Barium tantalate with a tetragonal tungsten bronze structure, Ba3Ta5O15, and compounds where Ba is partly substituted by various rare earths (R), Ba3-xRxTa5O15, were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta 5d orbital and the localized spins in the Eu 4f orbital.

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.

To study the photoinduced processes in strongly correlated electron systems occurring coherently on various time scales, we conducted the pump-probe optical spectroscopy of hexagonal YMnO3 with ranges of delay time between 0.1 and 5000 ps and the energy of light between 1 and 3 eV. We found that the photoinduced changes in reflectivity were dominated by the intra-atomic excitation of a d electron in Mn, by the relaxation of energy from electronic excitations to phonon excitations, by the propagation of a strain wave, and by thermal conduction inside a sample, which occur successively with time. We also found that the sound velocity and thermal conductivity of the sample including their anisotropy can be obtained by this technique.

We successfully grew bulk single crystals of VOx and TiOx with a sodium-chloride structure by the floating-zone method, and we performed optical reflectivity measurement and pump-probe spectroscopy to clarify the electronic structure of these series of compounds and their x dependence. We found that optical spectra can be discussed on the basis of the Mott-Hubbard model, although a disorder arising from the deficiencies of Vi or Ti and O plays an important role in the electronic structure as well as their x dependence.

We studied the slow dynamics of phase transitions in Ba1-xSrxV13O18, which is composed of quasitriangular lattices of V ions and exhibits a phase transition into a V orbital ordered state with V trimerization. We found a clear time dependence of magnetic susceptibility and strain on a phase transition, and the transformation time estimated from the time dependence exhibits a peculiar temperature dependence, consistent with nucleation-growth behavior. Detailed analysis indicates that there is a distribution of the transformation time in Ba1-xSrxV13O18, and the formation of twin structures associated with the phase transition may affect the nucleation-growth dynamics and causes such a distribution of the transformation time.

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 found a large negative magnetoresistance in Ba3-xEuxNb5O15 with a tetragonal tungsten bronze structure, which changes from a metallic state (x = 0) to an insulating state (x = 3) under variation of x. The magnitude of the magnetoresistance ρc(0)=ρc(H), where ρc(H) is the resistivity along the c axis with the applied magnetic field of H, increases with x and reaches ∼5000 near the metal–insulator phase boundary. Such a large negative magnetoresistance is likely to be due to the coupling between the itinerant 4d electrons in Nb and the localized 4 f spins in Eu, and the disorder arising from Eu ions in this series of compounds.

Orbital degrees of freedom appear in partially filled degenerate orbitals and are responsible for a variety of physical properties. Although many methods for directly observing orbital states in ordered states have been proposed, it is difficult to determine degenerate orbital states directly. We found that high-precision diffraction data obtained by synchrotron radiation X-rays can be used to observe only the valence electron density distribution with minimal analytical bias. The results obtained using this method clearly showed the anisotropy of the partially filled degenerate 3d orbital states in spinel vanadium oxide FeV2O4. The 3d orbital quantum state was successfully determined from this anisotropy.

In photoinduced phase transitions, the relationship between the crystal orientation and the direction of the polarization or wave vector of light seems important, but surprisingly, such directional properties in photoinduced phase transitions have not been well studied experimentally. Ti3O5 exhibits a structural phase transition involving the anisotropic migration of Ti and O ions, and its nanocrystals exhibit a persistent photoinduced phase transition. We found that a photoinduced phase transition occurs even in a single crystal of this compound, but it significantly depends on the orientation of the plane on which the light is irradiated. Such a selection rule for photoinduced phase transitions is dominated by the anisotropy of the strain associated with the phase transition.

We investigated single crystals of Ba3-xSrxNb5O15 with a tetragonal tungsten bronze (TTB) structure, which exhibits a metal-insulator transition with x. We found that there is a one-dimensional anisotropy with a higher dc conductivity along the c axis in the metallic phase, but it disappears in the insulating phase. The optical conductivity spectra of the metallic samples show similar anisotropy to the dc conductivity, but the anisotropy persists even in the insulating phase. These experimental results, together with the result of magnetic susceptibility, indicate that the disorder caused by the Sr ions loosely bound to the surrounding oxygen ions plays an important role in the metal-insulator transition.

In BaV10O15 with quasitriangular V lattices, we found unique superstructures characterized by (qq0)-type peaks in the momentum space, where q varies continuously with temperature between q = 0 and 1/3 and locks at 1/3. Such a threefold superstructure is likely caused by the dense arrangement of the V trimers associated with the orbital ordering of V. The results indicate that the superstructure of the V trimers, each of which is also a superstructure, gives rise to the ordered phases with peculiar characteristics.

The Fe2+/Fe3+ charge order (CO) in LuFe2O4 has been examined by Mossbauer quadrupole effect. The stoichiometric single crystal of LuFe2O4 was used to measure the paramagnetic spectrum at 282 K. The spectrum was analyzed by assuming the CO monoclinic structure (space group C2/m) with four inequivalent Fe sites, among which two are occupied by Fe3+ and other two are occupied by Fe2+. The obtained quadrupole parameters, quadrupole splitting QS and the intensity ratio of the two absorption lines I(+/- 3/2)/I(+/- 1/2), coincide well with those calculated from the crystal structure. On the other hand, the parameters obtained by assuming other two crystal structures previously proposed, the hexagonal structure with a single Fe site (R (3) over barm) and the monoclinic structure with a single Fe site (C2/m), are found to be inconsistent with the calculated ones. These results indicate that the CO monoclinic structure is most probable among the structural models proposed so far from the viewpoint of the Mossbauer quadrupole effect.

