substitution. Although the spectromicroscopy technique avoids the effect of microscale inhomogeneity for angle-resolved photoemission spectroscopy (ARPES), the ARPES spectral features are rather broad in the momentum space, indicating the likely effect of local disorder in the BiSe2 layer.

The electronic structure of phase separated KxFe2-y Se-2 has been studied by means of x-ray absorption spectroscopy (XAS) and photoemission spectroscopy. Both Fe L-2,L-3-edge XAS and Fe 2p photoemission spectra show a signature of the metallic phase that is embedded in the insulating texture. Valence-band spectra, measured with different excitation energies, reveal a finite density of states at the Fermi level driven by the Fe 3d orbitals. A combined analysis of the Se L-2,L-3-edge and the Se K-edge XAS provides further information on a significant density of states of Se 4p character near the Fermi level, consistent with a large hybridization of these states with the Fe 3d orbitals. The K L-2,L-3-edge XAS spectrum is also presented to show element specific partial density of states. The results are discussed in connection with the electronic phase separation in K-x Fe2-y Se-2 driven by the Fe 3d states.

We report on the structural phase transition found in Ca-10(Ir4As8)(Fe2-xIrxAs2)(5), which exhibits superconductivity at 16 K. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with the space group P4/n (No. 85) is unchanged at all temperatures measured. Our synchrotron X-ray diffraction study clearly shows iridium ions at a non-coplanar position shift along the z-direction at the structural phase transition. We discuss the fact that iridium displacements affect superconductivity in Fe2As2 layers.

We have studied the local structure of a KxFe2-ySe2 superconductor across the phase separation and iron-vacancy order-disorder temperatures (respectively at similar to 520 K and similar to 580 K). The combination of Fe K edge and Se K edge extended x-ray absorption fine-structure (EXAFS) measurements reveal the glassy local structure of KxFe2-ySe2, with anomalous behavior of local atomic displacements. We find that the Fe-Se distance remains temperature independent while the Fe-Fe distance suffers a substantial effect of the phase separation. Furthermore, the Fe-Se network shows a reduced disorder in the phase separation regime at lower temperature. The x-ray absorption near-edge structure features follow the local structure anomaly observed by EXAFS and reveal substantially reduced Fe 3d-Se 4p hybridization in the iron-vacancy disordered phase at higher temperature. The results provide further information on the role of nanoscale atomic displacements in the peculiar coexistence of different electronic phases in KxFe2-ySe2 with a filamentary superconducting state embedded in the iron-vacancy ordered magnetic texture.

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

We report a photoemission and x-ray absorption study on a Au1-xPtxTe2 (x = 0 and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au 4f and Te 3d core-level spectra of AuTe2 suggest a valence state of Au2+(Te-2)(2-), which is consistent with its distorted crystal structure with Te-Te dimers and compressed AuTe6 octahedra. On the other hand, valence-band photoemission spectra and preedge peaks of the Te 3d absorption edge indicate that Au 5d bands are almost fully occupied and that Te 5p holes govern the transport properties and the lattice distortion. The two apparently conflicting pictures can be reconciled by strong Au 5d/Au 6s-Te 5p hybridization. The absence of a core-level energy shift with Pt substitution is inconsistent with the simple rigid band picture for hole doping. The Au 4f core-level spectrum gets slightly narrow with Pt substitution, indicating that the small Au 5d charge modulation in distorted AuTe2 is partially suppressed.

We have used Bi and Ce L-3-edges extended x-ray absorption fine structure measurements to study local structure of CeO1-xFxBiS2 system as a function of F-substitution. The local structure of both BiS2 active layer and CeO1-xFx spacer layer changes systematically. The in-plane Bi-S1 distance decreases (Delta R-max similar to 0.08 angstrom) and the out-of-plane Bi-S2 distance increases (Delta R-max similar to 0.12 angstrom) with increasing F-content. On the other hand, the Ce-O/ F distance increases (Delta R-max similar to 0.2 angstrom) with a concomitant decrease of the Ce-S2 distance (Delta R-max similar to 0.15 angstrom). Interestingly, the Bi-S1 distance is characterized by a large disorder that increases with F-content. The results provide useful information on the local atomic displacements in CeO1-xFxBiS2, that should be important for the understanding of the coexistence of superconductivity and low temperature ferromagnetism in this system.

X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the unoccupied electronic states and local geometry of Ir1-xPtxTe2 (x = 0.0, 0.03 and 0.04) as a function of temperature. The Ir L-3-edge absorption white line, as well as high energy XANES features due to the photoelectron multiple scatterings with near neighbours, reveal clear changes in the unoccupied 5d-electronic states and the local geometry with Pt substitution. We find an anomalous spectral weight transfer across the known first-order structural phase transition from the trigonal to monoclinic phase in IrTe2, which characterizes the reduced atomic structure symmetry below the transition temperature. No such changes with temperature are seen in the Pt substituted superconducting samples. In addition, a gradual increase of the spectral weight transfer is observed in IrTe2 with a further decrease in temperature below the transition, indicating that the low temperature phase is likely to have a symmetry lower than the monoclinic one. The results suggest that the interplay between inter-layer and intra-layer atomic correlations should have a significant role in the properties of an Ir1-xPtxTe2 system.

We have used the combination of Fe K-edge and Ir L-3-edge x-ray absorption measurements as a function of temperature to investigate local atomic displacements in the newly discovered Ca10Ir4As8(Fe2As2)(5) superconducting system. We find relatively relaxed Fe-Fe atomic pair correlations with large displacements in the FeAs4 tetrahedra, revealed by Fe K-edge extended x-ray absorption fine structure (EXAFS) analysis. Similarly, the temperature dependence of Ir L-3-edge EXAFS shows nanoscale disorder in the IrAs layer that should have a significant effect on the active FeAs-layer characteristics. Furthermore, x-ray absorption near edge structure data are presented to discuss the evolution of the unoccupied electronic states revealing the marginal role of spin-orbit coupling, while the interlayer interactions and disorder should be important for describing the physics of the Ca10Ir4As8(Fe2As2)(5) system.

The coupled electronic-structural modulations of the ligand states in IrTe2 have been studied by x-ray absorption spectroscopy and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the TeM4,5 (3d -&gt; 5p) absorption edge, indicating the presence of a Te 5p-Ir 5d covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3d -&gt; 5p resonance at the Q=(1/5,0,-1/5) superlattice reflection is observed below the structural transition temperature Ts similar to 280 K. The analysis of the energy-dependent REXS line shape reveals the key role played by the spatial modulation of the covalent Te 5p-Ir 5d bond density in driving the stripelike order in IrTe2, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology recently observed in the high-temperature superconducting cuprates suggests that the iridates may harbor similar exotic phases.

Structural phase separation in A(x)Fe(2-y)Se(2) system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in KxFe2-ySe2, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high Tc superconductivity in the presence of a phase with a large magnetic moment in A(x)Fe(2-y)Se(2) materials.

Angle-resolved photoemission spectroscopy of Ca-10(Ir4As8)(Fe2-xIrxAs2)(5) shows that the Fe 3d electrons in the FeAs layer form the holelike Fermi pocket at the zone center and the electronlike Fermi pockets at the zone corners as commonly seen in various Fe-based superconductors. The FeAs layer is heavily electron doped and has relatively good two dimensionality. On the other hand, the Ir 5d electrons are metallic and glassy probably due to atomic disorder related to the Ir 5d orbital instability. Ca-10(Ir4As8)(Fe2-xIrxAs2)(5) exhibits a unique electronic state where the Bloch electrons in the FeAs layer coexist with the glassy electrons in the Ir4As8 layer.

We have performed Ce L-3-edge x-ray absorption spectroscopy (XAS) measurements on CeO1-xFxBiS2, in which the superconductivity of the BiS2 layer and the ferromagnetism of the CeO1-xFx layer are induced by the F doping, in order to investigate the impact of the F doping on the local electronic and lattice structures. The Ce L-3-edge XAS spectrum of CeOBiS2 exhibits coexistence of 4f(1) (Ce3+) and 4f(0) (Ce4+) state transitions revealing Ce mixed valency in this system. The spectral weight of the 4f(0) state decreases with the F doping and completely disappears for x &gt; 0.4 where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of the Ce-S-Bi coupling channel by the F doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS2 layer and the ferromagnetic CeO1-xFx layer.

We have studied the nature of the three-dimensional multiband electronic structure in the two-dimensional triangular lattice Ir1-xPtxTe2 (x = 0.05) superconductor using angle-resolved photoemission spectroscopy (ARPES), x-ray photoemission spectroscopy (XPS), and a band structure calculation. ARPES results clearly show a cylindrical (almost two-dimensional) Fermi surface around the zone center. Near the zone boundary, the cylindrical Fermi surface is truncated into several pieces in a complicated manner with strong three dimensionality. The XPS result and the band structure calculation indicate that the strong Te 5p-Te 5p hybridization between the IrTe2 triangular lattice layers is responsible for the three dimensionality of the Fermi surfaces and the intervening of the Fermi surfaces observed by ARPES.

We report an angle-resolved photoemission spectroscopy (ARPES) study on a triangular lattice superconductor Ir1-xPtxTe2 in which the Ir-Ir or Te-Te bond formation, the band Jahn-Teller effect, and the spin-orbit interaction are cooperating and competing with one another. The Fermi surfaces of the substituted system are qualitatively similar to the band structure calculations for the undistorted IrTe2 with an upward chemical potential shift due to electron doping. A combination of the ARPES and the band structure calculations indicates that the Te 5p spin-orbit interaction removes the p(x)/p(y) orbital degeneracy and induces p(x) +/- ip(y) type spin-orbit coupling near the A point. The inner and outer Fermi surfaces are entangled by the Te 5p and Ir 5d spin-orbit interactions which may provide exotic superconductivity with singlet-triplet mixing.

We have studied local structure of IrTe2 by Ir L-3-edge extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature to investigate origin of the observed structural phase transition at T-s similar to 270 K. The EXAFS results show an appearance of longer Ir-Te bond length (Delta R similar to 0.05 angstrom) at T &lt; T-s. We have found Ir-Ir dimerization, characterized by distinct Ir-Ir bond lengths (Delta R similar to 0.13 angstrom), existing both above and below T-s. The results suggest that the phase transition in IrTe2 should be an order-disorder-like transition of Ir-Ir dimers assisted by Ir-Te bond correlations, thus indicating important role of the interaction between the Ir 5d and Te 5p orbitals in this transition.

We have used V K-edge x-ray absorption spectroscopy to study local structures of bulk, nanoparticles and nanowires of V2O5. The extended x-ray absorption fine structure measurements show different local displacements in the three morphologically different V2O5 samples. It is found that the nanowires have a significantly ordered chain structure in comparison to the V2O5 bulk. In contrast, nanoparticles have larger interlayer disorder. The x-ray absorption near-edge structure spectra show different electronic structure that appears to be related with the local atomic disorder in the three V2O5 samples. (C) 2013 AIP Publishing LLC.

We have studied local structure of LiMnO2, LiMn0.65Cr0.35O2 and LiMn0.5Ni0.5O2 compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO2 is found to be consistent with Jahn Teller distorted MnO6 octahedra characterized by two different Mn-O bond distances. The Jahn Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn-O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn-Cr distance is longer than Mn-Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn-Ni distance is shorter than Mn Mn distance). Incidentally, Mn-O and Mn Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn0.65Cr0.35O2, that is different from the case of LiMn0.5Ni0.5O2 in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics. (C) 2013 Elsevier B.V. All rights reserved.

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca-10(Ir4As8)(Fe2As2)(5). This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe2As2, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

We have studied relationship between local lattice disorder, Ru 4d spin-orbit interaction, and global spin/orbital orders of Ca 2-xSrxRuO4 in the Sr- and Ca-rich regions of its phase diagram by using unrestricted Hartree-Fock calculation on a 8 × 8 RuO4 lattice model. The calculations show that the local elongation of RuO 6 octahedron in an antiferromagnetic insulator Ca2RuO 4 induces local orbital change with making the Mott gap narrower. On the other hand, local compression of RuO6 octahedron in a paramagnetic metal Sr2RuO4 induces global antiferromagnetic or ferromagnetic states. This result is consistent with a recent systematic μSR study by Carlo et al. which has revealed the static antiferromagnetic order at low temperature in the Sr-rich region. © 2013 The Physical Society of Japan.

We report an angle-resolved photoemission spectroscopy (ARPES) study on IrTe2 which exhibits an interesting lattice distortion below 270 K and becomes triangular lattice superconductors by suppressing the distortion via chemical substitution or intercalation. ARPES results at 300 K show multi-band Fermi surfaces with six-fold symmetry which are basically consistent with band structure calculations. At 20 K in the distorted phase, topology of the inner Fermi surfaces is strongly modified by the lattice distortion. The Fermi surface reconstruction by the distortion depends on the orbital character of the Fermi surfaces, suggesting importance of Ir 5d and/or Te 5p orbital symmetry breaking.

A combination of high resolution x-ray absorption and x-ray emission spectroscopy is used to investigate electronic properties of LiCoO2 nanoparticles with various sizes down to 8 nm. We find that the nanostructuring has direct influence on the electronic structure of the title system, similar to the effect of delithiation. In particular, an abrupt reduction (increase) of the intersite (intrasite) 4p-3d hybridization is observed for nanoparticles with size smaller than 15 nm. Since the electrochemical properties are known to degrade in nanoparticles below the critical size of 15 nm, the results indicate a direct relationship between the intersite-intrasite correlations and the cathode efficiency, limiting the use of LiCoO2 nanoparticles. (C) 2013 AIP Publishing LLC.

The fundamental electronic structure of the widely used battery material LixCoO2 still remains a mystery. Soft x-ray absorption spectroscopy of LixCoO2 reveals that holes with strong O 2p character play an essential role in the electronic conductivity of the Co3+/Co4+ mixed valence CoO2 layer. The oxygen holes are bound to the Co4+ sites and the Li-ion vacancy, suggesting that the Li-ion flow can be stabilized by oxygen hole back flow. Such an oxygen hole state of LixCoO2 is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.

We have studied spin-charge-orbital orderings in a Mn3+/Mn 4+ mixed valence state on a hollandite-type lattice using unrestricted Hartree-Fock calculation on a multi-band Mn 3d-O 2p lattice model. The calculations show that all the Mn3+-Mn4+, Mn 3+-Mn3+, and Mn4+-Mn4+ superexchange interactions are ferromagnetic and play important roles to stabilize the charge and orbital ordering patterns. The most stable charge and orbital ordering pattern is consistent with the 1 × 1 × 1 orthorhombic or monoclinic structure of K1.6Mn8O16. © 2013 The Physical Society of Japan.

We investigated the electronic structure of layered Mn oxide Bi3Mn4O12(NO3) with a Mn honeycomb lattice by x-ray absorption spectroscopy and model calculations. The valence of Mn was determined to be 4+ with a small charge-transfer energy of similar to 1 eV. The values of (J(1), J(2), J(3), J(c), J(c1), and J(c2)) obtained by unrestricted Hartree-Fock calculations on Mn 3d-O 2p lattice models show that intra-layer second and third neighbor superexchange interactions J(2) and J(3) as well as inter-layer superexchange interactions J(c), J(c1), and J(c2) are enhanced due to Mn-O-O-Mn pathways, which are activated by the smallness of charge-transfer energy. The present analysis indicates that the ferromagnetic J(c1) and antiferromagnetic J(c2) are responsible to the antiferromagnetic inter-layer coupling and that the intra-layer exchange interactions with the ferromagnetic J(2) and antiferromagnetic J(3) have no frustration effect. (C) 2013 Elsevier Ltd. All rights reserved.

When periodicity of crystal is disturbed by atomic disorder, its electronic state tends to be inhomogeneous and band dispersion is expected to be obscured. In case of Fe-based superconductors, disorder of chalcogen/pnictogen height is known to cause disorder of Fe 3d level splitting. Here, we report an angle-resolved photoemission spectroscopy study on FeSe1-xTe x with the chalcogen height disorder, showing that the disorder affects the Fe 3d band dispersions in an orbital-selective way instead of simple obscuring effect. The reverse of the Fe 3d level splitting due to the chalcogen height difference causes the splitting of the hole band with Fe 3d x2-y2 character around the Γ point. © 2013 The Physical Society of Japan.

In contrast to ordinary perovskites ABO(3), A-site ordered perovskites AA'3B4O12 include transition metals in both A' and B sites. The A'-O, B-O, and A'-B interactions in A-site ordered perovskites give rise to various physical properties. In this study, we report the O 1s and Cu 2p x-ray absorption spectroscopy (XAS) and resonant x-ray emission spectroscopy (RXES) study on the A-site ordered perovskite ACu(3)Ru(4)O(12) (A = Na, Ca, and La). Hartree-Fock calculation and cluster model calculation were carried out to analyze O 1s and Cu 2p XAS spectra. The O 1s RXES spectra indicate that the number of O 2p holes increases in going from La to Ca to Na. In addition, the Cu 2p RXES spectra reveal that the charge-transfer (CT) energy decreases in going from La to Ca to Na. The decrease of CT is consistent with our calculations and suggest an increase of the O 2p holes in the ground state. The substitution of the A site ion changes not only the carrier concentration in the Ru 4d and Cu 3d bands but also the O 2p to Cu 3d CT energy.

We have investigated the electronic structure of superconducting (SC) and nonsuperconducting (non-SC) KxFe2-ySe2 using x-ray photoemission spectroscopy (XPS). The spectral shape of the Fe 2p XPS is found to depend on the amount of Fe vacancies. The Fe 2p3/2 peak of the SC and non-SC Fe-rich samples is accompanied by a shoulder structure on the lower binding energy side, which can be attributed to the metallic phase embedded in the Fe2+ insulating phase. The absence of the shoulder structure in the non-SC Fe-poor sample allows us to analyze the Fe 2p spectra using a FeSe4 cluster model. The Fe 3d-Se 4p charge-transfer energy of the Fe2+ insulating phase is found to be ∼2.3 eV which is smaller than the Fe 3d-Fe 3d Coulomb interaction of ∼3.5 eV. This indicates that the Fe2+ insulating state is the charge-transfer type in the Zaanen-Sawatzky-Allen scheme. We also find a substantial change in the valence-band XPS as a function of Fe content and temperature. The metallic state at the Fermi level is seen in the SC and non-SC Fe-rich samples and tends to be enhanced with cooling in the SC sample. © 2013 American Physical Society.

We have studied surface electronic structure and light illumination effect of spinel-type MgTi2O4 with Ti3+(d1) electronic configuration. Ti4+ species are found at surfaces of MgTi2O4, which show photo-induced effect similar to the Ti4+-oxide semiconductors with surface depletion layer. MgTi 2O4 with the surface Ti4+ states reacts with water for H2 emission, which is largely enhanced by light illumination. © 2013 Author(s).

Temperature dependent Co K-edge extended X-ray absorption fine structure is used to investigate local disorder in LiCoO2 nanoparticles. We find that the nanostructuring has direct influence on the bondlength characteristics. The results reveal a substantial decrease in the force constant of Co-O bonds (the Co-O bonds becoming more flexible), while that for the Co-Co bonds showing hardly any change (or increases slightly) in LiCoO2 nanoparticles with respect to the bulk. Therefore, both random disorder and Co-O bondlength flexibility should be the factors to limit the battery characteristics of the LiCoO2 nanoparticles.