We studied the ferrimagnet La5Mo4O16 with Mo ions on quasisquare lattices both experimentally and theoretically. We found that in addition to the antiferromagnetic ordering at T-N = 200 K, a magnetic phase transition into a ferrimagnetic phase occurs at T-s1 = 60 K, which is caused by the orbital ordering of the Mo 4d states. We also found that the relatively small size of ferrimagnetic domains despite the large Ising anisotropy of the Mo spins are formed below T-s1, which are responsible for the relaxation behavior and the memory effect in the magnetization. Nonvolatile positive magnetoresistance observed in this compound below T-s1 can be attributed to the fact that such magnetic domains and the electrical conduction along the domain walls disappear with the applied magnetic field.

We performed a pump-probe optical reflectivity measurement with an applied uniaxial strain for bulk single crystals of BaV10O15 at room temperature. We found that with a compressive strain of Delta L/L= -3.0 x 10(-3) along the b-axis, the oscillation in the time dependence of the photoinduced reflectivity change, which arises from the propagation of shockwave, becomes longer in period, indicating that the sound velocity decreases on the order of 10(-2). This result implies that the elastic modulus of BaV10O15 is coupled with the correlation of the orbital degree of freedom of V t(2g) electrons.

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.

The dynamics of the first-order phase transitions involving a large displacement of atoms, for example, a liquid-solid transition, is generally dominated by the nucleation of the ordered phase and the growth of the nuclei, where the interfacial energy between the two phases plays an important role. On the other hand, electronic phase transitions seldom exhibit such a nucleation-growth behavior, probably because two-phase coexistence is not dominated by only the interfacial energy in such phase transitions. In the present paper, we report that the dynamics of a phase transition associated with an ordering of d orbitals in a vanadate exhibits a clear nucleation-growth behavior and that the interfacial energy between the orbital-ordered and -disordered phases dominated by the orbital-spin coupling can be experimentally obtained. The dynamics of many discontinuous phase transitions is determined by the nucleation and growth of regions of the new phase. Katsufuji et al. demonstrate that the orbital ordering transition of doped BaV10O15 presents an electronic example of nucleation-growth behaviour.

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.

Magnetotransport properties were investigated for single crystals of Ba1-xSrxV13O18(x = 0, 0.05, 0.2, 1), which exhibit an anomalous orbital-ordered state with electrical conduction at low temperatures. It was found that a positive magnetoresistance, which is 30% at a magnetic field of 14T, appears only in the low-temperature region (T < 10 K) with an anisotropy negligible in the direction of the magnetic field but a small anisotropy in the direction of the electric current. The behavior of the magnetoresistance hardly changes with x, except for a systematic decrease in its absolute value with increasing x. The fitting of the magnetoresistance by a two-carrier model reveals the existence of carriers with a relatively high mobility (similar to 10(3) cm(2).V--(1).S-1). It was also found by Hall measurement that the majority carriers in the orbital-ordered state are electrons and the number of carriers is much larger than that estimated from the magnetoresistance. On the basis of these experimental results, it was speculated that the ground state of this series of compounds is a semimetallic state with a small number of intrinsic electrons and holes with a high mobility, together with an additional larger number of electrons with a lower mobility.

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].

We have investigated the orbital states of the orbital-glassy (short-range orbital ordered) spinel vanadate Co1.21V1.79O4 using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and subsequent configuration-interaction cluster-model calculation. From the sign of the XMCD spectra, it was found that the spin magnetic moment of the Co ion is aligned parallel to the applied magnetic field and that of the V ion antiparallel to it, consistent with neutron scattering studies. It was revealed that the excess Co ions at the octahedral site take the trivalent low-spin state and induce a random potential to the V sublattice. The orbital magnetic moment of the V ion is small, suggesting that the ordered orbitals mainly consist of real-number orbitals.

By using a technique of thermoreflectance that can precisely measure the thermal conductivity of thin films, we found that the thermal conductivity of SrVO3-SrTiO3 multilayer thin films normal to the surface was substantially reduced by decreasing the thickness of each layer. This indicates that a large intrinsic thermal resistance exists at the interface between SrVO3 and SrTiO3 in spite of the similar phononic properties for these two compounds.

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 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 studied the physical properties of Ba1-xSrxV13O18 single crystals, which exhibit three different phases caused by the orbital and charge degrees of freedom in V t2g states. We found from the magnetic susceptibility and the NMR measurement that the low-temperature phase of this series of compounds is characterized by the spin-singlet state. We also found from the electrical resistivity and optical conductivity spectra that these compounds become heavily anisotropic in the low-temperature phase with V trimerization, indicating that the orbital ordering of V t2g states causes the anisotropy of the electronic structure of this series of compounds.

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.

We perform ultrasound velocity measurements on a single crystal of nearly metallic spinel Co1.21V1.79O4 which exhibits a ferrimagnetic phase transition at T-C similar to 165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above T-C but also the ferrimagnetic phase below TC, which should be driven by the nearly itinerant character of the orbitally degenerate V 3d electrons. In the paramagnetic phase above TC, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above T-C, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly itinerant character of the V3d electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally degenerate 3d electrons. In the ferrimagnetic phase below T-C, the elastic moduli exhibit distinct anomalies at T-1 similar to 95K and T-2 similar to 50 K, with a sign change of the magnetoresistance at T-1 (positive below T-1) and an enhancement of the positive magnetoresistance below T-2, respectively. These observations below TC suggest the successive occurrence of an orbital glassy order at T-1 and a structural phase transition at T-2, where the rather localized character of the V 3d electrons evolves below T-1 and is further enhanced below T-2.

Ba0.9Sr0.1V13O18 exhibits two ordered phases caused by the charge/orbital ordering of the d electrons on the V ions. We have succeeded in observing photoinduced phase transitions over three phases in this compound: a transition from the low-temperature phase to the high-temperature phase within 0.3 ps, followed by a transition to the intermediate-temperature phase within 2 ps. The relationship between the symmetry at each of the three phases plays a key role in the dynamics of the photoinduced phase transitions in such a compound.

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La5Mo4O16 were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the ab plane with antiferromagnetic stacking along the c-axis where the magnetic propagation vector is k = (0, 0, 1/2). The ordered magnetic moments are estimated to be 0.54(2)mu(B) for Mo5+ (4d(1)) ions and 1.07(3)mu(B) for Mo4+ (4d(2)) ions at 4K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the c-axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La5Mo4O16.

We found anomalous relaxation behaviors in a two-dimensional magnet La5Mo4O16, in a pure crystalline form in both the resistivity and magnetization, whose characteristics are similar to those of disordered materials. We also found that such anomalous relaxation behaviors disappear in the impurity-doped sample. We propose a possible scenario for the relaxation behavior that the charge rearrangement of Mo4+ and Mo5+ causes a change in the interlayer magnetic interaction in this compound.

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.

The simultaneous occurrence of the structural and magnetic phase transitions observed in MnV2O4 is one clear example of strong interplay among the spin, orbital and lattice degrees of freedom. The structure of MnV2O4 is switched by the magnetic field and the linear magnetostriction is very high. The orbital order mediates the interaction between the spin and the lattice generating these phenomena. In this work, we present experimental evidence of an orbital order in MnV2O4 and its reorientation under a rotating magnetic field as obtained by nuclear magnetic resonance(NMR). The shift in the resonance frequency of the V NMR spectrum is symmetrical with respect to 45 degrees as an external magnetic field of 7 T rotates from the c-axis to the b-axis, indicating that the initial easy axis flips to the orthogonal direction most parallel to the field direction. The spectrum of V3+ ions splits into four peaks with a maximum shift of 40 MHz. Our analysis revealed that this is the combined effect of the anisotropic hyperfine field due to an ordered orbital and the dipolar hyperfine field. Reorientation of the orbital order in response to an external magnetic field accompanies the macroscopically observed magnetostriction in MnV2O4 .

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.

The chiral crystal of enantiomeric (S)-N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in the enol form [enol-(S)-1] undergoes a reversible single-crystal-to-single-crystal (SCSC) phase transition at T-c approximate to 3 degrees C from the room temperature beta-form in orthorhombic space group P2(1)2(1)2(1) (Z' = 1) to the low temperature -form in the monoclinic space group P2(1) (Z' = 2) with a thermal hysteresis of approximately 1.7 degrees C. A detailed comparison of the crystal structures of the alpha- and beta-forms revealed that the 5-tert-butyl group of one molecule in the asymmetric unit of the beta-form rotated by ca. 60 degrees, and the dihedral angle between the phenyl and salicyl planes increased slightly in the beta-form crystal. However, the changes in the molecular conformation and packing arrangement are small, which leads to the reversible SCSC phase transition with no destruction of the crystal lattice. The dielectric constant along the b-axis was small, probably due to the weak intermolecular interactions in the crystals.

To clarify the dynamics of polarons in doped Mott insulators, we performed pump-probe reflectivity measurements for two barium titanates, Ba2Ti13O22 and BaxTi8O16, with a noninteger average number of d electrons per Ti. We found that the photoinduced dynamics in Ba2Ti13O22 with a three-dimensional electronic structure is characterized by a broadening of the polaron-excitation peak immediately after photoirradiation, whereas that in BaxTi8O16 with a one-dimensional electronic structure is characterized by a change in the intensities of the polaron-excitation peak and charge-gap-excitation peak with time. This indicates that a difference in dimensionality results in different photoinduced dynamics of polarons in doped Mott insulators.

Magnetoelectric coupling phenomena in EuTiO3 are of considerable fundamental interest and are also understood to be key to reported multiferroic behavior in strained films, which exhibit distinctly different properties to the bulk. Here, the magnetoelastic coupling of EuTiO3 is investigated by resonant ultrasound spectroscopy with in situ applied magnetic field and stress as a function of temperature ranging from temperatures above the structural transition temperature T-S to below the antiferromagnetic ordering temperature T-N. One single crystal and two polycrystalline samples are investigated and compared to each other. Both paramagnetic and diamagnetic transducer carriers are used, allowing an examination of the effect of both stress and magnetic field on the behavior of the sample. The properties are reported in constant field/variable temperature and in constant temperature/variable field mode where substantial differences between both data sets are observed. In addition, elastic and magnetic poling at high fields and stresses at low temperature has been performed in order to trace the history dependence of the elastic constants. Four different temperature regions are identified, characterized by unusual elastic responses. The low-temperature phase diagram has been explored and found to exhibit rich complexity. The data evidence a considerable relaxation of elastic constants at high temperatures, but with little effect from magnetic field alone above 20 K, in addition to the known low-temperature coupling.

We studied the thermal transport properties of transition-metal oxides, which have partially filled t(2g) orbitals of transition metals and exhibit intriguing phase transitions caused by orbital/charge ordering. We found that the temperature (T) dependence of the phonon thermal conductivity kappa(p) exhibits a universal behavior for all compounds; kappa(p)(T) decreases with decreasing T above the transition temperature T-c, whereas it is enhanced below T-c. This result implies that orbital/charge ordering in transition-metal oxides can be characterized by a unique T dependence of the phonon thermal conductivity.

Strain, magnetization, dielectric relaxation, and unpolarized and polarized neutron diffraction measurements were performed to study the magnetic and structural properties of spinel Co1-xV2+xO4. The strain measurement indicates that, upon cooling, Delta L/L in the order of similar to 10(-4) starts increasing below T-C, becomes maximum at T-max, and then decreases and changes its sign at T*. Neutron measurements indicate that a collinear ferrimagnetic order develops below T-C and upon further cooling noncollinear ferrimagnetic ordering occurs below T-max. At low temperatures, the dielectric constant exhibits a frequency dependence, indicating slow dynamics. These results indicate the existence of an orbital glassy state at low temperatures in this nearly metallic frustrated magnet.