In this paper it is shown that the "anti-Jahn-Teller effect" plays an essential role in giving rise to a small Fermi surface of Fermi pockets above T-c and d-wave superconductivity below T-c in underdoped cuprates. In the first part of the present paper, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn-Teller Physics and Mott Physics. It is shown that the feature of Fermi surfaces in underdoped LSCO is the Fermi pockets in the nodal region constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. In the antinodal region in the momentum space, there are no Fermi surfaces. Then it is discussed that the phonon-involved mechanism based on the Kamimura-Suwa model leads to the d-wave superconductivity. In particular, it is shown that the origin of strong electron-phonon interactions in cuprates is due to the anti-Jahn-Teller effect. In the second part a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below T-c is discussed. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and the real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

We have studied Jahn-Teller effects in Cs2Au2Br6, ACu(3)Co(4)O(12) (A=CaorY), and IrTe2 in which the ligand p-to-transition-metal d charge-transfer energy is small or negative. The Au+/Au3+ charge disproportionation of Cs2Au2Br6 manifests in Au 4f photoemission spectra. In Cs2Au2Br6 with negative Delta and intermediate U, the charge disproportionation can be described using effective d orbitals constructed from the Au 5d and Br 4p orbitals and is stabilized by the Jahn-Teller distortion of the Au3+ site with low-spin d(8) configuration. In ACu(3)Co(4)O(12), Delta s for Cu3+ and Co4+ are negative and Us are very large. The Zhang-Rice picture is valid to describe the electronic state, and the valence change from Cu2+/Co4+ to Cu3+/Co3+ can be viewed as the O 2p hole transfer from Co to Cu or d(9) + d(6)L -&gt; d(9)L+d(6). In IrTe2, both Delta and U are small and the Ir 5d and Te 5p electrons are itinerant to form the multi-band Fermi surfaces. The ideas of band Jahn-Teller transition and Peierls transition are useful to describe the structural instabilities.

We report a photoemission spectroscopy study on IrTe2 which exhibits an interesting lattice distortion below 270 K. Ir 4f core-level photoemission spectrum is broadened in going from 300K to 40K. This result indicates the charge moderation of Ir 5d electrons. Angle-resolved photoemission spectroscopy(ARPES) result at 300K shows multi-band Fermi surfaces with six-fold symmetry which are basically consistent with band structure calculations. At the distorted phase, the inner Fermi surfaces become quasi one-dimensional Fermi surface due to the lattice distortion.

The electronic structure of FeSe1-xTex chalcogenide superconductors has been studied by x-ray emission (XES) and x-ray absorption (XAS) as a function of Te substitution. The Fe K beta XES spectra reveal a relatively low spin state for Fe in FeSe1-xTex superconductors, persisting in the whole range of Te substitution. The Fe K-edge high-resolution XAS shows systematic spectral changes due to the evolving hybridization between the Fe 3d (4p) and chalcogen p (d) orbitals. The resonant inelastic x-ray scattering (RIXS) spectra hardly show any feature except the one due to charge transfer from occupied to unoccupied bands, that changes substantially from FeSe to FeTe. The results provide important information on the electronic states and their evolution in the FeSe1-xTex chalcogenides.

The role of Co substitution in the low-energy electronic structure of Ca(Fe0.944Co0.056)(2)As-2 is investigated by resonant photoemission spectroscopy and density-functional theory. The Co 3d state center of mass is observed at 250 meV higher binding energy than that of Fe, indicating that Co possesses one extra valence electron and that Fe and Co are in the same oxidation state. Yet, significant Co character is detected for the Bloch wave functions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2 and the inadequacy of a rigid-band shift description.

We have studied the local structure of LiCoO2 nanoparticles by Co K-edge x-ray absorption spectroscopy as a function of particle size. Extended x-ray absorption fine structure data reveal substantial changes in the near neighbor distances and the associated mean square relative displacements with decreasing particle size. X-ray absorption near edge structure spectra show clear local geometrical changes with decreasing particle size, similar to those that appear in the charging (delithiation) process. The results suggest that the LiCoO2 nanoparticles are characterized by a large atomic disorder confined to the Co-O octahedra, similar to the distortions generated during the delithiation, and this disorder should be the primary limiting factor for a reversible diffusion of Li ions when nanoparticles of LiCoO2 are used as cathode material in rechargeable Li ion batteries.

We report temperature-dependent angle-resolved photoemission spectroscopy measurement of Ta2NiSe5 which shows a semiconductor-semiconductor structural phase transition at around 330 K. Characteristically, flat band at the top of the valence band is observed, which is ascribed to the excitonic insulator effect. The top valence band shifts to higher binding energy and its bandwidth increases as the temperature decreases.
As the system exceeds the transition temperature, the flat feature of the valence band weakens though the exciton fluctuations remain finite.

We have studied the electronic structure of FeSe1-x Te (x) and Ir1-x Pt (x) Te-2 using photoemission spectroscopy. For FeSe1-x Te (x) , angle-resolved photoemission results indicate that the Fe 3d yz/zx orbital degeneracy at I" point and orbitally induced Peierls effect in the tetragonal lattice play important roles for the superconductivity. It is suggested that the Jahn-Teller instability of the yz/zx states couples with local lattice distortion derived from the Te substitution for Se and provides an inhomogeneous electronic state. Photoemission results of IrTe2 with triangular lattice are also consistent with the orbitally induced Peierls scenario. The Pt substitution for Ir suppresses the static band Jahn-Teller effect and induces an inhomogeneous electronic state in which orbital (or bond or nematic) fluctuations may help superconductivity through the Peierls effect.

We have studied the electronic structure of the triangular lattice Ir1-xPtxTe2 superconductor using photoemission spectroscopy and model calculations. Ir 4f core-level photoemission spectra show that Ir 5d t(2g) charge modulation established in the low-temperature phase of IrTe2 is suppressed by Pt doping. This observation indicates that the suppression of charge modulation is related to the emergence of superconductivity. Valence-band photoemission spectra of IrTe2 suggest that the Ir 5d charge modulation is accompanied by Ir 5d orbital reconstruction. Based on the photoemission results and model calculations, we argue that the orbitally induced Peierls effect governs the charge and orbital instability in Ir1-xPtxTe2.

We have studied electronic and magnetic properties of the K0.8Fe1.6Se2 superconductor by x-ray absorption and emission spectroscopy. Detailed temperature-dependent measurements along with a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with a high-spin Fe3+ state and a minority phase with an intermediate-spin Fe2+ state. The effect of high-temperature annealing suggests that the compressed phase with a lower-spin Fe2+ state is directly related to the high-T-c superconductivity in the title system. The results clearly underline the glassy nature of superconductivity in the electronically inhomogeneous K0.8Fe1.6Se2, similar to the superconductivity in granular phases.

Binary Ni-C thin-film alloys, which have been shown to be passive against corrosion in hot sulphuric acid solution whilst also being electrocatalytically active, were investigated by XPS to determine the oxidation state of the metal and carbon components. The Ni component produces a Ni 2p spectrum similar to that of metallic nickel (i.e., no oxidation occurs) but with a 0.3 eV shift to higher binding energy (BE) due to electron donation to the carbon matrix. The C 1s peak shows a shift to lower BE by accepting electrons from the Ni nanocrystals. A cluster-model analysis of the observed Ni 2p spectrum is consistent with the electron transfer from the nickel to the carbon. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4722785]

X-ray absorption spectroscopy is one of the most widely used experimental techniques to study the electronic and spatial structure of materials. Fluorescence yield mode is bulk-sensitive, but has several serious problems coming from saturation effects. In this study, we show the usefulness of partial fluorescence yields in addressing these problems. We discuss the different behaviors of La2NiMnO6 and LiMnO2 at the Mn 2p absorption edges. The total fluorescence yield produces misleading spectra for LiMnO2 due to the absence of high-Z (Z: atomic number) elements. We conclude that the measurement of the inverse partial fluorescence yield is essential in studies of LiMnO2, which is a hotly debated Li-ion battery material. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4711801]

Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K1.72Mn[Mn(CN)(6)](0.93)center dot square(0.07).0.65H(2)O (square: a [Mn(CN)(6)](4-) defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li+. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/discharge experiments revealed that the framework can accommodate a large amount of Li-ions.

Angle resolved photoemission spectroscopy of Ba(Fe1-xCo x)2As2 (x = 0:06, 0.14, and 0.24) shows that the width of the Fe 3dyz/zx hole band depends on the doping level. In contrast, the Fe 3dx2-y2 and 3z2 - r2 bands are rigid and shifted by the Co doping. The Fe 3dyz/zx hole band is flattened at the optimal doping level x = 0:06, indicating that the band renormalization of the Fe 3dyz/zx band correlates with the enhancement of the superconducting transition temperature. The orbital-dependent and doping-dependent band renormalization indicates that the fluctuations responsible for the superconductivity is deeply related to the Fe 3d orbital degeneracy. © 2011 The Physical Society of Japan.

We have studied the electronic structure of multiferroic BiCoO3 using x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and subsequent model calculations. The XAS results show that the Co3+ ion takes the high-spin d(6) configuration which usually prefers G-type antiferromagnetic state. The XPS results and model Hartree-Fock calculations show that, in case of BiCoO3, small charge-transfer energy plays an essential role to enhance the Co-O-O-Co superexchange pathways. It is found that the combination of ferro-type orbital ordering of Co 3d t(2g) and the Co-O-O-Co superexchange interaction gives rise to C-type antiferromagnetic state in BiCoO3. The present analysis suggests that, in addition to the Bi-O bonds responsible for the noncentrosymmetric deformation, the O-O bonds are important to stabilize the multiferroic phase of BiCoO3.

We report an angle-resolved photoemission spectroscopy study of BaFe 2-x Co x As 2. For x=0, above the structural and magnetic transition temperature (T s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn-Teller type instability is playing an important role in the tetragonal phase above T s . Below T s, the spectral weight of 0-100 meV is reconstructed to form flat bands at 70-100 meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron-lattice fluctuation due to the orbital degeneracy is important. © 2010 Springer Science+Business Media, LLC.

We report an x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) study on K(2)V(8-x) Ti(x)O(16) (x = 0.0, 0.3, and 4.0) in which the metal-insulator transition (MIT) and the tetragonal-to-monoclinic structural transition show different responses against the Ti doping. The spectral change of V 2p XAS across the MIT is similar to that observed in VO(2), indicating that the V-V pairing is responsible for the MIT. In the insulating phase, the V 2p XPS spectra are split into the V(3+) and V(4+) features showing the localized character of the V 3d electrons. In addition, the V 2p XAS spectra of the monoclinic insulating phase suggest that the V-V pairing is weakened, and, instead, the V(3+)-V(4+) coupling is strengthened by orbital ordering. In contrast to VO(2), the existence of the V(3+) component in K(2)V(8-x) Ti(x)O(16) gives additional orbital instability in the insulating phase, which competes with the V-V pairing.

Fe L-2,L-3-edge x-ray absorption spectroscopy has been used to study the electronic structure of FeSe1-xTex chalcogenides. The spectra are quite similar to those for the metallic Fe and Fe-based pnictides, consistent with weak correlation effects in the chalcogenides. A systematic change in the experimental spectra with Te substitution is observed and well reproduced by cluster calculations. A combined experimental and theoretical analysis reveals decreased hybridization of Fe 3d and chalcogen p states with increasing Te concentration, which could be important for the superconductivity and magnetism of the chalcogenides.

We report on an electronic structural study of BaFe2As 2 which has a spin-density-wave (SDW) transition and is a parent material of FeAs-based superconductors. Angle-resolved photoemission spectroscopy on BaFe2As2 shows that, across the SDW transition, the spectral weight at 0-100 meV below EF is reconstructed to form flat bands and Fermi surfaces in the SDW state. The coexistence of the Fermi surfaces and the flat band supports the orbital-dependent excitonic coupling between electron and hole pockets rather than the simple Fermi surface nesting as the origin of the SDW state. © 2009 Elsevier B.V. All rights reserved.

On the basis of unrestricted Hartree-Fock calculations on multi-orbital d-p Hamiltonians for a CuO layer and an FeAs layer, we give a qualitative discussion on the interplay between band Jahn-Teller effect and spin (and/or charge) density wave in the CuO and FeAs layers. In addition, we argue the relationship between the spin density wave, the pseudogap, and the superconductivity. (C) 2009 Elsevier B.V. All rights reserved.

We report on an electronic structural study of BaFe1.7Co 0.3As2 in heavily electron doped regime of the FeAs-based superconductors. Three hole bands around the zone center are clearly observed and two of them reach the Fermi level (EF) forming two hole pockets. Top of the lowest hole band is located at ∼30 meV below EF showing that it undergoes a pocket-vanishing type Lifshitz transition by electron doping. Around the zone corner, two electron pockets are clearly observed, and the smaller electron pocket is apparently nested with the larger hole pocket at the zone center. © 2009 Elsevier B.V. All rights reserved.

Fe K-edge and Se K-edge x-ray absorption near edge structure (XANES) measurements are used to study the FeSe1-xTex electronic structure of chalcogenides. An intense Fe K-edge pre-edge peak due to Fe 1s -&gt; 3d (and admixed Se/Te p states) is observed, showing substantial change with Te substitution and x-ray polarization. The main white line peak in the Se K-edge XANES due to Se 1s -&gt; 4p transition appears similar to the one expected for Se2- systems and changes with Te substitution. Polarization dependence reveals that unoccupied Se orbitals near the Fermi level have predominant p(x,y) character. The results provide key information on the hybridization of Fe 3d and chalcogen p states in the Fe-based chalcogenide superconductors.

We report a photoemission study of La8-xSrxCu8O20 which shows antiferromagnetic (AFM), weakly ferromagnetic (WFM), and paramagnetic (PM) phases. All the samples in the AFM, WFM or PM phases are found to have a sharp Fermi edge with finite density of states at the Fermi level (E-F), indicating the metallic nature of the samples at different doping and temperatures studied. In the WFM and AFM phases, the spectral weight near the E-F (up to similar to-200 meV) is partially suppressed. In the valence band spectra, the Cu 3d and O 2p derived states around similar to-5 and similar to-2.5 eV show different spectral weights in different magnetic phases. The observed changes in the electronic structure can be due to formation of the charge and/or spin density waves causing the anomalies in the electronic and magnetic transport properties of this system.

We report on an electronic structural study of LixCoO2 single crystals (x = 0.99, 0.71, 0.66, and 0.46) which have hole-doped CoO2 triangular lattices. The valence-band photoemission spectra show that the Fermi level is located near the top of the Co 3d t(2g) bands and that, by the reduction in x, the Co 3d t(2g) peak is shifted to the lower binding-energy side. This energy shift is consistent with the chemical-potential shift by the hole doping to the t(2g) bands. The fine structures near the Fermi level indicate the splitting of the t(2g) bands into the a(1g) and e(g)' components. The electronic structure parameters such as the charge-transfer energy Delta are obtained by the cluster-model analysis of the Co 2p core-level spectra. The unrestricted Hartree-Fock calculation using the obtained parameter values predicts that the doped holes are accommodated by the a(1g) band up to the doping level x of 0.46 which is consistent with the observation in the valence-band spectra. However, the valence-band spectra cannot be reproduced by the unrestricted Hartree-Fock calculation indicating that the correlation effect from the electron-electron and electron-phonon interactions is substantial in LixCoO2.

We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin states. We found that Li deficiencies occur in the case of Cr substitution, whereas there are no Li deficiencies in the case of Ni substitution. We conclude that the substitution of charge-transfer-type Ni or Cu is effective for LiMnO2 battery materials. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3463468]

The discovery of a new electrode material that provides a reversible Li ion insertion/extraction reaction is of primary importance for Li ion batteries. In this report, electrochemical Li ion insertion/extraction in valence tautomeric Prussian blue analogues A(x)Mn(y)[Fe(CN)(6)] (A: K, Rb) was investigated. Ex situ X-ray diffraction experiments revealed that the K salt without the valence tautomerism exhibits the Li ion insertion/extraction with a redox process of an Fe ion, while the Rb salt with the valence tautomerism exhibits that with a redox process of a Mn ion. Regardless of the redox-active metal ions, highly reversible Li ion storage was achieved. The electronic structure changes during the Li ion insertion/extraction are confirmed by XPS experiments.

An angle-resolved photoemission spectroscopy (ARPES) study is reported on a Mott insulator NiGa2S4 in which Ni2+ (S = 1) ions form a triangular lattice and the Ni spins do not order even in its ground state. The first ARPES study on the two-dimensional spin-disordered system shows that low-energy hole dynamics at high temperatures is characterized by wave vectors Q(E) which are different from wave vectors Q(M) dominating low-energy spin excitations at low temperatures. The unexpected difference between Q(E) and Q(M) is deeply related to charge fluctuation across the Mott gap in the frustrated lattice and is a key issue to understand the spin-disordered ground states in Mott insulators.

We have studied the electronic structure of A-site ordered perovskite oxides ACu(3)V(4)O(12) (A = Na, Ca, and Y) using photoemission spectroscopy. V and Cu valence fluctuations are clearly observed in the V 2p and the Cu 2p core-level spectra. In going from A = Na to Ca to Y, electrons doped by the A-site substitution are mainly captured by the Cu site which has a mixture of +2(d(9)) and +3 (d(9)L, L is an O 2p hole) and are partly distributed into the V site. The photoemission spectra near the Fermi level show a systematic change induced by the A-site substitution. The electronic structure of ACu(3)V(4)O(12) is very different from that in Ca1-xSrxVO3 due to the existence of the Cu 3d electrons, and the unusual correlation effect in ACu(3)V(4)O(12) can be related to the valence fluctuations observed in the core-level spectra.

We report on an electronic structure study of SrTiO3 under ultraviolet illumination. Photoemission measurements at 20 K under ultraviolet illumination show that the illumination induces electronic states within the band gap. The photo-induced in-gap states show the power-law energy distribution with exponent of 3.4-3.7 and weakly depends on the strength of illumination. The photo-induced in-gap states indicate that most of the photo-excited electrons are trapped by local lattice distortions with various size scale. We argue that the free carriers in the Ti 3d band and the localized Ti 3d electrons and O 2p holes within the band gap contribute to the photoconductivity and the ferroelectricity under the ultraviolet illumination, respectively.

In the heavily-electron-doped regime of the Ba(Fe,Co)2As 2 superconductor, three hole bands at the zone center are observed and two of them reach the Fermi level. The larger hole pocket at the zone center is apparently nested with the smaller electron pocket around the zone corner. However, the (π,0) Fermi surface reconstruction reported for the hole-doped case is absent in the heavily-electron-doped case. This observation shows that the apparent Fermi surface nesting alone is not enough to enhance the antiferromagnetic correlation as well as the superconducting transition temperature. © 2010 The American Physical Society.

X-ray photoemission spectroscopy measurements with ultraviolet laser illumination have been performed for anatase Ti1-xCoxO2-delta thin films with x=0.05 and 0.10 in order to investigate the interplay between the Co spins and the photoinduced carriers in the surface region. We have found that the surface band bending is removed by the ultraviolet illumination, indicating that photoinduced carriers are injected into the surface depletion layer. After the carrier injection, the position of the chemical potential is governed by the exchange splitting of the Ti 3d conduction band due to the magnetic interaction between the photoinduced carriers and the Co spins.

We report on a photoemssion study of A-site ordered perovskite oxides ACu(3)Co(4)O(12) (A=Ca and Y) which are metallic for A=Ca and insulating for A=Y. If the Cu valence is +2 as stabilized by the square planar coordination, the Co valence for A=Ca is formally +4 and the system could be a Mott insulator while that for A=Y should be +3.75 and the system is a doped Mott insulator. The photoemission study reveals that the Cu valence for A=Ca and Y is formally +3 with the d(9)L configuration (L: an O 2p hole) which corresponds to the Zhang-Rice singlet state. In addition, it is found that the Co valence for A=Y is formally +3 with the low-spin d(6) configuration consistent with its insulating behavior.

We have studied the effects of Madelung potentials on the shifts in the core-level photoemission spectra by investigating La0.6Sr0.4MnO3 thin films grown on three kinds of substrates, SrTiO3, (LaAlO3)(0.3)-(SrAl0.5Ta0.5O3)(0.7), and LaAlO3. The experimental shifts in the La 4d and Sr 3d core levels are almost the same as the calculated Madelung potentials, which we attribute to the nearly ionic character of these atoms. On the other hand, the experimental shifts in the O 1s and Mn 2p core levels are negligibly small. We consider that this is due to the strong covalent character of the Mn-O bonds.

We have performed a systematic tight-binding analysis of the angle-resolved photoemission spectroscopy (ARPES) spectra of a series of transition-metal (TM) oxides A(TM)O-3 and compared the obtained electronic structure parameters with those well established values obtained from cluster-model analyses of photoemission spectra. The values of epsilon(d) - epsilon(p) from ARPES are found to be much larger than the values which would be expected for the limit of weak p-d hybridization, indicating that the "effective" TM 3d hands are originated from the strong hybridization with the O 2p bands even for light TM oxides.