We studied the physical properties of hollandite titanates, BaxTi8O16+delta, which have double chains of edge-sharing TiO6 octahedra with d electrons in the t(2g) states. We found that there is an electronic phase transition at similar to 220 K, at which various properties exhibit anomalies. This phase transition is characterized by a modulation in the TiO6 chains and a spectral weight transfer of over 2 eV in the optical conductivity spectrum, which are presumably caused by charge and orbital ordering of the Ti t(2g) electrons.

We studied the optical properties of CoV2O4, in which a broad spectrum arising from the Mott excitation of the V d states and sharp peaks from the intra-atomic d-d transitions in the Co ions coexist. We found that the width of the d-d transition peaks decreases and that Fano resonance appears in the steady-state optical spectrum with decreasing temperature. We also found that the width of the d-d transition peaks increases less than 1 ps after a pump pulse is applied to the Mott excitation. These results indicate that the Mott excitation and the d-d transitions are strongly coupled in this compound.

We found that a layered magnet La5Mo4O16 exhibits a long-time relaxation of the magnetization from a ferromagnetic to an antiferromagnetic state. This unconventional relaxation in a pure crystal magnet far below the transition temperature arises from the extremely two-dimensional nature of the magnetic interactions, by which the Mo spins are strongly coupled and form a big spin within a layer.

The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below similar to 2.8K. Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls. open access Copyright (C) EPLA, 2015

Ultrasound velocity measurements were performed on a single crystal of the orbital-degenerate frustrated spinel CoV2O4 in all the symmetrically-independent elastic moduli of the cubic crystal. The measurements of temperature dependence of the elastic moduli observed discontinuous elastic anomalies due to a ferrimagnetic transition at TC = 165 K and another phase transition at T∗ = 50 K. Additionally, the measurements observed anomalous temperature dependence of the elastic moduli, specifically, non-monotonic temperature dependence in the magnetically-ordered phase below TC, and magnetic-field-sensitive elastic softening with decreasing temperature in the paramagnetic phase above TC. These anomalous temperature variations below and above TC should be driven by the coupling of lattice to magnetic excitations.

We studied the magnetoresistance of RTiO3 (R = Pr and Ce) with offstoichiometry as a doped Mott-Hubbard system and found positive magnetoresistance proportional to the absolute value of the magnetic field in the antiferromagnetic phase of both insulating and metallic samples. This result indicates that not only the coherent state that appears with doping and is responsible for the metallic conduction, but also the incoherent state of the Mott-Hubbard insulator is affected by the spin configuration.

We studied the thermal conductivity of various transition-metal oxides with the spinel structure (MnV2O4, FeV2O4, CoV2O4, and Mn3O4) upon varying the temperature and magnetic field. We found that for the spinel oxides having V3+ ions (two electrons in the t(2g) states) at the octahedral site, the orbital ordering suppresses the thermal resistivity (inverse thermal conductivity) in contrast to purely magnetic ordering, indicating that the orbital fluctuation of the V t(2g) states is the main factor affecting the thermal conductivity. On the other hand, for Mn3O4, which has Mn3+ ions (one electron in the e(g) states) at the octahedral site, the thermal resistivity is suppressed in association with the successive magnetic phase transitions with decreasing temperature and increasing magnetic field. This can be explained by the frustration of Mn3+ spins and the fluctuation of Mn3+ e(g) orbitals.

Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C44 and 12(C11-C12) by ∼20-30% occurs with falling temperature in a narrow interval through the transition point, Tc=284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3, which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at ∼140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below ∼80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at ∼5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO3. © 2014 American Physical Society.

Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C-44 and 1/2 (C-11 - C-12) by similar to 20-30% occurs with falling temperature in a narrow interval through the transition point, T-c = 284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3, which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at similar to 140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below similar to 80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at similar to 5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO3.

We studied single crystals of BaV13O18, in which V ions with an average valence of + 2.62 (3d(2.38)) form a quasi-fcc lattice. We found that the ground state of the stoichiometric crystal is insulating, whereas that of the off-stoichiometric crystal is metallic with a large effective mass. This metallic state can be explained by the Kondo coupling between the magnetic moment existing at the lone V ion and itinerant carriers in the V tetramers.

We studied the physical properties of Sr2VO4 with a K2NiF4 structure in polycrystalline forms synthesized under various different conditions. We found that stoichiometric samples or those with nominal hole doping into the d(1) state of the V ion exhibit insulating behavior with an antiferromagnetic interaction between V moments, whereas those with nominal electron doping exhibit metallic conduction and ferromagnetic interaction. A structural phase transition at T-c similar to 100 K, which is likely to be associated with the orbital ordering of the V4+ ion, is observed only in the former sample exhibiting a sharp drop in the magnetic susceptibility at the same temperature, whereas no structural anomaly was observed for the sample exhibiting a cusp in the magnetic susceptibility at similar to 50 K.

By pump-probe optical reflectivity measurements, we studied the photoinduced dynamics of MnV2O4, which is a d(2) Mott insulator and exhibits a simultaneous spin and orbital ordering accompanied by a structural phase transition at 57 K. We compared the present experimental results with the results of the steady-state optical measurement of the same compound and the steady-state and pump-probe optical measurements of Al-doped MnV2O4, in which the orbital ordering and structural phase transition are absent but the magnetic ordering exists. We found that the photoinduced dynamics of MnV2O4 can be separated into three components: (1) the suppression of the Mott-excitation peak, (2) the melting of the orbital ordering, and (3) the melting of the magnetic ordering. Furthermore, we found that the lattice distortion associated with the photoinduced structural phase transition from tetragonal to cubic induces a strain wave whose propagation velocity is discernibly smaller than that of a conventional shockwave.

The ferrimagnetic spinel MnV2O4 undergoes an orbital-induced cubic-to-tetragonal distortion at TYK=58 K, below which noncollinear commensurate ferrimagnetic ordering occurs. Using inelastic neutron scattering, we investigated low-energy dynamics in its cubic phase above TYK. We observed two types of coexisting short-range magnetic excitations: the dispersive spin waves centered around the Brillouin-zone centers, k0 = (0, 0, 0), and quasielastic spin fluctuations centered at incommensurate wave vectors, kincom = (ξ, ξ, 0). The coexistence of the two distinct features can be understood as a dynamic realization of the conical spiral order observed in cubic spinels such as CoCr2O4. © 2013 American Physical Society.