We have studied the d-electron heavy-fermion states in ACu(3)Ru(4)O(12) (A=Ca, Na, and La) using x-ray photoemission spectroscopy and high-resolution photoemission spectroscopy at bulk-sensitive photon energy (7 eV). The bulk-sensitive photoemission measurements for A=Ca, Na, and La show that a resonancelike structure within a pseudogap commonly evolves in the low-temperature range where the Kadowaki-Woods relation holds. The origin of the resonancelike peak and the heavy-fermion behavior is basically attributed to the hybridization between the Cu 3d and Ru 4d orbitals. The A-site substitution controls the filling of the Cu 3d and Ru 4d hybridized bands. Interestingly, while the resonancelike peak position roughly follows the rigid-band shift for A=Na and La, the resonancelike peak deviating from the rigid-band behavior is located just at the Fermi level as expected in a Kondo system. The difference in the resonancelike peak near the Fermi level is correlated with that in the Cu 2p core-level spectra.

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

We report on a new type of orbital symmetry breaking in the CoO2 triangular lattice with hole doped t(2g) bands. Unrestricted Hartree-Fock calculations on a multi-orbital d-p Hamiltonian show that, in the lightly hole doped t(2g) bands, the e(g)' orbitals are full occupied with a circular charge distribution, and the doped holes in the a(1g) band form a hole pocket sandwiched by two concentric Fermi surfaces with sixfold (and almost continuous) rotational symmetry. This symmetric state has an instability to an anisotropic state with twofold rotational symmetry, where the hole pocket is split into two pieces. This transition is governed by the anisotropic interaction between the a(1g) and e(g)' orbitals.

We have studied the electronic structure of the Fe and Ni triangular lattices in FeGa2S4, Fe2Ga2S5, and NiGa2S4 using photoemission spectroscopy measurements, configuration-interaction calculations on FeS6 and NiS6 cluster models, and unrestricted Hartree-Fock calculations on FeS2 and NiS2 triangular lattices. The cluster-model analysis of the Fe 2p core-level spectra shows that the S 3p to Fe 3d charge-transfer energy Delta is similar to 2.5 eV in FeGa2S4 and Fe2Ga2S5, in contrast to the small Delta (similar to-1 eV) found in NiGa2S4. The relationship between the Delta value and the superexchange pathway has been examined using the unrestricted Hartree-Fock calculations. In FeGa2S4 and Fe2Ga2S5, the superexchange interaction between the nearest-neighbor sites is dominant while that between the third-nearest-neighbor sites is enhanced in NiGa2S4.

Transition-metal compounds with spin, charge, and orbital degrees of freedom tend to have frustrated electronic states coupled with local lattice distortions and to show drastic response to external stimuli such as photo-excitation. We have studied the charge-orbital states in perovskite-type Pr-0.55(Ca1-ySry)(0.45)MnO3 thin films (PCSMO) and spinel-type CuIr2S4 using photoemission spectroscopy combined with additional laser illumination. PCSMO and CuIr2S4 are clear-cut examples of transition-metal compounds showing photo-induced metallic conductivities but the charge-orbital states in the two systems show contrasting responses to the photo-excitation. The charge-orbital states in PCSMO are stabilized by Jahn-Teller or Breathing-type lattice distortions and can be destroyed by photo-excitation. On the other hand, the charge-orbital states in CuIr2S4 are stabilized by dimer formation and tend to be robust against photo-excitation.

Transition-metal compounds with spin, charge, and orbital degrees of freedom tend to have frustrated electronic states coupled with local lattice distortions and to show drastic response against external stimuli such as optical excitation. By means of photoemission spectroscopy, we have studied the electronic states of transition-metal compounds with corner-sharing and edge-sharing MX(6) octahedra (M = transition metal, X = O, S, Se, Br) such as prerovskite-type Pr(0.55)(Ca(1-y)Sr(y))(0.45)MnO(3) and Cs(2)Au(2)Br(6), spinel-type CuIr(2)S(4), and quasi-one-dimensional Ta(2)NiSe(5). In the perovskite compounds with corner-sharing octahedra, the charge-orbital states are stabilized by Jahn-Teller or breathing-type lattice distortions and can be destroyed by optical excitations. On the other hand, the charge-orbital states in the edge-sharing systems are stabilized by dimer formation and tend to be robust against optical excitations. Based on the photoemission results, we will discuss effects of local lattice distortions on the excitonic states obtained by optical excitations as well as those in ground states.

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr2S4 using ultraviolet photoemission spectroscopy (UPS). The Ir3+/Ir4+ charge-ordered gap appears below the metal-insulator transition temperature. However, in the insulating phase, in-gap spectral features with softgap are observed in UPS just below the Fermi level (E-F), corresponding to the variable range hopping transport observed in resistivity. The spectral weight at E-F is not increased by light illumination, indicating that the Ir4+-Ir4+ dimer is very robust, although the long-range octamer order would be destructed by the photoexcitation. Present results suggest that the Ir4+-Ir4+ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr2S4.

We report on an electronic structure study of a quasi-two-dimensional Co oxide Ca3Co4O9 with Ca2CoO3 rocksalt layers and CoO2 triangular lattice layers by means of x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), configuration-interaction calculation on a CoO6 cluster model, and unrestricted Hartree-Fock calculation on a multiband d-p model. The Co 2p XPS spectrum shows that the Co valence of the rocksalt layer is similar to that of the triangular lattice layer. The cluster-model analysis of the Co 2p XPS spectrum indicates that the Co sites of the rocksalt and triangular lattice layers commonly have charge-transfer energy Delta of similar to 1.0 eV, d-d Coulomb interaction U of similar to 6.5 eV, and transfer integral (pd sigma) of similar to-2.3 eV. The Co 3d t(2g) peak in the valence-band XPS spectrum remains sharp even above the spin-state transition temperature at similar to 380 K, indicating that the spin-state transition is different from the low-spin to intermediate-spin or high-spin transitions in perovskite LaCoO3. The line shape of the UPS spectrum near the Fermi level can be reproduced by the combination of unrestricted Hartree-Fock results for the charge-ordered insulating (COI) and paramagnetic metallic (PM) states. The analysis shows that the phase separation between the COI and PM states plays important roles in Ca3Co4O9.

We have studied the electronic structure of the FeAs layer in LaFeAsO using unrestricted Hartree-Fock calculations on a multi-orbital d-p Hamiltonian. Based on the calculated results, we argue that the antiferromagnetism of LaFeAsO is derived from the excitonic effect between the hole pocket at the G point and the electron pocket at the M point that is coupled with the band Jahn-Teller effect due to the yz/zx orbital degeneracy at the G point.

A persistent photoinduced metal-to-insulator transition has been confirmed in a manganite thin film, Pr(0.55)(Ca(0.75)Sr(0.25))(0.45)MnO(3), near a multicritical point by monitoring with transport measurements and x-ray photoemission spectroscopy. Together with the previously reported reverse effect, the photoinduced insulator-to-metal transition, it is found that the relative stability of the metallic and insulating phases interchanges around 80 K in the middle of a very wide hysteresis loop, which is a manifestation of the large potential barrier due to the long-range elastic energy. It is shown that photons are much more effective in overcoming the barrier via the electronically excited intermediate states than via the heat mode.

In order to understand the nature of chain-driven superconductivity in Pr2Ba4Cu7O15-delta (Pr247), we have studied the electronic structure of as-sintered non-superconducting Pr247 and annealed superconducting Pr247 samples using photoemission spectroscopy. The O 1s, Ba 3d, Pr 4d, and Cu 2p core-level energy shifts by annealing indicate that electrons doped by the annealing mainly fill the localized Fehrenbacher-Rice (FR) state. The photoemission spectra rho(omega) near the Fermi level (E-F) are derived from the metallic Cu-O double chain and can be fitted to the power-law function plus step function A omega(alpha) + B theta(omega). rho(omega) near E-F is indistinguishable between the as-sintered and annealed Pr247 and the exponent a is similar to 1.5. These results indicate that the Cu-O double chain provides the spectral weight near E-F and that the annealing affects the localized FR state instead of the near-E-F state.

Perovskite-type BiNiO(3) is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for 0.05 &lt;= x &lt;= 0.1 a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of Bi(1-x)La(x)NiO(3) to investigate how the electronic structure changes across the MIT. From the Ni 2p XAS spectra of x=0.05, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for x=0.05 at a low temperature, whereas for x=0.1 and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the O 2p band and that the "insulating" phase below the MIT is indeed a mixture of insulating and metallic regions.

We have studied the charge-orbital states in spineltype CuIr2S4 and perovskite-type Pr-0.55(Ca1- ySry)(0.45)MnO3 (PCSMO, y = 0.25, 0.40) using X-ray photoemission spectroscopy (XPS) combined with laser illumination. The XPS data of CuIr2S4 are dramatically changed by the insulator metal transition (IMT). The line shape of CuIr2S4 in the insulating phase is consistent with the charge disproportionation of Ir3+: Ir4+ = 1: 1. Interestingly, a visible light irradiation at low temperature gives a resistance reduction but does not give any spectral change. In case of PCSMO, the IMT is also clearly observed on the XPS data and the peculiar temperature dependence for y = 0.25 suggests the coexisting of the ferromagnetic metallic phase and the charge-orbital ordered insulating phase. The spectra after visible light illumination show the enhancement of the density of states at the Fermi level, accompanied with the drop of the resistance.

We have studied the optical response and the electronic structure of Zn-doped MgAl2O4 using optical transmission, emission, and excitation spectroscopies, x-ray photoemission spectroscopy, and unrestricted Hartree-Fock calculation. Emission lines at 710, 650, and 470 nm observed in pure MgAl2O4 are related to the Mg vacancies and Mg-Al antisite defects. Interestingly, the intensities of these emission lines are enhanced by Zn doping. Unrestricted Hartree-Fock calculation for Zn-doped MgAl2O4 shows that in-gap impurity states are formed just above the valence-band maximum of MgAl2O4 when the Zn ion is substituted for the B-site Al ion. On the other hand, no in-gap state is formed when the Zn ion is substituted for the A-site Mg ion. The position of the Zn (3d) impurity level is identified by the photoemission measurement. The broad spectral features of the defect-induced states in pure MgAl2O4 is dramatically reduced by the Zn doping, indicating that holes supplied from the Zn ions at B site are trapped by the defect-induced states.

We report an x-ray photoemission spectroscopy study of spin-crossover complex [Fe(Ptz)(6)](BF4)(2) (ptz = 1-propyltetrazole) to identify the electronic changes due to temperature-induced and photoinduced spin transitions. The energy difference between the Fe 2p and F Is peaks is found to correlate with the low-spin (LS) to high-spin (HS) transition of [Fe(Ptz)(6)](BF4)(2). The Fe 2(P3/2) spectrum of the HS state is accompanied by a charge-transfer satellite and is analyzed by configuration-interaction cluster-model calculations. Based on the electronic-structure parameters from the cluster-model analysis, the stability of the LS state against the HS state is calculated as a function of the Fe-N bond length. A drastic bond-length reduction is required to stabilize the LS state and is also related to the observed core-level shift. The postillumination spectral shape well reproduces the spectrum before the laser illumination, suggesting that the surface of the Fe complex is chemically stable against the laser illumination for the photoinduced spin transition.

Strong emission bands in the visible region are observed in MgAl2O4 crystals doped with transition-metal ions under excitation at the band-to-band transitions. We report optical responses of Cr-, Co-, and Ni-doped MgAl2O4 and present optical models for M-doped MgAl2O4 (M=Ti, V, Cr, Mn, Co, and Ni) to describe the charge-transfer transitions and the transitions between multiplet levels of 3d electrons, which are observed competitively or coexisting, depending on the number of 3d electrons. While the optical responses of Cr- and Ni-doped MgAl2O4 are dominated by the multiplet-multiplet transitions, those of Ti- and V-doped MgAl2O4 are governed by the charge-transfer transitions. The two kinds of transitions coexist in the Mn- and Co-doped MgAl2O4. These behaviors are well understood based on the numerical results of unrestricted Hartree-Fock approximation.

We have studied the electronic structure of the Ni triangular lattice in NiGa2S4 using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2p core-level spectrum shows that the S 3p to Ni 3d charge-transfer energy is similar to-1 eV and the ground state is dominated by the d(9)L configuration (L is a S 3p hole). Cell perturbation analysis for the NiS2 triangular lattice indicates that the strong S 3p hole character of the ground state provides the enhanced superexchange interaction between the third-nearest-neighbor sites.

Perovskite manganite thin films, Pr-0.55(Ca1-ySry)(0.45)MnO3, have been studied using x-ray photoemission spectroscopy in order to clarify the consequence of the competition between ferromagnetic metal (FM) and charge-orbital-ordered insulator (COOI). Films with y=0.40 undergo a uniform paramagnetic insulator to FM transition. On the other hand, in films with y=0.25, the composition near the bicritical point, phase separation of COOI and FM domains is indicated by the spectral change below 125 K. Interestingly, between 50 K and 70 K, the visible laser illumination transfers the COOI-like spectra obtained in cooling process to the FM-like spectra obtained in warming process. This indicates that the photoinduced insulator to metal transition is governed by the increase of the FM volume fraction and is deeply related to the phase separation between the FM and COOI states.

We present high-temperature electrical transport and thermoelectric power of K and/or La-doped Ca3Co4O9-based ceramics. Temperature and doping dependence of electrical transport properties indicate that both K and La substitutions can increase electrical conductivity at a high temperature, which are mainly related to an increase in carrier concentration and carrier mobility. A metallic-to-semiconducting transition temperature (T-*) can be observed and shifts to a lower temperature as the doping content increases. However, activation energy is almost independent of the doping. All of the samples show positive thermoelectric power, and the temperature dependence of the thermoelectric power exhibits a simple linear behavior at T &gt; T-*. Our experimental data indicate that all of the Ca3Co4O9-based ceramic samples exhibit large thermoelectric power (similar to 120 to 180 mu V/K).

We have studied the electronic structure of the spinel-type V oxides LiV2O4, ZnV2O4, and CdV2O4 using x-ray photoemission spectroscopy. The p-d charge-transfer energy was estimated from the valence-band spectra and the exchange interaction due to d-d transfer T-sigma was found to be larger than that due to p-d transfer T-pi in these systems, consistent with the existing theoretical models. Based on the V 2p core-level spectra, we discuss the oxidation state of V ions in the surface region. Interestingly, only in ZnV2O4 does the oxidation state of V ions strongly depend on the procedure of sample preparation.

We have performed an angle-resolved photoemission spectroscopy study of La0.6Sr0.4FeO3 using in situ prepared thin films and determined its band structure. The experimental band dispersions could be well explained by an empirical band structure assuming the G-type antiferromagnetic state. However, the Fe 3d bands were found to be shifted downward relative to the Fermi level (E-F) by similar to 1 eV compared with the calculation and to form a gap of similar to 1 eV at E-F. We attribute this observation to a strong localization effect of doped holes due to polaron formation.

Nowadays it has become feasible to perform angle-resolved photoemission spectroscopy (ARPES) measurements of transition-metal oxides with three-dimensional perovskite structures owing to the availability of high-quality single crystals of bulk and epitaxial thin films. In this article, we review recent experimental results and interpretation of ARPES data using empirical tight-binding band-structure calculations. Results are presented for SrVO3 (SVO) bulk single crystals and La1-xSrxFeO3 (LSFO) and La1-xSrxMnO3 (LSMO) thin films. In the case of SVO, from comparison of the experimental results with calculated surface electronic structure, we concluded that the obtained band dispersions reflect the bulk electronic structure. The experimental band structures of LSFO and LSMO were analyzed assuming the G-type antiferromagnetic state and the ferromagnetic state, respectively. We also demonstrated that the intrinsic uncertainty of the electron momentum perpendicular to the crystal surface is important for the interpretation of the APRES results of three-dimensional materials.

We have investigated the electronic structure of Cr- and Mn-doped GaN using photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS). Cr and Mn XAS at the L-edge have indicated that the Cr and Mn ions are in the tetrahedral crystal field and that their valences are trivalent and divalent, respectively. Upon Cr and Mn doping into GaN, new states were found to form in the band-gap region of GaN. Resonant photoemission spectroscopy (RPES) has revealed that the main structure of the Cr 3d partial density of states (PDOS) appears within the band gap of GaN while that of the Mn 3d PDOS appears within the valence band of GaN and as a shoulder above the valence-band maximum of GaN, indicating that the character of the doping-induced states is different between Ga1-xCrxN and Ga1-xMnxN. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We analyse the ground state of the one-dimensional Ti-O network of NaTiSi2O6. First, we make the Hartree-Fock calculation of the original d-p model and obtain the ground-state phase diagram. We find that the orbital ferro and spin antiferro state is realized when the effect of lattice distortion is taken into account. We derive the effective spin-pseudospin Hamiltonian by the perturbation theory up to the fourth order of hoppings and its ground state is calculated by the density matrix renormalization (DMRG) method. We find that without the effect of distortion the orbital ordering occurs and the system is in the state of spin singlet, which indicates that the spin fluctuation plays an essential role in this phase transition. (c) 2006 Elsevier B.V. All rights reserved.

We have studied the electronic structure of the d-electron heavy-fermion system CaCu3Ru4O12 using x-ray photoemission spectroscopy and a cluster model calculation. The Ru 3d core level spectrum shows a double-peak structure as commonly observed in metallic Ru oxides. In CaCu3Ru4O12, the well-screened peak has dominating intensity, indicating that the Ru 4d electrons in CaCu3Ru4O12 are highly itinerant. On the other hand, the Cu 2p(3/2) core level peak is accompanied by a satellite and shows that the valence state of Cu is close to 3d(9) (Cu2+) with localized character. In addition, the main Cu 2p(3/2) peak shows an asymmetric line shape due to the screening effect, suggesting the hybridization effect between the Cu 3d and Ru 4d orbitals. The present results show that, among the d-electron heavy-fermion materials, the electronic structure of CaCu3Ru4O12 best resembles that of the f-electron Kondo system.

We have studied the electronic structure of the oxygen-deficient type perovskite TbBaCo2O5.5, NdBaCo2O5.5, and NdBaCo2O5 using x-ray photoemission spectroscopy. The valence-band photoemission spectrum of TbBaCo2O5.5 shows the gap opening at the Fermi level and the spectral-weight transfer from the e(g) band to the t(2g) band, across the metal-insulator transition at 340 K. The valence-band spectra of RBaCo2O6-delta commonly have broad Co 3d bands, suggesting that the Co 3d electrons are distributed to the t(2g) and e(g) bands even in the insulating phase. The present results indicate that the metallic versus insulating behaviors of RBaCo2O5.5 are related to the energy separation between the t(2g) and e(g) bands.

X-ray photoemission spectroscopy measurements were performed on thin-film samples of rutile Ti1-xCoxO2-delta to reveal the electronic structure. The Co 2p core-level spectra indicate that the Co ions take the high-spin Co2+ configuration, consistent with substitution on the Ti site. The high-spin state and the shift due to the exchange splitting of the conduction band suggest strong hybridization between carriers in the Ti 3d t(2g) band and the t(2g) states of the high-spin Co+2. These observations support the argument that room temperature ferromagnetism in Ti1-xCoxO2-delta is intrinsic.

A Ti-oxide compound in the pyroxene family NaTiSi2O6 containing a one-dimensional Ti-O network exhibits a phase transition at T=210K, for which it has been anticipated that the spin-singlet formation is driven by the orbital ordering. We develop a theory that describes the mechanism of this phase transition. We use the Hartree-Fock approximation to the multiband d-p model and derive the groundstate phase diagram in the parameter space. We also use the perturbation theory in the strong correlation limit up to the fourth order of electron hoppings to derive the effective spin-orbital Hamiltonian. We then apply the density-matrix renormalization group method to the derived Hamiltonian and obtain the groundstate phase diagram. Experimental relevance of our results is discussed.

We report high temperature thermoelectric properties of doped delafossite oxide CuRh0.9Mg0.1O2. The parent compound CuRhO2 is a band insulator and becomes metallic upon Mg doping for Rh. We found that polycrystalline CuRh0.9Mg0.1O2 shows a large thermoelectric power S similar to +270 mu V/K despite its low resistivity similar to 6 m Omega cm at high temperatures around 1000 K. Combining these with a thermal conductivity kappa similar to 80 mW/cm K, we estimate a large dimensionless figure of merit ZT similar to 0.15 at 1000 K.

We have studied the electronic structure of the oxygen-deficient type perovskite TbBaCo2 O5.5, NdBaCo2 O5.5, and NdBaCo2 O5 using x-ray photoemission spectroscopy. The valence-band photoemission spectrum of TbBaCo2 O5.5 shows the gap opening at the Fermi level and the spectral-weight transfer from the eg band to the t2g band, across the metal-insulator transition at 340 K. The valence-band spectra of R BaCo2 O6-δ commonly have broad Co 3d bands, suggesting that the Co 3d electrons are distributed to the t2g and eg bands even in the insulating phase. The present results indicate that the metallic versus insulating behaviors of R BaCo2 O5.5 are related to the energy separation between the t2g and eg bands. © 2006 The American Physical Society.

We have studied the electronic structure of the spinel-type compound CuIr2S4 using x-ray photoemission spectroscopy (XPS). CuIr2S4 undergoes a metal-insulator transition (MIT) at similar to 226 K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of similar to 0.15 eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir3+Ir4+=11. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr2S4 is very robust against photo-excitation in contrast to Cs2Au2Br6 which shows photo-induced valence transition.

Perovskite-type gold complex Cs2Au2Br6 is known to show a photoinduced valence transition as well as a pressure-induced valence transition from the mixed-valence Au+ and Au3+ state to the single-valence Au2+ state under high pressure of 6.5 GPa. Since photoemission spectroscopy permits direct observation of electronic states at the solid surface, it is well suited as a probe to identify electronic changes due to photoinduced phase transitions (PIPT) at the surface. By means of photoemission spectroscopy, we have succeeded to observe the photoinduced valence transition at a surface of Cs2Au2Br6 even under vacuum conditions (0 GPa). This observation shows that PIPT at the surface of transition-metal compounds, which have yet to be explored, can be different from those in the bulk, suggesting new possibilities to develop optical devices using PIPT at a solid surface.

We have investigated orbital ordering in the half-doped manganite La0.5Sr1.5MnO4, which displays spin, charge, and orbital ordering, by means of unrestricted Hartree-Fock calculations on the multiband p-d model. From a recent experiment, it has become clear that La0.5Sr1.5MnO4 exhibits a cross-type (z(2)-x(2)/y(2)-z(2)) orbital ordering rather than the widely believed rod-type (3x(2)-r(2)/3y(2)-r(2)) orbital ordering. The calculation reveals that cross-type (z(2)-x(2)/y(2)-z(2)) orbital ordering results from an effect of in-plane distortion as well as from the relatively long out-of-plane Mn-O distance. For the "Mn4+" site, it is shown that the elongation along the c axis of the MnO6 octahedra leads to an anisotropic charge distribution rather than the isotropic one.

We report on the results of x-ray absorption, x-ray magnetic circular dichroism (XMCD), and photoemission experiments on Zn1-xCoxO (x=0.05) thin film, which shows ferromagnetism at room temperature. The XMCD spectra show multiplet structures, characteristic of the Co2+ ion tetrahedrally coordinated by oxygen, suggesting that the ferromagnetism comes from Co ions substituting the Zn site in ZnO. The magnetic field and temperature dependences of the XMCD spectra imply that the nonferromagnetic Co ions are strongly coupled antiferromagnetically with each other.

Perovskite-type BiNiO3 is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray-absorption (XAS) spectra of Bi1-xLaxNiO3 to investigate how the electronic structure changes with La doping. From Ni 2p XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains similar to 4+ and that the Ni valence behaves as similar to(2+x)+, that is, La substitution results in a hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for x&gt;0, consistent with the metallic behavior of the La-doped compounds. The Ni 2p XAS, Ni 2p core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni2+-&gt; Ni3+ valence change.

We have studied the electronic structure of the diluted magnetic semiconductor Ga1-xMnxN (x=0.0, 0.02, and 0.042) grown on Sn-doped n-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn L-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new states were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2p core level and the Mn 3d partial density of states were analyzed using configuration-interaction calculation on a MnN4 cluster model. The deduced electronic structure parameters reveal that the p-d exchange coupling in Ga1-xMnxN is stronger than that in Ga1-xMnxAs.

We investigated the electronic structure of a layered Ca2-xSrxRuO4 in a metallic regime (0.15 &lt;= x &lt;= 2.0) using a polarization dependent O 1s x-ray absorption spectroscopy. The spectrum shows strong variations with the polarization especially in Ru 4d region, which enables us to identify the electronic states. The spectral line shape gradually changes with increase of Sr concentration, and agrees well with an unrestricted Hartree-Fock analysis, which suggests evolution of the orbital states and provides clues for how the lattice distortion affects the orbital occupations.

The photocarrier injection (PCI) effect is observed at the heterojunction of YBa2Cu3Oy (YBCO) thin film and Nb-doped SrTiO3 (STO:Nb) substrate. PCI is expected as a new carrier doping method in strongly correlated systems instead of chemical substitution. We have studied the nature of PCI in YBCO/STO:Nb using X-ray photoemission spectroscopy with pulsed laser excitation. In the core-level spectra, the energy shift to higher binding energy by 0.5 eV is observed under the laser illumination with frequency of 30 Hz. The energy shift corresponds to the photovoltage, which arises in the interface. In addition, we have measured the spectra varying frequency of the laser. The energy shift obviously depends on the frequency, which is related to the lifetime of the injected photoholes. (c) 2005 Elsevier B.V. All rights reserved.

The effects of photoexcitation on Sr14-xCaxCu24O41 and La2-2xSr1+2xMn2O7 single crystals were measured via X-ray photoemission spectroscopy. Changes observed in the Sr14-xCaxCu24O41 Cu 2p core-level line shape indicate that the cleaved surface is surprisingly unstable under vacuum and that changes in the surface stoichiometry are easily obtained with laser ablation. In La2-2xSr1+2xMn2O7, the O 1s core-level peak binding energy irreversibly increases upon optical excitation. The magnitude of the increase depends on the sample hole doping and is observed to be largest in a x = 0.5 sample, where phase competition between electronic ordered states is strongest. It appears that the photoinduced electronic structural change is permanent due to stabilization by strong electron-lattice coupling. (c) 2004 Elsevier B.V. All rights reserved.

An overview is given on the photoemission studies of the electronic structure of diluted magnetic semiconductors (DMS's), in particular of the prototypical ferromagnetic DMS Ga1-xMnxAs. Configuration-interaction cluster-model analyses of the photoemission data allow us to estimate the p-d exchange coupling constant and hence to predict how to increase the Curie temperature in new materials. Spectra near the Fermi level combined with the transport and optical properties suggest a highly incoherent metallic state for the ferromagnetic metallic phase. It is shown that new insight into the chemically and magnetically inhomogeneous states of DMS's can be gained by the temperature and magnetic field dependence of core-level magnetic circular dichroism signals. (c) 2005 Elsevier B.V. All rights reserved.

We have performed in situ photoemission and O 1s X-ray absorption studies of La1-xSrxFeO3 (0 &lt;= x &lt;= 0.67) epitaxial thin films grown by laser molecular beam epitaxy. We found that hole doping induces spectral weight transfer from below the Fermi level to above it across the gap in a highly non-rigid-band-like manner. We also observed the temperature dependence on the spectra of the x = 0.67 sample, which shows a charge disproportionation at 190 K. With decreasing temperature, we observed gradual changes of the spectra with spectral weight transfer primarily within the e, band. (c) 2005 Elsevier B.V. All rights reserved.

We developed a new inverse photoemission (IPES) machine based on a new idea to improve the energy resolution: off-plane Eagle mounting of the optical system in combination with dispersion matching between incoming electron and outgoing photon. In order to achieve dispersion matching, we have employed a parallel plate electron source and have investigated whether the electron beam is obtained as expected. In this paper, we present the principle and design of the new IPES method and report the current status of the high-energy resolution IPES machine. (c) 2005 Elsevier B.V. All rights reserved.

We have studied the electronic structure of the quasi-two-dimensional Co oxides NaCo2O4(NaxCoO2,x similar to 0.5-0.6), Ca3Co4O9, and Bi2Sr2Co2O9 using O 1s and Co 2p x-ray absorption (XAS) spectroscopy. We found that these Co-O triangular lattice systems have in common that their Co3+ and Co4+, ions are all low-spin, supporting the Koshibae-Tsutsui-Maekawa theory to explain the enhanced thermopower at elevated temperatures. The concentration of holes in the Co 3d t(2g) shell is estimated to be about 0.4, 0.6, and 0.33, respectively. The 0 Is XAS spectra strongly depend on the direction of polarization vector of the incoming x-ray. The polarization dependence indicates that the t(2g) orbital anisotropy of NaxCoO2 is different from that of Ca3Co4O9 and Bi2Sr2Co2O9, We argue that the difference of the orbital anisotropy and hole concentration are essential to explain why NaxCoO(2), Ca3Co4O9, and Bi2Sr2Co2O9 have different electric and magnetic properties at low temperatures.

We consider the superstructures, which can be formed in spinels containing on B sites the transition-metal ions with partially filled t(2g) levels. We show that, when such systems are close to the itinerant state (e.g., have an insulator-metal transition), there may appear in them an orbitally driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that a similar phenomenon should be observed in NaTiO2 and possibly in some other systems.

We have obtained Co 2p, Rh 3d, Ir 4f, and valence-band x-ray photoemission spectra of Ca3Co2O6, Ca3CoRhO6, Ca3CoIrO6, and Sr3ZnIrO6 in order to determine the valence states of Co. The spectral features are consistent with the picture that Co is in the trivalent state at both trigonal prismatic and octahedral sites with high- and low-spin states, respectively, in Ca3Co2O6, whereas Co is in the high-spin Co2+ state for Ca3CoRhO6 and Ca3CoIrO6. This conclusion resolves a recent controversy regarding the valence and spin states of Co in Ca3Co2O6 and Ca3CoRhO6. This is the first x-ray photoemission spectroscopic study of this family of oxides.

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Neel temperature. Using Ti L-2,L-3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t(2g) subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.

We have studied the electronic structure of epitaxially grown thin films of La1-xSrxFeO3 by in situ photoemission spectroscopy (PES) and x-ray-absorption spectroscopy (XAS) measurements. The Fe 2p and valence-band PES spectra and the O 1s XAS spectra of LaFeO3 have been successfully reproduced by configuration-interaction cluster-model calculation and, except for the satellite structure, by band-structure calculation. From the shift of the binding energies of core levels, the chemical potential was found to be shifted downward as x was increased. Among the three peaks in the valence-band spectra of La1-xSrxFeO3, the peak nearest to the Fermi level (E-F), due to the "e(g) band," was found to move toward E-F and became weaker as x was increased, whereas the intensity of the peak just above E-F in the O 1s XAS spectra increased with x. The gap at E-F was seen for all values of x. These results indicate that changes in the spectral line shape around E-F are dominated by spectral weight transfer from below to above E-F across the gap and are therefore highly nonrigid-bandlike changes.

Heterojunctions of Nb-doped SrTiO3 substrate and YBa2Cu3Oy thin films show photoconductivity and photovoltaic effects due to photocarrier injection. Photocarrier injection is expected to be a new carrier doping method in strongly correlated systems instead of chemical substitution. We have studied the nature of photocarrier injection in YBa2Cu3Oy/SrTiO3:Nb using x-ray photoemission spectroscopy with pulsed laser excitation. The core-level spectra shift to higher binding energy by 0.78 eV under pulsed laser illumination at 30 Hz. The energy shift corresponds to the photovoltage, which arises at the interface. In addition, we have observed that the energy shift strongly depends on the frequency of the laser. The lifetime of the injected photoholes has been estimated to be 40 ms by analyzing the frequency dependence.

We have studied the electronic structure of Sr2RuO4 and Ca2RuO4 using x-ray photoemission spectroscopy (XPS) and subsequent model calculations. While the t(2g) band of Sr2RuO4 has substantial spectral weight at the Fermi level, that of Ca2RuO4 has no spectral weight at E-F and shows a peak at -1.8 eV. In the valence-band XPS spectrum of Sr2RuO4, a satellite structure of the t(2g) band is observed. In order to explain the spectral features, we have carried out band-structure calculations using the unrestricted Hartree-Fock (HF) approximation and found good agreement with the experimental result for Ca2RuO4. In order to explain the satellite structure of Sr2RuO4, we have performed second-order perturbation calculations of the self-energy corrections around the unrestricted HF solution of Sr2RuO4.

We have studied the electronic structure of La2-2xSr1+2xMn2O7 with x=0.4 and 0.5 by means of x-ray photoemission spectroscopy (XPS). In going from x=0.4 to x=0.5, the O 1s, Mn 2p, and valence-band spectra show an energy shift of similar to0.2 eV toward the Fermi level that is consistent with the hole doping in the e(g) band. The spectral weight at the Fermi level is considerably suppressed for x=0.4 and 0.5, indicating that the e(g) electron has polaronic character as pointed out by Dessau Also, photoemission measurements have been done for the samples illuminated with laser light to probe photoinduced change of the electronic structure. While the O 1s spectrum of the x=0.4 sample is shifted by 0.1 eV to the higher binding energy side, that of x=0.5 is shifted by 0.7 eV. This would be related to the phase competition between the CE-type and A-type antiferromagnetic states and the unusual electron-lattice coupling found in x=0.5.

We have studied the electronic structure of a hole-doped Cu-O single chain, double chain, and ladder using model Hartree-Fock calculations. In the single chain, the hole doping destroys the antiferromagnetic and insulating state of the undoped Cu-O chain and stabilizes a charge-ordered and insulating (COI) state. The COI states are lower in energy than antiferromagnetic and metallic (AFM) states for the hole-doped Cu-O chain. The energy difference between the COI and AFM states takes a maximum around x=1/2, indicating that the 1/4-filled case of x=1/2 is the most favored, where x is the hole concentration. In the Cu-O double chain, the COI state at x=1/2 is unstable and the COI at x=1/3 is favored, which is related to the COI state in the triangular lattice. In the Cu-O ladder, the COI at x=1/2 is as stable as that in the Cu-O single chain. In the COI of x=1/2, two different COI states, namely metal-centered charge-ordered (MCO) and oxygen-centered charge-ordered (OCO) states, are stable and degenerate in energy. Some of the COI states obtained for various x can be viewed as a mixture of the MCO and OCO states. In the COI states, the in-gap band with spectral weight of 2x is formed between the remnants of the upper and lower Hubbard bands.

We have studied the electronic structure of the Mn-based diluted magnetic semiconductors and the ordered Mn surface alloys using photoemission spectroscopy and configuration interaction (CI) calculation. As for the Mn-based diluted magnetic semiconductors, the electronic-structure parameters obtained from the Cl analysis show systematic variation with expected chemical trends. The Cl picture gives a systematic description of the d-d absorption spectra, the exchange constants between the 3d spins and host band states, and the donor and acceptor ionization energies of the diluted magnetic semiconductors. The photoemission spectra of the ordered surface alloys CuMn/Cu(100) and NiMn/Ni(100) show correlation satellites similar to the impurity Mn atom in solids. It is shown that the CI analysis can describe the correlation satellites driven by the reduced dimensionality in the ordered surface alloy systems. (C) 2004 Elsevier B.V. All rights reserved.

The filling-controlled metal-insulator transition (MIT) in a two-dimensional Mott-Hubbard system La1.17-xPbxVS3.17 has been studied by photoemission spectroscopy. With Pb substitution x, chemical potential mu abruptly jumps by similar to0.07 eV between x=0.15 and 0.17, indicating that a charge gap is opened at xsimilar or equal to0.16 in agreement with the Mott insulating state of the d(2) configuration. When holes or electrons are doped into the Mott insulator of xsimilar or equal to0.16, the gap is filled and the photoemission spectral weight at mu, rho(mu), gradually increases in a similar way to the electronic specific-heat coefficient, although the spectral weight remains depressed around mu compared to that expected for a normal metal, showing a pseudogap behavior in the metallic samples. The observed behavior of rho(mu)--&gt;0 for x--&gt;0.16 is contrasted with the usual picture that the electron effective mass of the Fermi-liquid system is enhanced towards the metal-insulator boundary. With increasing temperature, the gap or the pseudogap is rapidly filled up, and the spectra at T=300 K appears to be almost those of a normal metal. Near the metal-insulator boundary, the spectra around mu are consistent with the formation of a Coulomb gap, suggesting the influence of long-range Coulomb interaction under the structural disorder intrinsic to this system.

We have investigated the electronic structure of Zn1-xMxO (M: 3d transition metal) by x-ray absorption spectroscopy. Using configuration-interaction cluster-model analyses, electronic structure parameters have been deduced and their chemical trend is discussed. Results show that the p-d exchange constant Nbeta is negative and large in cases of Mn, Fe, and Co, which is consistent with the enhancement of magnetic circular dichroism. (C) 2004 American Institute of Physics.

The nature of metal-insulator (MI) transition in a t(2g)-electron system Ca2-xSrxRuO4 has been investigated using O 1s x-ray absorption spectroscopy (XAS). The O 1s XAS data strongly depend on temperature and demonstrate that the hysteresis of the MI transition is accompanied by the hysteresis of the orbital change. An unrestricted Hartree-Fock analysis indicates that the amount of yz/zx hole is discontinuously reduced at the MI transition. In the insulating phase, the amount of 4d yz/zx hole gradually decreases on heating, suggesting that partial orbital fluctuation increases with temperature.

Ga1-xMnxAs, x=0.043, has been grown ex situ on GaAs(100) by low-temperature molecular-beam epitaxy. On the reprepared p(1x1) surface, resonant photoemission of the valence band shows a 20-fold enhancement of the Mn 3d contribution at the L-3 edge. The difference spectrum is similar to our previously obtained resonant photoemission at the Mn M edge, in particular a strong satellite appears and no clear Fermi edge ruling out strong Mn 3d weight at the valence-band maximum. The x-ray absorption lineshape differs from previous publications. Our calculation based on a configuration-interaction cluster model reproduces the x-ray absorption and the L-3 on-resonance photoemission spectrum for model parameters Delta, U-dd, and (pdsigma) consistent with our previous work and shows the same spectral shape on and off resonance thus rendering resonant photoemission measured at the L-3 edge representative of the Mn 3d contribution. At the same time, the results are more bulk sensitive due to a probing depth about twice as large as for photoemission at the Mn M edge. The confirmation of our previous results obtained at the M edge calls recent photoemission results into question which report the absence of the satellite and good agreement with local-density theory.

The electronic structure of pyrochlore-type Tl2Ru2O7, which undergoes a metal-semiconductor transition at T(t)similar to120 K, has been studied by ultraviolet photoemission spectroscopy (UPS) and inverse-photoemission spectroscopy [bremsstrahlung isochromat spectroscopy (BIS)]. The temperature dependence of the UPS intensity around the Fermi level (E-F), which is largely due to the Ru 4d states, reflects the transition. In the BIS spectra, changes occur across the metal-semiconductor transition not only near E-F but also several eV above E-F where the Ru 4d e(g) states and the Tl 6s states are dominant. It is therefore concluded that the transport properties across the metal-semiconductor transition in Tl2Ru2O7 are mainly determined by the Ru 4dt(2g) states around E-F but that the changes in the Tl 6s states also make contributions to the transition.

We have studied the electronic structure of the layered Kogome lattice compound Rb2Ni3S4 by photoemission spectroscopy and comparison of the results with band-structure and configuration-interaction cluster-model calculations. The band-structure calculation reproduced the gross features of the experimental spectra but could not reproduce the weak satellite feature, and predicted the narrow peak of Ni 3d character just below the Fermi level (E-F) to be too strong. On the other hand, the cluster-model calculation assuming the low-spin (S=0) ground state reproduced the satellite but failed to predict the same peak just below E-F at the correct energy position. Angle-resolved photoemission measurements revealed momentum dispersions for the S 3p-derived and Ni 3d-S 3p hybridized bands, in agreement with the band-structure calculation, whereas the Ni 3d-derived band just below E-F was found to be dispersionless, unlike the result of the band-structure calculation. We conclude that Rb2Ni3S4 is a moderately correlated, strongly p-d hybridized system.