We studied the photoinduced dynamics of spinel MnV1.8Al0.2O4 and pyrochlore Yb2V2O7 as prototypical Mott-Hubbard insulators with magnetic ordering. We found that the Mott excitation spectrum is suppressed much sooner than 1 ps after photoirradiation, and subsequently it shows an energy shift over several or several tens of picoseconds, which is caused by the melting of magnetic ordering. This indicates that the Mott excitation spectrum is dominated not only by the Coulomb repulsion energy but also by magnetic interactions, and their different energy scales lead to the different time scales in the photoinduced dynamics.

We found that Ba2Ti13O22 with Ti3 + (3d1) ions on a triangle-based lattice exhibits a phase transition at Tc∼200 K, below which the increase of electrical resistivity and decrease of magnetic susceptibility were observed. Transmission electron microscopy and optical reflectivity measurements indicate that the low-temperature phase of the present compound shares characteristics in common with a charge-density-wave state with remnant carriers, although a commensurate wave vector of the modulation and a linear temperature dependence of the magnetic susceptibility below Tc suggest an exotic ordered state. © 2013 American Physical Society.

The temperature dependence of the photoinduced dynamics by the irradiation of a femtosecond laser pulse was studied for the cleaved surface of BaV10O15, which shows V trimerization with orbital ordering at T-c = 123 K. For T &gt; T-c, the photoinduced reflectivity change exhibits oscillation with time, indicating that a photoinduced state propagates with time in the crystal. For T &lt; T-c, almost time-independent behavior after the instantaneous change of the reflectivity is observed over several tens of picoseconds, indicating the appearance of a stable photoinduced phase. The different photoinduced dynamics above and below T-c can be attributed to the absence or existence of V trimerization with orbital ordering. We also found that the absolute values of the photoinduced reflectivity change are several times larger on the cleaved surface than those on the polished surface, indicating that remnant stress on the polished surface affects the photoinduced dynamics.

Heat conduction in the system that releases or absorbs latent heat caused by the first-order phase transition gives rise to an intriguing problem. A typical example is seen in the system showing melting and solidification, where the domain boundary between the liquid and solid phase moves with time, known as moving boundary problems. We report in the present paper that a similar moving domain boundary is observed in an all-solid-state system, spinel MnV2O4, where there is a first-order magnetic phase transition and the transition temperature changes with applied magnetic field. We found that in the sample attached to a heat bath with a constant temperature, thermal current flows (without external heat source) when the magnetic field is increased or decreased, associated with the release or absorption of the latent heat. We also found that the magnitude of the spontaneous heat current depends on the sweep rate of the magnetic field. We show that the experimental results, which have a nonlinear nature, can be reproduced by a simple model and simulation.

Tetragonal ferrimagnetic spinel Mn3O4 shows complex magnetic phase transitions due to competing antiferromagnetic interactions involving Mn2+/3+ ions with difference valences. Using magnetization and neutron diffraction measurements, we propose that its tetragonal lattice possesses an instability towards orthorhombic distortion in zero field well below T-N. The direction of the observed instability suggests that the anomaly is related to lifting of the persistent geometrical frustration via spin-lattice coupling.

We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO3. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment
we hence deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials. © 2013 American Physical Society.

We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO3. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment; we hence deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials. DOI: 10.1103/PhysRevB.87.064103

Spinel MnV2O4 exhibits orbital ordering at T-c = 57 K accompanied by a structural phase transition from cubic to tetragonal and ferrimagnetic ordering. We measured the temperature dependence of the optical reflectivity spectra on the cleaved surface of a MnV2O4 single crystal with controlled twin structures belowT(c). We found that there are two peaks at 1.6 and 2.3 eV, corresponding to the Mott excitations, in the optical conductivity spectra obtained by the Kramers-Kronig transformation of the reflectivity, and that the two peaks exhibit a similar anisotropic change below T-c. Furthermore, we found that these two peaks exhibit a different temperature dependence above T-c. We analyzed these results by a model in which the multiplet structures of the d state for the Mott excitation are fully taken into account, and discussed the occupation of the triply degenerate t(2g) orbitals both below and above T-c. DOI: 10.1103/PhysRevB.87.054424

In Mn3O4, the magnetization along the c axis is different from that along the ab plane even in the strong field of 30 T. To investigate the origin of the huge magnetic anisotropy, Mn2+ and Mn3+ nuclear magnetic resonance spectra were measured in the 7-T magnetic field. The canting angle of the magnetic moments was estimated for various directions of field by rotating a single-crystalline Mn3O4 sample. One of the main results is that Mn3+ moments lie nearly in the ab plane in the external field perpendicular to the plane, meaning that the macroscopic magnetic anisotropy of Mn3O4 originates from the magnetic anisotropy of Mn3+ in the ab plane. The anisotropy field is estimated to be about 65 T. It is obvious that the Yafet-Kittel structure made of Mn2+ and Mn3+ spins lies in the ab plane due to this huge magnetic anisotropy, contrary to the previous reports. By the least-squares fit of the canting angle data for various field directions to a simple model, we obtained that J(BB) = 1.88J(AB) - 0.09 meV and K-A = -14.7J(AB) + 2.0 meV, where J(AB), J(BB), and K-A are the exchange interaction constants between Mn2+ moments, Mn2+ and Mn3+ moments, and an anisotropy constant of Mn2+, respectively. DOI: 10.1103/PhysRevB.86.224420

We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L 2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTiO 3. Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a,b-plane diagonal. Recent reports of a novel transition at 2.75 K with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Néel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible. © 2012 American Physical Society.