FexCo1-xSi is a metal with helical spin structure in the concentration range 0.2&lt;x&lt;0.95 although the end members are a narrow-gap semiconductor (FeSi) and a diamagnetic semimetal (CoSi). We have studied the electronic structure of FexCo1-xSi (x=0.5, 0.8) using ultraviolet photoemission spectroscopy. With Fe substitution for Co, the structure due to the transition-metal 3d states is shifted toward lower binding energies, qualitatively consistent with the prediction of the rigid-band model. Although the superposition of the spectra of FeSi and CoSi better describes the x dependence of the global spectral features than the rigid-band model, the x dependence near the Fermi level (E-F) is better described by the rigid-band model. The appearance of magnetic order in FexCo1-xSi may thus be explained by the rigid-band model, which predicts that the density of states at E-F is low or zero for CoSi and FeSi but becomes large for intermediate x. We also find a weak temperature dependence around the Fermi level, qualitatively consistent with the increase in the electrical resistivity below the Neel temperature (T-N).

We have investigated doping dependence of Fermi surface, band dispersion, and magnetic structure in high-T-c cuprates by using Hartree-Fock calculation on a Cu 3d-O 2p tight-binding model. In the slightly electron-doped regime, electron pockets emerge around (pi,0), which agrees with the photoemission study of Nd2-xCexCuO4. In the slightly hole-doped regime, hole pockets emerge around (pi/2,pi/2), which might explain the recent photoemission result of Ca2-xNaxCuO2Cl2.

A small amount of Sc substitution for Y in the antiferromagnetic metal YMn2 results in a paramagnetic metal with a Strongly enhanced electronic specific heat. We have studied the electronic structure of YMn2 and Y0.97SC0.03Mn2 by photoemission spectroscopy. The spectra of YMn2 taken above and below the Neel temperature did not show appreciable difference, and the spectra of YMn2 andY(0.97)Sc(0.03)Mn(2) were similar to each other. The photoemission spectra of the antiferromagnetic phase are well explained by band-structure calculation on the antiferromagnetic state while those of the paramagnetic phase are not explained by band-structure calculation on the paramagnetic state. These observations suggest that there are strong antiferromagnetic correlations in the paramagnetic phase. For the paramagnetic phase, agreement between experiment and calculation could be considerably improved by applying a model self-energy correction to the band density of states. (C) 2003 Elsevier Science Ltd. All rights reserved.

Lightly doped La2-xSrxCuO4 in the so-called "insulating" spin-glass phase has been studied by angle-resolved photoemission spectroscopy. We have observed that a "quasiparticle" (QP) peak crosses the Fermi level in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface, which explains the metallic behavior at high temperatures of the lightly doped materials. The QP spectral weight of the arc smoothly increases with hole doping, which we attribute to the nsimilar tox behavior of the carrier number in the underdoped and lightly doped regions.

Lightly doped La2-xSrxCuO4 in the so-called "insulating" spin-glass phase has been studied by angle-resolved photoemission spectroscopy. A quasi-particle (QP) peak crossing the Fermi level has been observed in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface consistent with the recently observed metallic transport behavior at high temperatures of lightly doped materials. The spectral weight of the nodal QP smoothly increases with hole doping, corresponding to the n similar to x behavior of the carrier number in the underdoped and lightly doped regions., (C) 2003 Elsevier Science B.V. All rights reserved.

We have measured the photoemission spectra of Nd1-xSmxNiO3, where the metal-insulator transition and the Néel ordering occur at the same temperature for x≲0.4 and the metal-insulator transition temperature (TMI) is higher than the Néel temperature for x≳0.4. For x≤0.4, the spectral intensity at the Fermi level is high in the metallic phase above TMI and gradually decreases with cooling in the insulating phase below TMI while for x&gt
0.4 it shows a pseudogaplike behavior above TMI and further diminishes below TMI. The results clearly establish that there is a change in the nature of the electronic correlations in the middle (x∼0.4) of the metallic phase of the RNiO3 system.

We have measured the photoemission spectra of Nd1-xSmxNiO3, where the metal-insulator transition and the Neel ordering occur at the same temperature for xless than or similar to0.4 and the metal-insulator transition temperature (T-MI) is higher than the Neel temperature for xgreater than or similar to0.4. For xless than or equal to0.4, the spectral intensity at the Fermi level is high in the metallic phase above T-MI and gradually decreases with cooling in the insulating phase below T-MI while for x&gt;0.4 it shows a pseudogaplike behavior above T-MI and further diminishes below T-MI. The results clearly establish that there is a change in the nature of the electronic correlations in the middle (xsimilar to0.4) of the metallic phase of the RNiO3 system.

Iron perovskites CaFeO3 and La0.33Sr0.67FeO3 show charge disproportionation, resulting in charge-ordered states with Fe3+:Fe5+ = 1: 1 and =2: 1, respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of CaFeO3 and compared it with La0.33Sr0.67FeO3. With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO3 as in La0.33Sr0.67FeO3 although lattice distortion occurs only in CaFeO3. Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in CaFeO3, while no lattice distortion is necessary for the charge disproportionation in La0.33Sr0.67FeO3.

Iron perovskites CaFeO3 and La0.33Sr0.67FeO3 show charge disproportionation, resulting in charge-ordered states with Fe3+:Fe5+=1:1 and =2:1, respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of CaFeO3 and compared it with La0.33Sr0.67FeO3. With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO3 as in La0.33Sr0.67FeO3 although lattice distortion occurs only in CaFeO3. Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in CaFeO3, while no lattice distortion is necessary for the charge disproportionation in La0.33Sr0.67FeO3.

The x-ray photoemission spectrum of the valence states of 3d transition-metal systems is spin polarized when using circularly polarized photons. The integral of the spin-orbit spectrum is proportional to the expectation value of the angular part of the 3d spin-orbit operator in the initial state. We show that this quantity can be used to get an estimate of the atomic orbital moment. While the measurement is sensitive to the magnetization axis, it does not require a net macroscopic magnetization nor the presence of a long-range magnetic order, and is therefore suitable for any transition-metal systems being antiferromagnetic or paramagnetic or magnetically disordered. In the case of full 3d shell the integral of the spin-orbit spectrum is zero, but the spectral shape can give a direct estimate of the 3d spin-orbit energy splitting DeltaE(SO). We have used Cu and CoO to experimentally test this technique. As expected Cu provides a vanishing result for &lt;L-z&gt;, whereas for Co2+ in CoO we find &lt;L-z&gt;=1.36h at 0 K. On the other hand we find DeltaE(SO)similar or equal to280 meV for Cu.

We have studied the electronic structure of MnSi, which shows a helical spin ordering below 30 K, by ultraviolet photoemission spectroscopy. We have attempted to separate the spectra into surface and bulk components by considering the electron mean-free path and the thickness of the Outermost atomic layer. The 'bulk' spectra thus obtained are compared with the density of states given by band-structure calculation corrected for model self-energies and reasonable agreement was obtained. This indicates that both band structure and electron correlation effects are important to understand the physical properties of MnSi. The spectra in the entire valence-band region do not show appreciable temperature dependence, but the spectral weight in the vicinity of the Fermi level is slightly decreased with decreasing temperature, which we tentatively attribute to the exchange splitting of the Mn 3d bands.

We have performed a photoemission study of La1-xSrxTiO3+y/2 near the filling-control metal-insulator transition (MIT) as a function of hole doping. The spectral intensity at the chemical potential (mu) and the mass enhancement factor deduced from the bandwidth show qualitatively the same doping dependence as the specific coefficient gamma except for near the MIT, where additional mass renormalization may occur in the vicinity of mu. Upon antiferromagnetic ordering, spectral weight transfer occurs from the coherent to the incoherent parts, which we associate with the partial gap opening at mu

Ca2-xSrxRuO4 is a quasi-two-dimensional Mott transition system. We have studied the relationship between the metal-insulator transition and the changes of orbital states using model Hartree-Fock calculations. As the d-d Coulomb interaction increases, the ground state changes from paramagnetic metal (PM) to antiferromagnetic metal (AFM) to antiferromagnetic insulator (AFI). Magnetic anisotropy is strong in the AFI state due to the spin-orbit interaction while it is weak in the AFM state. It has been found that the magnetic anisotropy is controlled by the Jahn-Teller distortion. We have also investigated the effect of the tilting and rotation of the RuO6 octahedron. The calculated result is consistent with the electronic phase diagram of Ca2-xSrxRuO4.

The shift of the chemical potential as a function of band filling provides useful insight into the electronic structure of solids. In this article, we describe how the chemical potential shift can be measured using core-level photoemission spectroscopy and what information can be obtained from the measured chemical potential shift, in particular on strongly correlated systems. We present results on various transition-metal oxides, which include materials showing typical Fermi-liquid behavior, pseudogap behavior, charge-density wave formation, 'stripe' formation, etc. (C) 2002 Elsevier Science B.V. All rights reserved.

RNiO3 (R: rare earth) is one of the typical bandwidth-control metal-insulator transition systems. Its metal-insulator transition temperature (T-M1) is the same as the antiferromagnetic transition temperature (T-N) for R = Pr and Nd, while T-M1 is higher than T-N for R = Sm and Eu. The boundary between the two types of phase transitions is located between NdNiO3 and SmNiO3. We have measured photoemission spectra of Nd(1-x)SmNio(3) at various temperatures above and below T-M1. The spectra show quite different temperature dependences between x less than or equal to 0.4 and x greater than or equal to 0.6. For x less than or equal to 0.4, the spectral intensity near the Fermi level (EF) is temperature-dependent even well below T-M1, while for x 0.6 it shows a significant temperature dependence only near T-M1. Also, the spectra near EF above T-M1 show a pseudo-gap like behavior for x greater than or equal to 0.6. From these results, we conclude that the metallic phase has different natures between x less than or equal to 0.4 and x greater than or equal to 0.6. (C) 2002 Elsevier Science Ltd. All rights reserved.

We have studied the electronic structure of metallic Cu-O double chains in Zn-doped PrBa2Cu4O8 (Pr124) using angle-resolved photoemission spectroscopy. The single-particle spectral function of Zn-doped Pr124 is compatible with a power-law spectral function expected for a Tomonaga-Luttinger liquid. This is in contrast to the fact that the photoemission spectrum of pure Pr124 shows a Fermi step and can be interpreted as a Fermi liquid realized in coupled metallic chains. The exponent alpha is estimated to be 0.6+/-0.1 that is one of the smallest values among those obtained for various one-dimensional metals.

We have investigated the electronic structure of the p-type diluted magnetic semiconductor In1-xMnxAs by photoemission spectroscopy. The Mn 3d partial density of states is found to be basically similar to that of Ga1-xMnxAs. However, the impurity-band-like states near the top of the valence band have not been observed by angle-resolved photoemission spectroscopy unlike Ga1-xMnxAs. This difference would explain the difference in transport, magnetic and optical properties of In1-xMnxAs and Ga1-xMnxAs. The different electronic structures are attributed to the weaker Mn 3d-As 4p hybridization in In1-xMnxAs than in Ga1-xMnxAs.

CuS2 and CuSe2 with pyrite structures were systematically studied by transport, magnetization, and specific-heat measurements. In remarkable contrast to other 3d transition-metal pyrites, a clear indication of strong electron correlations was absent in the electronic properties of Cu pyrites. We interpret this as a consequence of the dominant chalcogen p character rather than copper d character at the Fermi level. Photoemission results indeed support this picture, indicating that the Cu is predominantly monovalent. We therefore conclude that Cu pyrites, CuS2 and CuSe2, can be viewed as anion conductors.

We have investigated the electronic structure of the p-type diluted magnetic semiconductor In1-xMnxAs by photoemission spectroscopy. The Mn 3d partial density of states is found to be basically similar to that of Ga1-xMnxAs. However, the impurity-band-like states near the top of the valence band have not been observed by angle-resolved photoemission spectroscopy unlike Ga1-xMnxAs. This difference would explain the difference in transport, magnetic and optical properties of In1-xMnxAs and Ga1-xMnxAs. The different electronic structures are attributed to the weaker Mn 3d-As 4p hybridization in In1-xMnxAs than in Ga1-xMnxAs.

CuS2 and CuSe2 with pyrite structures were systematically studied by transport, magnetization, and specific-heat measurements. In remarkable contrast to other 3d transition-metal pyrites, a clear indication of strong electron correlations was absent in the electronic properties of Cu pyrites. We interpret this as a consequence of the dominant chalcogen p character rather than copper d character at the Fermi level. Photoemission results indeed support this picture, indicating that the Cu is predominantly monovalent. We therefore conclude that Cu pyrites, CuS2 and CuSe2, can be viewed as anion conductors.

Using spin-resolved resonant photoemission we have probed the singlet vs. triplet character of the two-hole state in the layered cuprates Bi2Sr2CaCu2O8+delta La2-xSrxCuO4 and Sr2CuO2Cl2. The combination of the photon circular polarization with the photoelectron spin detection gives access to the character of the photoemission final states, which correspond to the two-hole configurations localized at a (CuO4) site. In particular, the lowest energy state is found to have a very high singlet character in all the measured compounds. This can be considered as a strong indication of the existence and stability of the so-called Zhang-Rice singlets in the layered cuprates. (C) 2002 Elsevier Science B.V. All rights reserved.

The electronic structure of the La2-xSrxCuO4 (LSCO) system has been studied by angle-resolved photoemission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap, and the band dispersion around the extended saddle point k=(pi,0) with hole doping in the superconducting and metallic phases. As hole concentration x decreases, the flat band at (pi,0) moves from above the Fermi level (E-F) for x &gt; 0.2 to below E-F for x &lt; 0.2, and is further lowered down to x = 0.05. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around (pi,0) is found to monotonically increase as x decreases from x = 0.30 down to x = 0.05 even though T-c. decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at x = 0.05. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample (x=0.30), the band dispersion and the ARPES spectral line shape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor m*/m(b)similar or equal to 2 has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region (x less than or similar to 0.10) are consistent with numerical works on the stripe model.

The electronic structure of the La2-xSrxCuO4 (LSCO) system has been studied by angle-resolved photo-emission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap, and the band dispersion around the extended saddle point k = (π,0) with hole doping in the superconducting and metallic phases. As hole concentration x decreases, the flat band at (π,0) moves from above the Fermi level (EF) for x&gt
0.2 to below EF for x&lt
0.2, and is further lowered down to x = 0.05. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around (π,0) is found to monotonically increase as x decreases from x = 0.30 down to x = 0.05 even though Tc decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at x = 0.05. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample (x = 0.30), the band dispersion and the ARPES spectral line shape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor m*/mb≃2 has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region (x≤0.10) are consistent with numerical works on the stripe model.

We have studied the electronic structure of Zn1-χMnχO using photoemission spectroscopy measurements and configuration-interaction (CI) calculations on a MnO4 cluster model. It is shown that the CI calculation can give a consistent description of the photoemission and d-d optical absorption spectra of Zn1-χMnχO as well as those of other II-VI- and III-V-based diluted magnetic semiconductors such as Cd1-χMnχTe and Ga1-χMnχAs. The CI approach predicts that the magnitude of the p-d exchange constant in Zn1-χMnχO is much larger than that in Ga1-χMnχAs.

We have studied the electronic structure of Zn1-xMnxO using photoemission spectroscopy measurements and configuration-interaction (CI) calculations on a MnO4 cluster model. It is shown that the CI calculation can give a consistent description of the photoemission and d-d optical absorption spectra of Zn1-xMnxO as well as those of other II-VI- and III-V-based diluted magnetic semiconductors such as Cd1-xMnxTe and Ga1-xMnxAs. The Cl approach predicts that the magnitude of the p-d exchange constant in Zn1-xMnxO is much larger than that in Ga1-xMnxAs.

We have studied the electronic structure of (formula presented) using photoemission spectroscopy measurements and configuration-interaction (CI) calculations on a (formula presented) cluster model. It is shown that the CI calculation can give a consistent description of the photoemission and (formula presented) optical absorption spectra of (formula presented) as well as those of other II-VI- and III-V-based diluted magnetic semiconductors such as (formula presented) and (formula presented) The CI approach predicts that the magnitude of the (formula presented) exchange constant in (formula presented) is much larger than that in (formula presented). © 2002 The American Physical Society.

From a spin-resolved photoemission study on the Bi2Sr2CaCu2O8+delta superconductor, we show experimentally that the first ionization state is of nearly pure singlet character. This is true both above and below the superconducting transition and in the presence of doping and band formation. This provides direct support for the existence and stability of Zhang-Rice singlets in high-temperature superconductors, justifying the ansatz of single-band models. Moreover, we establish this technique as an important probe for a wide range of cuprates and strongly correlated materials.

We have deposited Mn on the (110) surface of Ni and discover ordering into a c(2 x 2) superstructure for coverages of 0.35-0.5 monolayer Mn. Mn 2p photoemission spectra show distinct satellite structures which disappear for higher Mn coverage. Calculations using configuration-interaction theory including multiplet effects on a model cluster representing the local geometry of a surface alloy identify the features as correlation satellites and give model parameters as follows: charge-transfer energy Delta =1 eV, Coulomb energy U = 3 eV, and transfer integral T = 1.2 eV. A detailed comparison to the case of c (2 x 2) Mn/Cu(100) leads to the conclusion that c (2 x 2) Mn/Ni (110) is a new magnetic surface alloy.

Valence-band dispersions in Ga1-xMnxAs along the Γ-Δ-X line (k∥[001]) are obtained by angle-resolved photoemission spectroscopy. Compared with the spectra of GaAs, a small energy shift caused by Mn doping is observed for one of the valence bands. In addition, states are observed near the Fermi level in Ga1-xMnxAs. These states show no clear dispersions and behave like an impurity band induced by Mn doping.

We present a photoemission and x-ray-absorption study of the misfit-layered (Bi,Pb)-Sr-Co-O compounds which have a Co-O triangular lattice with a mixed valence of Co3+ and Co4+. The valence-band photoemission as well as the O 1s and Co 2p x-ray absorption spectra indicate that Co3+ and Co4+ have the low-spin t(2g)(6) and t(2g)(5) configurations, respectively. The angle-resolved photoemission spectra show that the dispersion of the t(2g) feature is very small compared to its width at each angle, suggesting that the electron-lattice coupling energy is much larger than the kinetic energy of the t(2g) electrons and that the carriers in the Co-O triangular lattice are essentially polarons formed by Co4+ in the nonmagnetic Co3+ background.

Valence-band dispersions in Ga1-xMnxAs along the Gamma-Delta -X line (k parallel to [001]) are obtained by angle-resolved photoemission spectroscopy. Compared with the spectra of GaAs, a small energy shift caused by Mn doping is observed for one of the valence bands. In addition, states are observed near the Fermi level in Ga1-xMnxAs. These states show no clear dispersions and behave like an impurity band induced by Mn doping.

O 1s x-ray absorption study of the Mott insulator Ca2RuO4 shows that the orbital population of the 4d t(2g) band dramatically changes with temperature. In addition, spin-resolved circularly polarized photoemission study of Ca2RuO4 shows that a substantial orbital angular momentum is induced in the Ru 4d t(2g) band. Based on the experimental results and model Hartree-Fock calculations, we argue that the cooperation between the strong spin-orbit coupling in the Ru 4d t(2g) band and the small distortion of the RuO6 octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature.

Photoemission in the system of linear charged chains has been studied within the one-step model incorporating the dipole transition matrix. from which its dependencies on the photoelectron momentum (k), photon polarization ((E) over cap), and photon energy (omega) can be explored. The employed model is the three-dimensional array of noninteracting chains. which is so simple as to allow an analytic approach. Motivation of the study is for the doped CuO3 chain in PrBa2CU3O7 and the doped static stripe phase in La2-x-yNdySrxCuO4. From chains having one-dimensional dispersion exhibiting spin-charge separation. spectral dependences on the momentum perpendicular to the chain and photon polarization are discussed for PrBa2Cu3O7. For La2-x-yNdySrxCuO4, the anomalous spectral distribution formed by two sets of stripes perpendicular to each other is investigated. The geometric effects resulting from the transition matrix including the interference of photocurrents from different chains are found to change the simple one-dimensional feature drastically. We find these changes are consistent with experiment for the chain system PrBa2Cu3O7, but less satisfactory for the stripe phase in La2-x-yNdySrxCuO4. This means that in the stripe phase much two-dimensional character still exists unlike the chain system.

We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+delta The data analysis using energy distribution curves, momentum distribution curves, and intensity maps all show evidence of an electronlike Fermi surface, which is well explained by band-structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around ( pi ,0), where the band is predicted to lie above E-F.

Oxides of Ti and V belong to the Mott-Hubbard regime of the Zaanen-Sawatzky-Allen classification scheme of transition-metal compounds and have simple electronic structures which allow us to study the effect of electron correlation in a transparent way. In this article, we make an overview of our recent photoemission studies on Ti and V oxides. with special emphasis on metal-insulator transitions induced by the control of the width and the filling of the Ti and V 3d bands. Spectroscopic data yield spectral weight transfer between the coherent and incoherent parts of the d band, the spectral intensities at the Fermi level and the chemical potential shifts as functions of band filling. We show that such spectroscopic information well corresponds to the thermodynamic and transport properties and is necessary to understand electron correlation phenomena from a fundamental viewpoint. (C) 2001 Elsevier Science B.V. All rights reserved.

Using photoemission spectroscopy, the electronic structure around the two-dimensional filling-control metal-insulator transition (MIT) of La1.17-xPbxVS3.17 has been studied. The density of state at the Fermi level disappears in going to the insulating phase, i.e., as x approaches similar to 0.17 or as the temperature decreases at x = 0.15. The chemical potential shift with carrier doping shows a finite jump between the electron- and hole-doped samples and is unusually suppressed around x = 0.17. The results are contrary to a rigid band picture and show the presence of a pseudogap in the metallic phase. 2001 Elsevier Science B.V. All rights reserved.

We have studied the electronic structure of Ga1-xMnxAs by angle-resolved photoemission spectroscopy. The effect of Mn doping in GaAs was revealed as the formation of new states near the Fermi level, which originate from the Mn aceptor state, and are split from the valence-band maximum of the host GaAs. These states would be responsible for the anomalous transport properties of Ga1-xMnxAs. (C) 2001 Elsevier Science B.V. All rights reserved.

We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn4+ sites) are surrounded by nearest neighbor Mn3+ sites in which the occupied 3d orbitals have their lobes directed towards the central hole (Mn4+) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn4+) and Jahn-Teller type (Mn3+) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.

We have obtained the Mn 3d partial density of states in Ga1-xMnxAs and energy-band dispersion by photoemission techniques to study the Mn doing effects in GaAs. We found that states near the Fermi level largely derived from As 4p states with some admixture of Mn 3d character. We also observed non-dispersive impurity-band like states near the Fermi level, which may give a key to understand the anomalous properties of Ga1-xMnxAs.

Photoemission results on La2-xSrxCuO4 (LSCO) are overviewed and discussed in the context of the formation of large and small pseudogaps and the effects of stripe fluctuations in the underdoped regime. (C) 2000 Elsevier science Ltd. All rights reserved.

Doping dependence of the electronic structure of La2-xSrxCuO4 (LSCO) has been systematically studied in a series of photoemission measurements. The unusual spectral features in the underdoped regime are attributed to the formation of dynamical stripes and the opening of large and small pseudogaps.

We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+δ. The data analysis using energy distribution curves, momentum distribution curves, and intensity maps all show evidence of an electronlike Fermi surface, which is well explained by band-structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around (π,0), where the band is predicted to lie above EF.

We present spectral weight mapping in the momentum space for La2-chiSrchiCuO4 using angle-resolved photoemission spectroscopy. The spectral weight of the nodal states, which shows suppression in previous results, is enhanced and shows sharp peaks in the second Brillouin zone for the overdoped sample due to matrix element effects, although they remain much weaker than the other cuprates.

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare-earth iron Laves phase compounds. Our experimental results, obtained using core-level and valence-band photoemission, inverse photoemission and soft x-ray absorption techniques, indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f-Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.

We have studied the electronic structure of the stripe phase in La2-xSrxCuO4 (LSCO) and the Cu-O chains in PrBa2Cu3O7 (Pr123) and PrBa2Cu4O8 (Pr124) using angle-resolved photoemission spectroscopy (ARPES). In LSCO with x = 0.12, the spectral feature near the Fermi level (E-F) is almost flat from (pi; 0) to (pi, pi /4), namely, along the stripe direction in LSCO. While the 1/4-filled chain in Pr123 has a band gap because of charge ordering, the metallic chain in Pr124 shows a dispersive feature which reaches E-F at similar to (pi, pi /4) and a flat feature near E-F which is similar to that observed in the stripe phase of LSCO. Although Pr124 shows spectral-weight suppression near E-F due to the instability toward charge ordering, the suppression is imperfect and Pr124 has a substantial spectral weight at E-F probably due to the chain-chain coupling. This is in sharp contrast to LSCO with the perfect gap opening near (pi; pi /4). The difference between the stripe phase in LSCO and the coupled chains of Pr124 would be derived from the interaction between the stripe and the neighboring Cu spins which suppresses the charge ordering along the stripe.

The electronic structure of La2-xSrxCuO4 (LSCO) has been revealed by a series of photoemission spectroscopic studies. The unusual electronic states in the underdoped regime are attributed to the large and small pseudogap formation, which has been observed in other high-T-C cuprates, and stripe fluctuations, which are particularly pronounced in LSCO.

We compare the angle-resolved photoemission spectra of the hole-doped Cu-O; chains in PrBa2Cu3O7 (Pr123) and in PrBa2Cu4O8 (Pr124). While, in Pr123,: a dispersive feature from the chain takes a band maximum at k(b) (momentum along the chain) similar to pi /4 and loses its spectral weight around the Fermi level, it reaches the Fermi level at k(b) similar to pi /4 in Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4 filled, they show contrasting behaviors: While the chains in Pr123 have an instability to charge ordering, those in Pr124 avoid it and show an interesting spectral feature of a metallic coupled-chain system.

We report on the result of angle-resolved photoemission study of La2-xSxCuO4 (LSCO) from an optimally doped superconductor (x=0.15) to an antiferromagnetic insulator (x=0). Near the superconductor insulator transition x similar to 0.05, spectral weight is transferred with hole doping between two coexisting components, suggesting a microscopic inhomogeneity of the doped-hole distribution. For the underdoped LSCO (x less than or equal to 0.12), the dispersive band crossing the Fermi level becomes invisible in the (0,0)-(pi,pi) direction unlike Bi2Sr2CaCu2O8+gamma These observations may be reconciled with the evolution of holes;in the insulator into fluctuating stripes in the superconductor.

We have studied the chemical potential shift in La2-xSrxNiO4 and the charge ordering transition in La1.67Sr0.33NiO4 by photoemission spectroscopy. The result shows a large (similar to 1 eV/hole) downward shift of the chemical potential with hole doping in the high-doping regime (delta greater than or similar to 0.33) while the shift is suppressed in the low-doping regime (delta less than or similar to 0.33). This suppression is attributed to a segregation of doped holes on a microscopic scale when the hole concentration is lower than delta similar or equal to 1/3. In the delta=1/3 sample, the photoemission intensity at the chemical potential vanishes below the charge ordering transition temperature T-CO=240 K.

We have investigated possible spin and charge ordered states in 3d transition-metal oxides with small or negative charge-transfer energy, which can be regarded as self-doped Mott insulators, using Hartree-Fock calculations on d-p-type lattice models. It was found that an antiferromagnetic state with charge ordering in oxygen 2p orbitals is favored for relatively large charge-transfer energy and may be relevant for PrNiO3 and NdNiO3. On the other hand, an antiferromagnetic state with charge ordering in transition-metal 3d orbitals tends to be stable for highly negative charge-transfer energy and can be stabilized by the breathing-type lattice distortion; this is probably realized in YNiO3.

We have explored spin, charge, and orbitally ordered states in La1-xSrxMnO3 (0&lt;s&lt;1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x = 1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8.

We have performed an angle-resolved photoemission study on PrBa2Cu3O7 (Pr123) and PrBa2Cu4O8 (Pr124) which have hole-doped Cu-O chains. Dispersive features with one-dimensional (1D) character are observed in Pr123 and Pr124 and are attributed to signals from the hole-doped Cu-O chains which are approximately 1/4-filled. While, in Pr123, the 1D feature loses its spectral weight near the Fermi level (E-F), it crosses E-F in Pr124. These results suggest that, while the hole-doped Cu-O single chains in Pr123 have instability to charge ordering, the hole-doped Cu-O double chains in Pr124 can get rid of it.

We have performed an angle-resolved photoemission study on untwinned PrBa2Cu3O7, which has low resistivity but does not show superconductivity. We have observed a dispersive feature with a band maximum around (pi/2, pi/2), indicating that this band is derived from the undoped CuO2 plane. We have observed another dispersive band exhibiting one-dimensional character, which we attribute to signals from the doped CuO3 chain. The overall band dispersion of the one-dimensional band agrees with the prediction of the t-J model calculation with parameters relevant to cuprates except that the intensity near the Fermi level is considerably suppressed in the experiment. [S0163-1829(99)04541-5].

La1−xSrxMnO3 is a perovskite-type magnetic material. LaMnO3 and SrMnO3 are antiferromagnetic insulators while the system becomes a ferromagnetic metal in a certain Sr concentration. In ∼0.2&lt
x&lt
∼0.5, it shows a so-called colossal magnetoresistance (CMR) which is a phenomenon of a huge resistivity drop when a magnetic field is applied. This property has a potential application to the magnetic storage industry. The aim of this research was to understand the electronic structure of this system using photoemission spectroscopy and to elucidate the origin of the CMR phenomenon. The samples were polycrystalline and were prepared by solid-state reaction. XPS measurements were carried out using a Mg Kα source at liquid nitrogen temperature (∼80 K). In order to obtain a fresh, clean surface, the samples were scraped with a diamond file in situ, and surface aging was monitored by observing the intensity of a tail at the higher binding-energy side of the O 1s peak. The surface typically lasted for 1-2 h.

We have studied the interplay between orbital ordering, Jahn-Teller and GdFeO3-type lattice distortions in perovskite-type transition-metal oxides using model Hartree-Fock calculations, It has been found that the covalency between e-site cations and oxygens causes interaction between the Jahn-Teller and GdFeO3-type distortions. The present calculations explain why the d-type Jahn-Teller distortion and orbital ordering compatible with it are realized in LaMnO3, YVO3, and YTiO3. [S0163-1829(99)12633-X].

We have studied the "charge disproportionation" phenomenon in La1-xSrxFeO3 by photoemission spectroscopy and unrestricted Hartree-Fock band-structure calculations. We find a systematic decrease of the spectral intensity near Fermi level (E-F) toward x = 0.67, where the charge disproportion is most stabilized. The spectra for x = 0.67 show the dearest temperature-dependent changes in the vicinity of E-F across the transition corresponding to the charge ordering of ''Fe3+'':''Fe5+'' = 2:1. The Hartree-Fock calculations indicate that ordering of oxygen holes indeed occurs in the charge disproportionated state. [S0163-1829(99)07831-5].

We have studied the electronic structure of Y(Co1-xAlx)(2) with x = 0.0, 0.12, and 0.18, which compositions cover the paramagnetic, metamagnetic, and ferromagnetic phases, by means of photoemission and inverse-photoemission spectroscopy. With Al substitution, broad peaks near the Fermi level (EF) are shifted upwards and one of them crosses EF as predicted by band-structure calculation, although the peaks art: much broader than predicted by the calculation. The overall Co 3d band is narrow and has a high binding energy tail compared to the band-structure calculation, indicating significant electron correlation effects in the Co 3d band. In order to explain these observations, model self-energy corrections have been applied to the band density of states. The lower energy scale of the self-energy is found to lie in the same range as the characteristic temperature of the magnetic susceptibility.[S0163-1829(99)09725-8].

We have measured photoemission and inverse-photoemission spectra of the ferromagnetic metal SrRuO3. The observed Ru 4d-derived conduction band above and below the Fermi level (E-F) is found to be widely spread and the emission intensity at E-F to be weakened compared to band-structure calculations. Here, the calculations have been made for the ferromagnetic state with the actual distorted perovskite structure. We compare the spectra with the spectral density of states (DOS) obtained by modifying the band DOS with a phenomenological self-energy correction, which is chosen to be compatible with the measured electronic specific heat coefficient gamma. Although the mass enhancement factor is only moderately large, similar to 4.4, the renormalization factor Z is found to be as small as similar to 0.08 (Z(-1)similar to 13). These indicate a significant momentum dependence of the self-energy and a highly incoherent metallic state in SrRuO3. We have also compared the photoemission and inverse-photoemission spectra taken below and above the Curie temperature, but significant changes have not been identified.

The results of measurements of X-ray photoelectron (XPS), X-ray emission (XES) and X-ray absorption spectra and LSDA band structure calculations of Pr0.5Sr0.5MnO3 are presented. The excitation energy dependence of Mn L-2.3 and O K-alpha X-ray emission spectra of Pr0.5Sr0.5MnO3 is measured using tunable synchrotron radiation. Comparing XPS and XES measurements and the LSDA band structure calculations, one concludes that the Mn 3d states are localized near the top of the valence band. Some differences in the Mn 3d-distribution in the upper part of the XPS valence band and Mn L-3 XES are due to spin selection rules. (C) 1999 Elsevier Science B.V. All rights reserved.

The occupied and unoccupied states of ferrimagnetic metals Fe7S8 and Fe7Se8, and a paramagnetic metal Co7Se8 have been studied by photoemission and X-ray inverse photoemission spectroscopies. Compared with the density of states (DOS) given by the band-structure calculation, the photoemission spectra of Fe 3d and Co 3d are narrowed. While the Fe 3d photoemission spectrum has a strong intensity at the higher binding energy side, it is significantly suppressed near the Fermi level. Self-energy corrections to the DOS's have been done for Fe7Se8 and Co7Se8. Disagreements between experimental and theoretical spectra have become small, indicating the importance of electron correlation effect. (C) 1999 Elsevier Science B.V. All rights reserved.

We have performed x-ray-absorption spectroscopy (XAS) and x-ray-photoemission spectroscopy (XPS) studies of single crystal Ba1-xKxBiO3 (BKBO) covering the whole composition range 0 less than or equal to x less than or equal to 0.60. Several features in the oxygen 1s core XAS spectra show systematic changes with x. Spectral weight around the absorption threshold increases with hole doping and shows a finite jump between x=0.30 and 0.40, which signals the metal-insulator transition. We have compared the obtained results with band-structure calculations. Comparison with the XAS results of BaPb1-xBixO3 has revealed quite different doping dependences between BKBO and BPBO. We have also observed systematic core-level shifts in the XPS spectra as well as in the XAS threshold as functions of x, which can be attributed to a chemical potential shift accompanying the hole doping. The observed chemical potential shift is found to be slower than that predicted by the rigid band model based on the band-structure calculations. [S0163-1829(99)01023-1].

The results of measurements of x-ray photoelectron (XPS), x-ray emission (XES), and x-ray absorption spectra and local spin-density approximation band structure (LSDA) calculations of Pr0.5Sr0.5MnO3 are resented. The excitation energy dependence of Mn L-2,L-3 and O K alpha x-ray emission spectra of Pr0.5Sr0.5MnO3 is measured using tunable synchrotron radiation. The XES measurements yielded no photon energy dependence for the O K alpha spectra, but the Mn L-2,L-3 spectra yielded inelastic scattering losses of 2 and 6 eV, corresponding to features in the structure of the occupied part of the valence band. Comparing XPS and XES measurements with LSDA band-structure calculations, one concludes that the electronic structure of the compound consists mainly of Mn 3d and O 2p states. States of 3d character localized at the Mn site predominate near the top of the valence band (VB). Some differences in the Mn 3d distribution in this part of the XPS valence band and Mn L-3 XES with d symmetry due to spin-selection rules that govern the Mn L-3 XES. In addition, the Mn 3d states distribution is hybridized with the O 2p part of the VB. Mn L-3 XES spectra were determined relative to the Fermi energy by assuming normal x-ray emission begins from the lowest level of the p(5)d(n+1)L intermediate state (which is the Mn 2p ionizatation threshold). From the local spin-density approximation, the orbital character of the Mn 3d electrons can be assigned eg symmetry at the top of the valence band T-2g in the central part of the VB, and equal contributions of e(g) and T-2g states at the bottom of the valence band. [S0163-1829(99)00620-7].

Using angle-resolved photoemission spectroscopy (ARPES), we observe the band structure, the Fermi surface and their doping dependences in La2-xSrxCuO4. The results reveal that the Fermi surface undergoes a dramatic change: it is holelike and centered at (pi,pi) in underdoped (x = 0.1) and optimally doped (x = 0.15) samples as in other cuprates, while it is electronlike and centered at (0.0) in heavily overdoped (x = 0.3) ones. The peak in the ARPES spectra near (pi/2,pi/2) is broad and weak unlike that in other cuprates. In the underdoped and optimally doped samples, a superconducting gap (Delta = 10-15 meV) is observed near (pi, 0).

We have obtained the Mn 3d partial density of states in Ga1-xMnxAs using the resonance photoemission technique as well as by means of the difference between Ga1-xMnxAs and GaAs. We have observed a strong satellite structure on the higher binding energy side of the main peak, as in Mn-doped II-VI compounds such as Cd1-xMnxTe. Based on analysis using configuration-interaction calculation for a MnAs4 cluster, we could ascribe the spectral features to strong Mn 3d-As 4p hybridization and Mn 3d-3d Coulomb interaction. [S0163-1829(99)50904-1].

We have made an angle-resolved photoemission study of a one-dimensional Mott-Hubbard insulator Na0.96V2O5 and found that the spectra of the V 3d lower Hubbard band are strongly dependent on the temperature. We have calculated the one-particle spectral function of the one-dimensional t-J model at finite temperatures by exact diagonalization and compared them with the experimental results. Good overall agreement is obtained between experiment and theory. The strong finite temperature effects are discussed in terms of the existence of the "Fermi surface" of the spinon band.

We have performed an angle-resolved photoemission study on untwinned PrBa2Cu3O7, which has low resistivity but does not show superconductivity. We have observed a dispersive feature with a band maximum around (π/2,π/2), indicating that this band is derived from the undoped CuO2 plane. We have observed another dispersive band exhibiting one-dimensional character, which we attribute to signals from the doped CuO3 chain. The overall band dispersion of the one-dimensional band agrees with the prediction of the t-J model calculation with parameters relevant to cuprates except that the intensity near the Fermi level is considerably suppressed in the experiment. ©1999 The American Physical Society.

The electronic structure of pyrite-type MnTe2 has been investigated by photoemission spectroscopy. The Mn 3d partial density of states was obtained from resonance photoemission experiments in the Mn 3p--&gt;3d core excitation region. The experimental results were compared with a linear-muffin;tin-orbital atomic-sphere approximation ab initio calculation, and some discrepancy was found. To reveal a role of electron correlation, the results were analyzed in terms of the configuration-interaction model by considering a (MnTe6)(10-) cluster, and parameter values for a model Hamiltonian were estimated (U=5.5 eV, Delta = 1.5 eV and T = 1.4 eV). X-ray photoemission spectra of the Mn 2p core level were also measured to confirm this assignment. [S0163-1829(98)05227-8].

We have studied the electronic structure of NiS2-xSex, which undergoes a metal-insulator transition as functions of composition x and temperature, by means of photoemission and inverse-photoemission spectroscopy. Spectral changes across the transition near the Fermi level (E-F) (particularly within similar to 100 meV of E-F) have been interpreted as due to a "semimetallic" closure of the band gap in going from the insulating phase to the antiferromagnetic metallic phase. On the other hand, there is also composition- and temperature-dependent spectral weight transfer over a wider energy range of similar to 0.5-1 eV, indicating significant correlation effects. Photoemission intensity just below EF remains high in the insulating phases, indicating that the carrier number is large at high temperatures and that the activation-type transport is due to the activated mobility rather than the activated carrier number. [S0163-1829(98)07536-5].