We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L-2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTiO3. Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a, b-plane diagonal. Recent reports of a novel transition at 2.75 K with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Neel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible.

We measured the thermal conductivity of spinel MnV2O4 and Al-doped samples with an applied magnetic field, and they exhibited orbital and spin ordering at low temperatures. We found that the thermal resistivity (reciprocal thermal conductivity) in the orbital-disordered phase is enhanced when the system approaches the critical point of the orbital ordering under the variation of both temperature and magnetic field, but it is suppressed when it crosses the critical point and goes into the orbital-ordered phase. This result indicates that the thermal conductivity of these compounds is dominated by the fluctuation of V (t2g) orbitals.

BaV10O15 shows a structural phase transition associated with the trimerization of V ions at T-s = 123 K. We investigated the orbital states of V trimers using the technique of resonant x-ray scattering (RXS) near the V K-edge absorption energy. The ratio of the intensity between the pre-edge structure (1s to 3d transition) and the main-edge structure (1s to 4p transition) strongly depends on the reciprocal Q position of the scattering. We found that the Q position dependence is consistent with the model of orbital ordering proposed by Pen et al., [Phys. Rev. Lett. 78, 1323 (1997)].

We used infrared ellipsometry to study phonons in a spinel MnV2O4 single crystal. At 300 K, the imaginary part of the complex dielectric function spectra show four phonon peaks. Below the cubic-tetragonal structural transition temperature at 58 K, small peaks appear around the cubic modes and the number of phonons becomes more than 12. The group analysis predicted 12 infrared modes for the I4(1/a) symmetry. The observed modes support the I4(1/a) or lower crystal symmetry, which reflects the antiferro ordering of V3+ t(2g) orbitals.

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO 3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO 6 octahedra tilting, in analogy to the case of SrTiO 3. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T A ∼ 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T c, the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO 3 properties. © 2012 American Physical Society.

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I 4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T-c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T-A similar to 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T-c, the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO3 properties.

We investigated Raman scattering of BaV10O15, which exhibits a structural phase transition with V trimerization at T-c = 123 K possibly dominated by the orbital ordering of V t(2g) states. It was found that phonon Raman spectra drastically change at T-c. According to the analysis of the phonon spectra based on the bond-polarizability model, the force constant of the V-V bond increases by 75% and the peak width is reduced by half below T-c. This result can be explained by the existence of disorder originating from orbital degrees of freedom above T-c, which is suppressed by the long-range orbital ordering below T-c.

We have fabricated an oxide thermal rectifier made of La1.98Nd0.02CuO4 and MnV2O4. By utilizing a jump of a thermal conductivity originated in the structural phase transition accompanied by the orbital ordering in MnV2O4, a rectifying coefficient of 1.14 has been achieved in the presence of a small temperature difference of 2 K. A thermal rectifier operating under a small temperature difference will play an important role for realizing heat-current control in electronic devices. (C) 2012 The Japan Society of Applied Physics

We measured the optical conductivity spectra of SrV10O15 with V2.8+, which exhibits an insulating behavior without any phase transition, and compared the results with those of BaV10O15, which shows orbital ordering below T-c = 123 K. We found that a soft gap opens at the lowest temperature but it collapses with increasing temperature in SrV10O15, in contrast to a finite charge gap (similar to 0.3 eV) in BaV10O15 below T-c. We also found that the difference of the conductivity spectra between SrV10O15 and BaV10O15 evolves even above T-c of Ba, indicating that the fluctuation of orbital ordering produces a pseudogap in BaV10O15.

We report V-51 NMR spectra in the triangular-lattice vanadates, AV(10)O(15) (3d(2.2)), AV(13)O(18) (3d(2.38)), and A(2)V(13)O(22) (3d(1.92)) (A = Ba, Sr). These compounds have orbital degrees of freedom and itinerant electrons arising from the partially occupied t(2g) bands. We show microscopic observations of spin-singlet trimers and antiferromagnetic tertramers with orbital orders in BaV10O15, but no indication of the orbital order and the spin singlet in SrV10O15. The paramagnetic and nonmagnetic spins coexist in BaV13O22 but only the nonmagnetic spins are present in AV(13)O(18) (A = Ba, Sr). The spin singlet formation in the incommensurate band filling is understood as the incommensurate charge-density wave induced by the 3d orbital order.

We studied the dynamics of photoinduced phenomena in AV 10O 15 (A=Ba, Sr), where the ground state changes from an insulating state with V trimerization (Ba) to another insulating state with no structural anomaly (Sr). We found that the time dependence of photoinduced reflectivity change is similar up to several picoseconds, but is critically different in the range of several tens of picoseconds between the two compounds. These different behaviors can be attributed to the coupled mode of the photoexcited electronic state and the lattice distortion, which propagates in the crystal of SrV 10O 15 but is trapped in the local minimum of the free energy in BaV 10O 15. © 2011 American Physical Society.

We studied the dynamics of photoinduced phenomena in AV(10)O(15) (A = Ba, Sr), where the ground state changes from an insulating state with V trimerization (Ba) to another insulating state with no structural anomaly (Sr). We found that the time dependence of photoinduced reflectivity change is similar up to several picoseconds, but is critically different in the range of several tens of picoseconds between the two compounds. These different behaviors can be attributed to the coupled mode of the photoexcited electronic state and the lattice distortion, which propagates in the crystal of SrV10O15 but is trapped in the local minimum of the free energy in BaV10O15.

We investigated Raman scattering of spinel MnV2O4, for which orbital ordering of Vt(2g) states below T-c = 57K was proposed. Several peaks evolve in the Raman spectra below T-c, and they show a peculiar polarization dependence, which cannot be explained only by a group theory analysis. We found that it can be explained by considering Mott excitations in the presence of orbital ordering as the dominant intermediate states in the Raman process. This result gives the evidence of orbital ordering in this compound from the viewpoint of electronic structure.

Local spin and orbital textures are investigated by V-51 NMR measurements on a triangular-lattice compound BaV10O15 with orbital degrees of freedom and itinerant electrons. The Knight shift shows the spin-singlet V trimer formation in a itinerant phase above the semiconductor-insulator transition temperature T-SI = 140 K. Below T-SI, the observation of the asymmetric electric hyperfine coupling tensor agrees with the orbital order giving direct d-d bonds of the trimer. The remaining paramagnetic spins on a V tetramer exhibit an antiferromagnetic long-range order accompanied by a ferro-type order. The coexistence of the trimer and tetramer with the two opposite magnetism is ascribed to the orbitally induced Peierls instability on a non-half-filling triangular lattice with nonuniform V-V separation.

We have investigated spin-doping effects on the valence-band electronic structure of Sr1-(x+y)Lax+yTi1-xCrxO3 at y = 0.2 using high-resolution photoemission spectroscopy and first-principles band structure calculations. The Cr 3d photoemission spectral weight located between 2.0 and 3.5 eV increased with x, indicating that x works as a rough measure of the spin density. However, the increase in the spectral weight was actually more rapid than the band theory prediction. The near-Fermi level (E-F) weight due to the Ti 3d states showed significant reduction with x. Considering that the band theory predicted nearly constant density of states near E-F, this reduction of the near-E-F weight should be interpreted as a manifestation of strong Ti-Cr site disorder effects. (C) 2011 Elsevier B.V. All rights reserved.

We studied the transport, magnetic, and structural properties of Ba1-xSrxV13O18, in which V and O ions form a sodium-chloride lattice with periodically missing sites. We found that the high-temperature insulating phase is characterized by spin-singlet V tetramers, whereas the low-temperature conducting phase is characterized by V trimers, and these two phases are competing in this series of compounds. Various properties substantially change at the phase transition between the two, indicating a drastic reconstruction of the electronic structure.

We found that La5Mo4O16 with Mo4+ and Mo5+ ions (S = 1 and S = 1/2 spins) on a quasisquare lattice exhibits a distinct magnetoresistance for the current perpendicular to the square-lattice layers below the antiferromagnetic ordering temperature T-AF = 190 K. This magnetoresistance occurs well below 1 T, and can be attributed to a metamagnetic transition from antiferromagnetically aligned moments between the layers to ferromagnetically aligned ones. The magnetoresistance changes its characteristic with the change of the magnetic state below T-F = 70 K, where spontaneous magnetization appears.

We have investigated characteristic ferroelectric and structural antiphase domain structures in single crystals of hexagonal RMnO3 (R=Y, Ho, Lu, and Yb) by obtaining various electron diffraction patterns, dark-filed images and high-resolution lattice images. In the ferroelectric phase RMnO3 characteristic domain structures consisting of six ferroelectric and structural antiphase domains, which can be identified as the "cloverleaf" pattern, is found in the (110) plane, in addition to the (001) plane, and are inherent to the ferroelectric phase of hexagonal RMnO3. In domain configuration with the cloverleaf pattern in the (110) plane, the structural antiphase boundaries are inclined to be parallel to the [001] direction. © 2012 Materials Research Society.

BaV10O15 having a bilayer triangular lattice exhibits a semiconductor-insulator transition accompanied by the structural phase transition at Tc ∼130 K. We show the microscopic properties of the phase transition investigated by 51V NMR measurements on a single crystal. We have observed 51V NMR spectra from two of the three inequivalent V sites at high temperatures. The Knight shift tensors are determined at 300 K for the two V sites. The local spin susceptibility strongly depends on the V sites and temperature despite the nearly independent magnetic susceptibility. Each spectrum is split into two below 200 K far above T c, suggesting a charge/order ordering transition. The local spin susceptibilities of the three sites gradually vanish with decreasing temperature, consistent with the V trimer formation. The microscopic evidence for the spin singlet state is clearly observed below Tc. © 2011 Published under licence by IOP Publishing Ltd.

We investigated the magnetic and the thermodynamic properties of a spinel MnV2O4 single crystal. The magnetization and susceptibility show the signature of ferromagnetic ordering at similar to 61 K. A sharp peak in the magnetization and the specific heat indicate the structural phase transition at 58 K, consistent with the scattering experiments. The specific-heat data exhibits the signature of double transitions. In addition to the higher-temperature transition at T-S, the second transition occurs at a lower temperature T*, which divides the phase below T-S into a low-magnetic-field and a high-magnetic-field phase. Based on the magnetic susceptibility and the specific heat, a wide-range temperature and magnetic-field phase diagram was constructed. The low-magnetic-field phase shows the characteristics of a spin-glass-like state. A comparison with the magnetic and thermodynamic characteristics of similar materials suggests that the low temperature and low-magnetic-field, spin-glass-like phase in MnV2O4 may originate from the competition between the interchain and the intrachain exchange interaction owing to the fluctuating V-ion orbitals.

We studied the multiferroic properties of the incommensurate spiral phase of Mn1-xCoxWO4 using single-crystal neutron-diffraction and ferroelectric measurements. Whereas the rotational plane of the spin spiral is parallel to the b axis for x &lt;= 0.05, it flops by 90 degrees for x=0.10. As a result, the direction of the ferroelectric polarization also flops from the b to the a axis. These results show that the orientation of the ferroelectric polarization in MnWO4 can be controlled via chemical doping.

We studied AV(13)O(18) (A = Sr, Ba), in which V ions form a face-centered-cubic lattice with periodically missing ions. It was found that there is a phase transition in both SrV13O18 (T-c = 270 K) and BaV13O18 (T-c = 200 K), where anomalies are observed in resistivity and magnetic susceptibility. Resistivity shows a contrasting behavior in the low-temperature phase between SrV13O18 (a metallic behavior) and BaV13O18 (an insulating behavior). By synchrotron x-ray powder-diffraction measurement, it was found that V trimerization occurs at the phase transition of SrV13O18 . On the other hand, BaV13O18 does not show a large change in the x-ray powder diffraction but superlattice peaks at (01/21/2) was observed in the electron diffraction. Possible orbital states of the t(2g) electrons in the V ions are discussed.