We have made photoemission studies of La2-xSrxCuO4 (LSCO) and found the following: (1) suppression of the chemical potential (mu) shift fur small x indicates a breakdown of the Fermi-liquid picture in the underdoped regime and suggests the opening of a pseudogap: (2) valence band spectra have indeed shown a pseudogap whose magnitude increases with decreasing x. simultaneously the density of states (DOS) at mu diminishes;(3) ARPES measurements have revealed changes from an electron-like Fermi surface in the overdoped regime to a hole-like one in the underdoped regime. (C) 1998 Elsevier Science Ltd. All rights reserved.

We have made a high-resolution photoemission study of La2-xSrxCuO4 in a wide hole concentration (x)range from a heavily overdoped metal to an undoped insulator. As x decreases, the spectral density of states at the chemical potential (mu) is suppressed with an x dependence similar to the suppression of the electronic specific heat coefficient. In the underdoped region, the spectra show a pseudogap structure on the energy scale of 0.1 eV. The width of the pseudogap increases with decreasing x following the x dependence of the characteristic temperatures of the magnetic susceptibility and the Hall coefficient.

We have studied the electronic structure of Mn impurities in GaAs by Mn 2p core-level photoemission spectroscopy. From cluster-model analysis assuming the neutral (Mn3+) or negatively ionized (Mn2+) ground state, electronic structure parameters have been obtained. In either case, the Mn d electron number is evaluated to be similar to 5 using the obtained parameters, meaning that the neutral Mn3+ impurity, if it exists, consists of the Mn 3d(5) configuration and a valence hole bound to it through p-d hybridization and/or Coulomb interaction. We discuss the exchange interaction between the Mn local spin and the Valence hole as well as the stability of the neutral impurity against the ionization of the valence hole.

We have studied the effect of varying temperature and Co substitution on the low-energy electronic structure of FeSi by photoemission spectroscopy. We have observed that the density of states (DOS) of FeSi is reduced at low temperatures over a wide energy range of similar to 50 meV below the Fermi level (E-F), which is the energy scale observed by the magnetic, transport and optical measurements. The reduced DOS is completely recovered with increasing temperature but only partly, i.e., predominantly near E-F, With Co substitution, again consistent with the transport properties. [S0163-1829(98)00428-7].

The electronic structure of the ferromagnetic transition-metal oxide SrRuO3 has been studied by magnetic circular dichroism (MCD) in the valence-band photoemission spectra. MCD signals have been obtained by changing the direction of sample magnetization as well as the helicity of incident photons. Strong MCD signals were observed near the Fermi level. We have attempted to explain the MCD spectra using a band structure calculated with the unrestricted Hartree-Fock method. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

In order to study the effect of intercluster interaction on the high-energy spectroscopy of formally Cu3+ oxides, we have performed X-ray absorption spectroscopy (XAS) and Cu 2p-3d resonant photoemission spectroscopy (RPES) studies of rhombohedrally distorted LaCuO3. In the Cu 2p XAS spectrum, the Cu 2p(3/2) main peak is split into two structures, indicating that the interaction between the CuO6 octahedral clusters is strong. The valence-band RPES spectrum shows an enhanced satellite structure at similar to 12.5 eV, similar to that of Cu2+ oxides for the incident photon energies which create intermediate states well screened by the intercluster interaction. For the incident photon energy for the poorly screened intermediate state, the enhanced satellite structure becomes broad and has a maximum at similar to 13 eV. (C) 1998 Elsevier Science B.V. All rights reserved.

We have studied chemical potential shifts in filling-control transition-metal compounds from the shifts of photoemission and inverse photoemission spectra. Implications of the results for the electronic structure and electron correlation in these systems are discussed. In addition to the conventional Fermi-liquid behavior we find unusual shifts caused by a pseudo-gap formation and gap-opening resulting from stripe order in two-dimensional systems. (C) 1998 Elsevier Science B.V. All rights reserved.

We have made an X-ray photoemission and angle-resolved photoemission (ARPES) study of NaV2O5 at room temperature, i.e. in the paramagnetic phase well above its spin-Peierls transition temperature. The V 2p core level spectra well reflect its mixed valence nature. The result of the ARPES spectra clarifies the experimental band structure, reflecting the one-dimensionality of this compound. The lower binding energy side of the V 3d band shows a momentum-dependent modulation with the periodicity of pi, suggesting the existence of antiferromagnetic correlations or holon excitations in the one-dimensional Mott insulator. (C) 1998 Elsevier Science B.V. All rights reserved.

The electronic structures of the antiferromagnetic semiconductor FeS and ferrimagnetic metals Fe7S8 and Fe7S8 have been studied by spin-integrated and spin-resolved photoemission spectroscopy and inverse-photoemission spectroscopy. The overall Fe 3d bandwidth in the photoemission spectra is 25-30 % narrower than the density of states (DOS) predicted by first-principles band-structure calculations and is accompanied by an intense tail on the high-binding-energy side, indicating the correlated nature of electrons in the Fe 3d band. Deviation from the band DOS is more significant in Fe7S8 than in Fe7Se8, and in the minority-spin spectra than in the majority-spin spectra. Cluster-model calculation for FeS has shown satellite structures at high binding energies, but the calculated spectral line shape is not in good agreement with experiment compared to the band DOS. By introducing a self-energy correction to the band DOS, we could explain the narrowing of the overall Fe 3d bandwidth and the high-binding-energy tail shape but not for the unusual broadening of the Fe 3d band within similar to 1 eV of the Fermi level. [S0163-1829(98)02415-1].

We have made an angle-resolved photoemission study of NaV2O5 at room temperature, i.e., in the paramagnetic phase well above the spin-Peierls transition temperature. The obtained results reflect the one dimensionality of the electronic structure. The lower binding energy side of the V 3d band shows a clear momentum-dependent modulation with the periodicity of pi, indicating the existence of antiferromagnetic correlations or holon excitations in the one-dimensional Mott insulator. We compare the results with recent theoretical works on the one-dimensional Hubbard and t-J models and discuss consistency between model parameters.

The electronic structure of the formally Cu3+ metallic LaCuO3 has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the valence-band and Cu 2p core-level photoemission spectra using a CuO6 cluster model, the charge-transfer energy is estimated to be -1 eV, indicating that the ground state is dominated by the d(9) (L) under bar configuration with which the d(8) configuration is strongly hybridized, where (L) under bar denotes a ligand hole. However, agreement between the experimental results and the cluster-model calculations is not satisfactory for the detailed line shape of the main peaks. Especially, the Cu 2p x-ray-absorption spectrum cannot be well explained by the single-site cluster-model calculation, suggesting the importance of intercluster interaction. On the other hand, the existence of the charge-transfer satellite in the Pauli-paramagnetic state is explained by Hartree-Fock band-structure and self-energy calculations using parameters deduced from the analysis of the photoemission spectra. [S0163-1829(98)00315-4].

Various electron-spectroscopic results including core-level X-ray photoemission, X-ray absorption, Ce 3d-4f and 4d-4f resonant photoemission, and inverse-photoemission spectroscopy on the valence fluctuating ferromagnet CeFe2 are presented and analyzed with model calculations using the Anderson-impurity Hamiltonian. Surface effects on the spectra are taken into account in the analysis. The valence-band spectra are also compared with the band-structure density of states calculated using the local-spin-density approximation. The validity of describing the Ce 4f states as a localized impurity level and in the 4f band picture is discussed. (C) 1998 Elsevier Science B.V.

We have measured soft X-ray inverse-photoemission, Ce 3d-4f resonant photoemission and Ce 3d X-ray absorption spectra of CeNiSn, which is known as a Kondo semimetal, in order to clarify the possible difference of the surface Ce valency from the bulk one and its effect on the photoemission spectra. We have analyzed the spectra by model calculations based on the Anderson-impurity model with the assumption that the spectra are the superposition of the bulk and surface components and obtained the bulk and surface parameter sets of the model Hamiltonian. High-resolution photoemission spectra are also presented to observe the low-energy-scale electronic structure. (C) 1998 Elsevier Science B.V.

Spin and charge ordered stares with domain walls (DW) in two- and three-dimensional perovskite-type 3d transition-metal oxides have been investigated. We show that the relative stability of the DW perpendicular to the (1,0,0), (1,1,0), and (1,1,1) directions in three dimensions and those along the (1,0) and (1,1) directions in two dimensions is systematically and qualitatively understood in terms of superexchange interaction.

We have investigated the electronic structures of one-dimensional antiferromagnetic insulators Ca2CuO3 and Sr2CuO3 combining electron spectroscopic measurements and various calculations. While calculations based on a local-spin-density approach Tor the real magnetic structures fail to yield an insulating state, from our experiments we estimate the intrinsic band gaps in these materials to be about 1.7 eV (Ca2CuO3) and 1.5 eV (Sr2CuO3). Analysis of the core-level and the valence-band spectra in terms of model many-body Hamiltonians show that the charge-transfer energy Delta for these one-dimensional systems is significantly smaller than other cuprates, such as the high-T-c oxides (two-dimensional) and CuO (three-dimensional). Such a small Delta suggests the presence of the bare upper Hubbard band within the oxygen p bandwidth, and thus provides an example of a correlated covalent insulator.

We have studied the electronic structure of CrO2 by photoemission spectroscopy and specific heat measurements. The Cr 3d band shows a splitting into the upper and lower Hubbard bands with a small but finite density of states at the Fermi level, consistent with its metallic behavior. The small renormalization factor (z similar to 0.1 much less than 1) signifies strong electron correlation, but is still large compared with many 3d transition-metal oxides.

We have studied the effect of Lu substitution on the Kondo insulator YbB12 by high-resolution photoemission. Comparison of the spectra of YbB12, Yb0.5Lu0.5B12, and LuB12 reveals that the density of states (DOS) of the B sp-derived conduction band near the Fermi level is reduced in YbB12 over a rather wide (similar to 40 meV) energy region. Lu substitution (i) recovers the reduced B sp DOS, (ii) shifts the Yb 4f-derived Kondo peak towards higher binding energy, and (iii) decreases the Yb valence. These results are consistently analyzed using the Anderson-impurity model, and imply interaction between the Yb 4f ions mediated by the Yb 4f-B sp hybridization in YbB12.

Charge ordering in La2-xSrxMO4 (M=Cu and Ni) has been studied by means of unrestricted Hartree-Fock calculations. The present calculations show that, while the vertical charge stripes along the (1,0) direction of the CuO2 square plane are favored in the cuprates with a charge-transfer energy Delta similar to 2 eV, the diagonal stripes are stable in the nickelates with Delta similar to 4 eV. It has been found that the metal-centered and oxygen-centered stripes are nearly degenerate in energy both in the cuprates and in the nickelates. In the metal-centered diagonal stripe of the nickelates, the coupling between Ni2+ and Ni3+ is calculated to be ferromagnetic. The stabilization of the various charge-ordered states by the elongation and tilting of the MO6 octahedra has also been studied. [S0163-1829(97)00442-6].

We have studied the temperature dependence of the photoemission spectra of La1-xSrxMnO3 (x=0.0, 0.2, and 0.4) and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for x=0.2 and 0.4. By contrast, the spectra of La0.6Sr0.4CoO3 show no significant temperature dependence. From comparison between the temperature-and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La1-xSrxMnO3, we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations.

We have investigated the exchange interaction between the localized d electrons of 3d transition-metal impurities and the delocalized host band electrons in LI-VI semiconductors based on the configuration-interaction scheme. The exchange constant N beta in the Kondo-type effective Hamiltonian is evaluated from Ti to Ni. For Mn, Fe, and Co, N beta is negative (antiferromagnetic), and is in good agreement with experimental values. The exchange coupling of the Ti impurity is predicted to be positive (ferromagnetic) due to Hund&apos;s coupling. For V, Cr, and Ni, in which orbital degrees of freedom is important, the sign of N beta depends on the Jahn-Teller distortion.

We have measured photoemission and oxygen Is x-ray absorption spectra of the ferromagnetic metal SrRuO3 and compared them with a first-principles band-structure calculation. The overall distribution of Ru 4d and O 2p spectral weight is in good agreement with that predicted by the band-structure calculation. However. the observed spectral line shape of the Ru 4d band is spread over a wide energy range and the emission intensity at the Fermi level is weakened compared to the band-structure calculation. This implies the importance of electron correlation in the Ru oxide.

The downward shift of the electron chemical potential mu with hole doping in La2-xSrxCuO4 has been deduced from the shifts of photoemission and inverse-photoemission spectra. While the shift is large (similar to 1.5 eV/hole) in overdoped samples, it is suppressed (&lt;0.2 eV/hole) in underdoped samples, implying a divergent charge susceptibility near the metal-insulator transition. In the overdoped regime, the mu and the electronic specific heat coefficient gamma are consistently explained within Fermi-liquid theory, whereas the same analysis gives unphysical results in the underdoped regime, indicating the breakdown of the Fermi-liquid picture in the underdoped regime.

Photoelectron spectra of the ordered surface alloy c(2 x 2) CuMn/Cu(100) show distinct valence band spectral features at higher binding energy, which are most pronounced for the optimal CuMn c(2 x 2) superstructure at half-monolayer Mn deposition. Our analysis comprising resonant photoemission, core level photoemission, inverse photoemission, first-principles local-density calculations, and configuration-interaction cluster-model calculations shows that the data represent the first observation of a valence band satellite caused by the increased electron correlation in an ultrathin metal film.

We have studied the electronic structure of Y(Co1-xAlx)(2) with x = 0.0, 0.12 and 0.18, which compositions cover the paramagnetic, metamagnetic and ferromagnetic phases, by photoemission and inverse-photoemission spectroscopy. The spectra are compared with the density of states given by band-structure calculations. An agreement between experiment and theory was considerably improved by applying a self-energy correction to the band density of states.

The electronic structures of FeS, Fe7S8 and Fe7Se8 have been studied by spin-integrated and spin-resolved photoemission spectroscopy and inverse-photoemission spectroscopy. Electron correlation in Fe 3d bands is found to be important in these compounds.

The spin, charge, and orbital ordering in R(0.5)A(0.5)MnO(3) and R(0.5)A(1.5)MnO(4) (R=rare earth, A=Sr, Ca) has been studied by means of unrestricted Hartree-Fock calculations on the multiband p-d model. Since the superexchange interaction between the Mn3+ and Mn4+ sites depends on which type of e(g) orbital is occupied at the Mn3+ site, antiferromagnetic states such as A type and CE type favor a specific orbital ordering. It is shown that the Jahn-Teller distortion consistent with the orbital ordering plays an essential role in stabilizing the experimentally observed antiferromagnetic states.

An overview is given on the effects of strong d-d Coulomb repulsion on the electronic structure of charge-transfer (CT)-type 3d transition-metal (TM) compounds, especially of 3d TM chalcogenides. Mean-field effects determine the groundstate properties of the system including magnetic and orbital ordering whereas correlation effects are necessary to correctly describe excitation properties such as photoemission spectral weight distribution. Because of the strong d-d repulsion even in the metallic state of CT-type compounds, the overall photoemission spectral line shapes remain relatively unchanged across metal-insulator transitions. In the vicinity of the Fermi level, high-resolution photoemission spectra exhibit unusual line shapes and spectral weight transfer, which implies strong renormalization of quasi-particles due to electron correlation.

The electronic structures of pyrite-type transition-metal chalcogenides MS2-xSex (M = Fe, Co, Ni) has been investigated by photoemission and inverse-photoemission spectroscopy. The valence-band spectrum of ferromagnetic CoS2 does not show exchange splitting of the Co 3d peak, in disagreement with band-structure calculations. High-resolution photoemission spectra of NiS1.55Se0.45 shows spectral weight transfer from low (similar or equal to 50 meV) to high (0.2-0.5 eV) binding energies, in going from the metallic to the insulating phase.

We have studied the effects of hole doping on the electronic structure of La1-xSrxCoO3 by photoemission and x-ray-absorption spectroscopy. The Co 2p core-level and the valence-band spectra show charge-transfer satellites, which have been more obviously observed in 3p-3d resonant-photoemission spectra. By Sr substitution for La in LaCoO3, the valence-band spectra do not show rigid-band behavior but change systematically and reflect the semiconductor-to-metal transition which occurs with hole doping. Only small changes with x have been observed in the resonant-photoemission spectra. Combined with configuration-interaction cluster-model calculations, this observation suggests that the intermediate-spin state is realized in the ferromagnetic phase.

We have studied the electronic structure and its changes across the metal-insulator transition in the spinel-type compound CuIr2S4 using photoemission and inverse-photoemission spectroscopy Photoemission spectra near the Fermi level show a gap; opening of similar to 20 meV in the insulating phase, consistent with the transport activation energy. Core-level spectra indicate that the Cu ion is monovalent, End hence Ir is in the intermediate valence state of +3.5. Comparison between the spectra and band-structure calculation reveals that the Ir 5d density of states is strongly distorted, probably due to electron correlation in spite of the general belief of weak correlation in 5d-electron systems.

The hole-doped ladder compound La1-xSrxCuO2.5 has been studied by photoemission and x-ray-absorption spectroscopy. Cluster-model analysis of photoemission spectra and subsequent Hartree-Fock band-structure calculation for LaCuO2.5 indicate weak ferromagnetic coupling between the ladders. Changes in the overall electronic structure induced by hole doping are similar to those in La2-xSrxCuO4. Spectral weight around the Fermi level (EF) increases with x and a weak but finite Fermi edge is established at x=0.20.

We investigate the electronic structure of the antiferromagnetic insulator Sr2CuO3 combining photoemission measurements and various calculations. The results suggest that the charge transfer energy, Delta, is unusually small, placing the bare upper Hubbard band within the oxygen p bandwidth, thereby implying that this compound is the first example of a correlated covalent insulator with a finite local moment.

We have studied the electronic structure of LaCoO3 by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Co 2p core-level and valence-band photoemission spectra display satellite structures indicating a strong electron-correlation effect. The Co 2p core-level photoemission, the valence-band photoemission, and the 0 1s XAS spectra have been analyzed using a configuration-interaction cluster model for the initial-state configurations of the low-spin (LS: (1)A(1)), intermediate-spin (IS: T-3(1)) and high-spin (HS: T-5(2)) states and their mixtures. The ground state of LaCoO3 in the LS state is found to have heavily mixed d(6) and d(7)L character, reflecting the strong covalency. The magnetic susceptibility has been analyzed for various level orderings of the LS, IS, and HS states. From the analyses of the photoemission spectra and the magnetic susceptibility data, the temperature-induced paramagnetism in LaCoO3 above similar to 90 K is most likely due to a gradual LS-to-IS transition.

The electronic structure of pyrite-type NiS2, CoS2, and FeS2 has been studied by photoemission spectroscopy. From resonant photoemission studies and configuration-interaction cluster-model analysis of the spectra, NIS2 is found to be a charge-transfer-type insulator, the band gap of which is formed between the occupied S 3p and the empty Ni 3d states. Cluster-model calculations indicate that the short Fe-S distance favors the low-spin (S = 0) ground state in FeS 2 compared to the high-spin FeS. Resonant photoemission results indicate a sign of electron correlation in the nonmagnetic semiconductor FeS2.

We have studied the low-energy electronic structure of a Kondo insulator YbB12 by high-resolution photoemission spectroscopy. A ''Kondo peak'' is observed similar to 25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (greater than or similar to 75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak.

We have studied Y1-xCaxTiO3 by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak similar to 1.4 eV below the Fermi level (E(F)), which evolves into the lower Hubbard band in the x=0 (d(1)) limit. The spectra show quasiparticle emission at E(F) with an extremely small spectral weight, z similar to 0.01, which vanishes as the system approaches either the Mott insulator limit (x=0) or the band insulator limit (x=1). Correspondingly, inverse-photoemission spectra show the upper Hubbard band and a quasiparticle feature in the unoccupied state. The fact that the observed quasiparticle spectral weight is smaller than that of La1-xSrxTiO3 is attributed to the larger U/W, where U is the an-site d-d Coulomb energy and W is the d-band-width. The presence of the similar to 1.4-eV peak for a marry empty d band (x similar to 1) end the small spectral weight at E(F) cannot be explained within the Hubbard model, indicating the importance of interactions which are not included in the model, such as the long-range Coulomb interaction and the electron-phonon interaction.

We have studied transition-metal 3d-oxygen 2p lattice models, where full degeneracy of transition-metal 3d and oxygen 2p orbitals and on-site Coulomb and exchange interactions between 3d electrons are taken into account, by means of a spin- and orbital-unrestricted Hartree-Fock (HF) approximation. The electronic-structure parameters deduced from the cluster-model analyses of the photoemission spectra are used as input. We have applied this method to perovskite-type 3d transition-metal oxides, which exhibit various electrical and magnetic properties. It is shown that the HF results can explain the ground-state properties of insulating oxides. The relationship between spin- and orbital-ordered solutions and the Jahn-Teller-type and GdFeO3-type distortions in RTiO(3), RVO(3), RMnO(3), and RNiO(3) (R is a rare earth atom or Y) is extensively studied. Single-particle excitation spectra calculated using Koopmans&apos; theorem give us an approximate but relevant picture on the electronic structure of the perovskite-type 3d transition-metal oxides. As a drawback, the HF calculations tend to overestimate the magnitude of the band gap compared with the experimental results and to predict some paramagnetic metals as antiferromagnetic insulators.

We have studied the electronic structure of Ni borocarbides by means of photoemission and inverse-photoemission spectroscopy. The core-level and valence-band spectra of superconducting YNi2B2C and nonsuperconducting LaNi2B2C are presented and are compared with band-structure calculations. The core-level spectra well reflect their highly covalent bonding character. The Ni core-level spectra show weak but distinct satellites due to two-hole bound states, indicating significant electron correlation in both compounds. Although the gross electronic structure of both compounds is in agreement with the band-structure calculations except for the two-hole bound-state satellites, spectra near the Fermi level (E(F)) are quite different from those predicted by the calculations. That is, high-resolution photoemission spectra do not show a peak at E(F) in YNi2B2C and that at similar to 0.1 eV below E(F) in LaNi2B2C, which have been predicted by the calculations, indicating that electron correlation and/or electron-phonon interaction may Play a significant role in the low-energy excitations in the Ni borocarbides. A similar behavior in the spectra of A15-type superconductors is also pointed out.

We have extended the single impurity cluster model to include the effects of charge transfer to the conduction band. By introducing a few well defined parameters to describe the interaction between the single impurity and the conduction band we find an improved agreement for the asymmetric line shapes of the metal 2p core-level photoemission spectra of the 3d-transition metal pyrites FeS2 and CoS2 and of NiAs-type NiS.

Itinerant ferrimagnetic metals Fe7S8 and Fe7Se8 and antiferromagnetic semiconductor FeS have been studied by spin-polarized and spin-integrated photoemission and inverse-photoemission spectroscopy. Comparison with the density of states given by band-structure calculations indicates that the Fe 3d band is narrowed with spectral weight transfer towards higher binding energies and that the intensity at the Fermi level is suppressed. The band narrowing and the spectral weight transfer is reproduced by including the self-energy correction, suggesting the importance of electron correlation. Spin-polarized photoemission spectra of Fe7S8 and Fe7Se8 show negative spin-polarization near the Fermi level and spin-dependent correlation effect.

We present the results of systematic photoemission and inverse-photoemission studies of strongly correlated metallic systems which have one-, two- and three-dimensional crystal structures. The spectral weight distribution exhibits characteristic evolution as a function of band width and band filling as well as of mass anisotropy. Mass enhancement is deduced from the weight transfer between the coherent and incoherent parts of the spectral function. Chemical potential shift as a function of band filling exhibits an unusual suppression near Mott transition.

The electronic structure of PrNiO3 has been studied by photoemission and x-ray absorption spectroscopy and subsequent configuration-interaction calculations on a NiO6 cluster model. It has been found that the charge-transfer energy Delta is similar to 1 eV and that the Ni 3d and O 2p orbitals are strongly hybridized in the ground state. We have also performed unrestricted Hartree-Fock calculations on a Ni 3d-O 2p perovskite-type lattice model using the parameters obtained from the cluster-model analysis.

Electronic structures of Magneli-phase eta- and gamma-Mo4O11 have been studied by means of ultraviolet photoemission and inverse-photoemission spectroscopies. Resonant photoemission spectra have been also measured in the photon energy range from 32 eV to 90 eV The energy positions of Mo 4d states can be qualitatively explained in terms of a simple band structure model.

The electronic structure of La1-xSrxCoO3 has been studied by photoemission and x-ray absorption spectroscopy. The valence-band spectra for LaCoO3 show features characteristic of the low-spin state. The spectra systematically change with x, showing a gradual transition from the low-spin to high-spin states. The difference between the on- and off-resonant photoemission spectra shows small but detectable changes, corresponding to the magnetic transition. The absence of significant changes with temperature in the valence-band spectra of La0.6Sr0.4CoO3 clearly demonstrates a distinction between the Co and Mn perovskites.

We have reinterpreted the metal 2p core-level x-ray photoemission spectra of the 3d transition-metal pyrites FeS2, CoS2 and NiS2 using a new version of the single-impurity cluster model which includes the effects of charge transfer to the conduction band. By accounting for the effective screening of the strong core-hole potential involving the unfilled S 3p sigma* orbitals on the ligand sites, we can successfully model the highly asymmetric lineshapes and weak satellite structures found for FeS2 and CoS2. Reduced screening in NiS2 leads to the appearance of strong satellite structures and the spectrum is best interpreted using the screening channel to the narrow upper Hubbard band. These results indicate the importance of the empty ligand orbitals when interpreting the physical properties of the pyrite-type chalcogenides.

We report temperature-dependent redistribution of spectral weight in photoemission spectra of La0.6Sr0.4MnO3 over a wide energy range of several eV. We attribute this to a change in the electron correlation strength induced by the changing degree of ferromagnetic order with temperature. We discuss the relationship between the temperature-dependent spectral weight close to the Fermi level and the unusual transport properties.

In order to explain the excitation properties of 3d transition-metal oxides in a unified framework, we have performed second-order perturbation calculations of the self-energy corrections around the unrestricted Hartree-Fock solution of lattice models using the electronic-structure parameters deduced from photoemission spectroscopy. The self-energy modifies the magnitude of the band gap and causes substantial spectral weight transfer over a wide energy range both in insulating and metallic compounds of the Mott-Hubbard type as well as of the charge-transfer type, resulting in an improved agreement between theory and experiment.

The electronic structures of a wide range of early transition-metal (TM) compounds, including Ti and V oxides with metal valences ranging from 2+ to 5+ and formal d-electron numbers ranging from 0 to 2, have been investigated by a configuration-interaction cluster model analysis of the core-level metal 2p x-ray photoemission spectra (XPS). Inelastic energy-loss backgrounds calculated from experimentally measured electron-energy-loss spectra (EELS) were subtracted from the XPS spectra to remove extrinsic loss features. Parameter values deduced for the charge-transfer energy Delta and the d-d Coulomb repulsion energy U are shown to continue the systematic trends established previously for the late TM compounds, giving support to a charge-transfer mechanism for the satellite structures. The early TM compounds are characterized by a large metal d-ligand p hybridization energy, resulting in strong covalency in these compounds. Values for Delta and U suggest that many early TM compounds should be reclassified as intermediate between the charge-transfer regime and the Mott-Hubbard regime.

In order to explain the excitation properties of (Formula presented) transition-metal oxides in a unified framework, we have performed second-order perturbation calculations of the self-energy corrections around the unrestricted Hartree-Fock solution of lattice models using the electronic-structure parameters deduced from photoemission spectroscopy. The self-energy modifies the magnitude of the band gap and causes substantial spectral weight transfer over a wide energy range both in insulating and metallic compounds of the Mott-Hubbard type as well as of the charge-transfer type, resulting in an improved agreement between theory and experiment. © 1996 The American Physical Society.

The electronic structure of pyrite-type (Formula presented), (Formula presented), and (Formula presented) has been studied by photoemission spectroscopy. From resonant photoemission studies and configuration-interaction cluster-model analysis of the spectra, (Formula presented) is found to be a charge-transfer-type insulator, the band gap of which is formed between the occupied S 3p and the empty Ni 3d states. Cluster-model calculations indicate that the short Fe-S distance favors the low-spin (S=0) ground state in (Formula presented) compared to the high-spin FeS. Resonant photoemission results indicate a sign of electron correlation in the nonmagnetic semiconductor (Formula presented). © 1996 The American Physical Society.

We report temperature-dependent redistribution of spectral weight in photoemission spectra of (Formula presented)(Formula presented)(Formula presented) over a wide energy range of several eV. We attribute this to a change in the electron correlation strength induced by the changing degree of ferromagnetic order with temperature. We discuss the relationship between the temperature-dependent spectral weight close to the Fermi level and the unusual transport properties. © 1996 The American Physical Society.

We have studied (Formula presented)(Formula presented)(Formula presented) by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak ∼1.4 eV below the Fermi level ((Formula presented)), which evolves into the lower Hubbard band in the x= 0 ((Formula presented)) limit. The spectra show quasiparticle emission at (Formula presented) with an extremely small spectral weight, z∼0.01, which vanishes as the system approaches either the Mott insulator limit (x=0) or the band insulator limit (x=1). Correspondingly, inverse-photoemission spectra show the upper Hubbard band and a quasiparticle feature in the unoccupied state. The fact that the observed quasiparticle spectral weight is smaller than that of (Formula presented)(Formula presented)(Formula presented) is attributed to the larger U/W, where U is the on-site d-d Coulomb energy and W is the d-band-width. The presence of the ∼1.4-eV peak for a nearly empty d band (x∼ 1) and the small spectral weight at (Formula presented) cannot be explained within the Hubbard model, indicating the importance of interactions which are not included in the model, such as the long-range Coulomb interaction and the electron-phonon interaction. © 1996 The American Physical Society.

We have studied the electronic structure of rare-earth Ni boro-carbides by high-resolution photoemission. The spectra of superconducting YNi2B2C and non-superconducting LaNi2B2C are remarkably similar in spite of large differences predicted by band-structure calculations. The disappearance of the peak in the density of states at the Fermi level is attributed to spectral weight transfer towards higher energies concomitant with the conduction-band mass renormalization.

We have studied metallic SrVO3 and CaVO3 by inverse photoemission and high-resolution photoemission. In going from Sr to Ca, considerable spectral weight is transferred from the coherent band to the upper and lower Hubbard bands. Meanwhile, the overall intensity rather than the width of the coherent band decreases, implying that the bandwidth remains finite as the system approaches the Mott transition. The result implies that the effect of long-range Coulomb interaction as well as short-range interaction becomes increasingly important towards the transition.

The electronic structure of PrNiO3 has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the spectra using configuration-interaction calculations on a NiO6 cluster model, it has been found that the charge-transfer energy Delta is similar to 1 eV and the Ni 3d and O 2p orbitals are strongly hybridized in the ground state. From the cluster-model calculation, the magnetic moment of Ni 3d is estimated to be similar to 0.9 mu(B), which is close to the ionic value of Ni3+ and in good agreement with that obtained from neutron-diffraction experiments. Using the electronic-structure parameters deduced from the cluster-model analysis, we have performed unrestricted Hartree-Fock calculations on a Ni 3d-O 2p perovskite-type lattice model in order to study the effect of GdFeO3-type distortion on the orbital polarization and band gap.

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band. It is found that these effects can have a considerable influence on the line shapes of core-level spectra of small-gap transition-metal compounds. Using a few additional, well-defined parameters to describe the interactions between the local cluster and the conduction band, and retaining the original intracluster parameters from previous single-impurity models, we find an improved agreement for the Ni 2p spectrum of NiS. The asymmetric line shape is well reproduced, keeping the correct satellite to main peak ratio, which is not possible using the three-peak structure of previous models. The new model is less successful in explaining effects beyond standard single-impurity models in more ionic compounds, such as the double-peak stucture of the main line of the core-level spectrum of NiO, where the interactions between the metal d site and the conduction band are expected to play a lesser role.

We have studied the systematic changes of the electronic structure in 3d transition-metal oxides and sulfides within a configuration-interaction cluster model including multiplet effects. Parameters of the model have been deduced from analyses of the 2p core-level photoemission spectra of these compounds. We have calculated the magnitudes of the band gaps, the net d-electron numbers, and the character of doped carriers (and hence of band gaps) within the cluster model. The variation of the calculated magnitudes of the band gaps is in good agreement with experiment, especially with those derived in a recent optical study by Arima et al. of the LaMO(3) series, where M denotes a transition-metal element.

We have studied the electronic structure of FeSi by high-resolution photoemission. At room temperature the spectra show no energy gap while a gap opening was observed below similar to 150 K accompanied by the gradual paramagnetic-to-nonmagnetic transition. From analysis of the spectral lineshape using a model self-energy, we find that considerable band narrowing occurs near the band edge, probably due to electron correlation, giving a qualitative explanation to the unusual electronic and magnetic properties.

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band for the interpretation of the core level photoemission spectra of highly correlated, small-gap semiconducting and metallic transition metal compounds. The model includes the charge fluctuations between the local transition metal site and the conduction band and has been successfully applied to reinterpret the spectra of some 3d transition metal chalcogenides, reproducing well the large asymmetry of the main peaks, as well as the other satellite structures.

The studies of ultra-violet photoemission spectroscopy and soft X-ray absorption spectroscopy of a perovskite-type 3d(1) Mott-Hubbard system Ca1-xSrxVO3, which is metallic in the whole composition range (0 &lt; x &lt; 1), are reported. As we control electron-electron interaction of the system by changing the Sr concentration (x), we have observed a systematic change of the spectra, which clearly manifests how the single-particle density of states around Fermi level (E-F) behaves near a Mott transition.

Valence-band and conduction-band structures of NiAs-type MnTe have been investigated by means of ultraviolet photoemission and inverse-photoemission spectroscopy. We estimate the Mn 3d spin-exchange splitting energy to be 6.6 +/- 0.2eV. The Mn 3d-derived spectra of MnTe have been calculated in terms of a configuration interaction theory.

By using ultra-violet photoemission spectroscopy (UPS) and soft x-ray absorption spectroscopy (XAS), we have observed a systematic change of the spectral function around the Fermi level in the perovskite-type conductive 3d(1) system, Ca1-xSrxVO3. When we decrease the Sr concentration (x) from 1 to 0 in Ca(1-x)SrxVO(3), UPS and XAS intensities near the Fermi level decrease, and the reduced spectral weight is transferred to high energy part about 2 eV below and above Fermi level. The high energy features are thought to correspond to the remnants of lower and upper Hubbard bands, and the spectral weight within similar to 1eV of Fermi level is assigned to the quasiparticle excitations or renormalized 3d-band. That is, the observed systematic spectral change indicates the opening of Mott-Hubbard gap.

Valence-band and conduction-band structures of NiAs-type MnTe have been investigated by means of ultraviolet photoemission and inverse-photoemission spectroscopies. Based on the comparison with the results of band-theory, features observed at -3.7 and 2.9 eV relative to the valence-band maximum (VBM) are assigned to emission from the Mn 3d up arrow and 3d down arrow states with fairly localized character, providing a spin-exchange splitting energy of 6.6+/-0.2eV. On the other hand, the Mn 3d photoemission and inverse-photoemission spectra have been calculated in terms of a configuration interaction theory using a MnTe6 model cluster, to interpret the whole features of the experimental spectra including multielectron satellites. The Mn 3d spectral features at -12 to -6, -6 similar to 0 and 2.9 eV relative to the VBM are attributed predominantly to transitions into the d(4), d(5) L and d(6) final state configurations, respectively, where L represents a ligand hole.

We have studied the electronic structure of the superconductor YNi2B2C by photoemission and inverse-photoemission spectroscopy. The spectra show a prominent Ni 3d band centered around 1.5 eV below the Fermi level (E(F)), whose top crosses E(F), and B, C 2sp-derived states at higher binding energies, generally consistent with band-structure calculations. However, the Ni 3d-derived conduction bands are narrower than calculated and are accompanied by a satellite, signaling electron-correlation effects within the d bands. The density-of-states peak at E(F) predicted by band-structure calculations is suppressed and its spectral weight is transferred away from E(F), presumably due to an electron-phonon interaction and/or electron correlation.

A measure of the Mn 3d partial density of states (DOS) of NiAs-type MnTe has been obtained from resonant photoemission experiments in the Mn 3p-3d core excitation region. On the basis of an analysis of the Mn 3d partial DOS spectrum in terms of a configuration interaction on a Mn2+ (Te2-)6 model cluster, we find that the spectral intensities in the top 6 eV are predominantly due to transitions to the d5L final-state configuration, where L represents a ligand hole. On the other hand, those at 6-14 eV originate mainly from transitions to the d4 configuration.

The electronic structures of the chalcopyrite-type CuFeS2 and CuAl0.9Fe0.1S2 are studied by x-ray photoemission (XPS), resonance photoemission, Auger-electron, optical reflectance, and electron-energy-loss (EELS) spectroscopies. The Fe 3d-derived states are revealed by the valence-band XPS spectra and the Fe 3p core resonance photoemission spectra. The spectra are analyzed by configuration-interaction calculation on the FeS4 cluster model; the analysis yields the S 3p --&gt; Fe 3d charge-transfer energy DELTA close to zero, indicating strong covalency between the Fe 3d and S 3p orbitals. This situation is reflected upon the reduced Fe magnetic moment and the high Neel temperature of CuFeS2. The S 3p --&gt; Fe 3d charge-transfer excitation is resolved in the optical reflectance and EELS spectra, which explains the larger binding-energy tails of the core-level photoemission spectra of CuFeS2. The Cu 3d two-hole bound state is studied through the Cu L3M4,5M4,5 Auger and Cu 3p core resonance photoemission spectra, from which the effective Coulomb energy U(eff)(1G) between the two holes and the Cu 3d --&gt; 4sp promotion energY DELTA(d-sp) are evaluated. The Cu 2p core XPS spectrum of CuFeS2 has revealed a mixing of the d9 (''Cu2+'') ConfigUration into the formally monovalent Cu. This is interpreted as due to the Cu 3d-Fe 3d hybridization mediated by the S 3sp valence states.

The insulating oxide NaCuO2 has been studied by x-ray photoemission spectroscopy and subsequent cluster-model analysis. It is found that the d8 --&gt; d9L charge-transfer energy (L:ligand hole) is negative and the ground state is dominated by the d9L configuration. Using the Anderson impurity model, it is shown that strong 3d-ligand hybridization opens a band gap for a negative charge-transfer energy. This band pp corresponds to charge fluctuations mainly of the p-p type, d9L + d9L --&gt; d9 + d9L2, with a considerable mixture of d character into the p states, and not of the conventional Mott-Hubbard (d-d) type nor of the charge-transfer (p-d) type. The magnitude of the pp is strongly affected by the geometrical arrangement of metal-oxygen local units, giving a natural explanation for the difference between the insulating NaCuO2 and metallic LaCuO3. The electronic structures of Fe4+ and Ni3+ oxides and their insulating versus metallic behaviors, which are expected to resemble those of the Cu3+ oxides, are also discussed. To generalize the above conclusions, a modification of the metal-insulator boundaries in the Zaanen-Sawatzky-Allen diagram is proposed to include compounds with small or negative charge-transfer energies.

A measure of the Fe 3d partial density of states (DOS) in the valence bands of Cd0.89Fe0.11Se single-phase single crystals has been obtained using synchrotron-radiation photoemission. Based on an analysis of the Fe 3d DOS spectrum in terms of a configuration-interaction calculation on a Fe2+(Se2-)(4) model cluster, we find that the spectral intensities in the top 6 eV are predominantly due to transitions to d(6)L final-state configuration, where L represents a ligand hole. On the other hand, those at 7-15 eV originate mainly from transitions to the d(5) configuration. The model calculation provides the Fe 3d Coulomb correlation energy of 4.5 eV.