We found that a crossover behavior of the crystal structure at high temperatures dominates the magnetic state at low temperatures in the series of RTiO3 (R is rare earth). At the crossover temperature, which exist only in the samples that become antiferromagnetic at lower temperatures, the temperature derivative of lattice constants changes its sign, and the crossover temperature approaches zero as the Neel temperature approaches zero in the phase diagram of the ionic radius of R vs temperature. This result indicates that the orbital state of the Ti ion dominates the low-temperature magnetic state in RTiO3.

We found that in A(2)V(13)O(22) (A = Ba, Sr), which contains a trilayer slab of VO in the sodium-chloride structure with periodically missing ions, the trimerization of V ions occurs at 290 K (A = Ba) and 380 K (A = Sr). V trimers form a three-dimensional network, but some V ions remain untrimerized in these compounds. The suppression of magnetic susceptibility with trimerization and the existence of a Curie tail at low temperatures, together with the result of NMR measurement, indicate that the V trimers are spin singlet, whereas the untrimerized V ions have a magnetic moment; i.e., there is a spontaneous separation between nonmagnetic and magnetic ions in the crystal.

The third-order E2 H2 -type magnetoelectric (ME) response of polycrystalline EuTiO3 changes sign under magnetic bias and shows a large anomaly at the antiferromagnetic (AF)-paramagnetic phase boundary below TN ≈5.3 K. It is attributed to critical fluctuations of the AF order parameter reinforced by quantum paraelectric polar correlations. The underlying biquadratic spin-lattice coupling involves electric field induced Dzyaloshinskii-Moriya interaction as described within mean-field approximation. Single domaining by ME annealing (or cooling) significantly enhances the response by additional EH and E H2 effects. © 2010 The American Physical Society.

The third-order (EH2)-H-2-type magnetoelectric (ME) response of polycrystalline EuTiO3 changes sign under magnetic bias and shows a large anomaly at the antiferromagnetic (AF)-paramagnetic phase boundary below T-N approximate to 5.3 K. It is attributed to critical fluctuations of the AF order parameter reinforced by quantum paraelectric polar correlations. The underlying biquadratic spin-lattice coupling involves electric field induced Dzyaloshinskii-Moriya interaction as described within mean-field approximation. Single domaining by ME annealing (or cooling) significantly enhances the response by additional EH and EH2 effects.

A structural phase transition at T-c similar to 130 K in BaV10O15 containing modified V3+ (3d(2)) triangular lattices was studied. It was found that the electrical resistivity jumps by three orders of magnitude and a gap opens up in the optical conductivity spectrum at T-c. It was also found from synchrotron x-ray powder-diffraction measurement that the trimerization of the V ions occurs below T-c. These results indicate that the orbital ordering of V t(2g) states occurs at T-c and that induces a charge gap near the Fermi level in the electronic states of BaV10O15.

We developed an experimental system for measuring temperature-modulated optical reflectivity with a frequency up to 20 kHz to Study the dynamics of inhomogeneity in a crystal. By this system, we studied doped perovskite manganites, in which a ferromagnetic metallic phase and a (short-ranged) charge-ordered phase coexist near the Curie temperature (T-C). We found an appreciable frequency dependence of the temperature-modulated reflectivity around T-C. This result can be explained by the dynamics of the domains in the two coexisting phases, i.e., by the relaxation time to reach a new equilibrium state of the two-phase coexistence when the temperature is modulated.

The spinel crystal system, AB(2)O(4), has been fertile ground in studying the effects of magnetic and orbital frustration. The experimental findings, with primary focus on neutron and synchrotron x-ray scattering techniques, are hereby reviewed in spinels with magnetic B ions. Highlighted are novel collective phenomena, such as the zero-energy excitation mode in the spin liquid phase, zero-field and field-induced novel phase transitions, the emergence of complex local spin entities, and heavy fermionic behaviors. Such a diversity in the exotic properties stems from their close proximity to critical points among degenerate states and a delicate balance among different degrees of freedom such as spin, orbital, and lattice.

Thin-film transistors (TFTs) with amorphous InGaZnO4 as a channel layer are known to exhibit high field-effect mobility. We studied the characteristic of the accumulated carriers in this TFT by optical spectroscopy and observed Drude absorption and oscillation in the transmittance spectrum of the channel region appearing with an applied gate voltage. The scattering rate of the carriers in the accumulation layers estimated from the optical spectrum is (h) over bar/tau(AL) = 0.19eV. and this small value is the origin of the high mobility of the TFT with amorphous InGaZnO4.

We have investigated electron-doping effects on the electronic structure of Sr0.8-yLa0.2+yTi0.8Cr0.2O3 using high-resolution photoemission spectroscopy and first-principles band structure calculations. Photoemission results showed that the Cr 3d peak intensity is independent of the carrier concentration (y), while the spectral weight near the Fermi level, composed of the Ti 3d states, is proportional to y at a fixed spin concentration. Detailed comparisons between the photoemission spectra and the band structure calculations including magnetic-moment calculations reveal that the large energy separation between the Cr 3d states and the Ti 3d states plays an important role to control the electric and magnetic properties in this system.

We studied kagome-like magnets, SrVxGa12-xO19 with V3+ (3d(2) in the triply degenerate t(2g) states), as a geometrically frustrated spin system with orbital degrees of freedom. We observed a suppression of low-temperature spin entropy and an anomalous contraction of the V triangles on the kagome lattice, which can be regarded as the V trimerization with spin-singlet formation. This result, which is in clear contrast to their Cr3+ (3d(3)) analogs exhibiting a spin-liquid behavior, indicates a substantial impact of orbitals on geometrically frustrated magnets.

By the microspectroscopic imaging of optical reflectivity for the La1-xSrxFeO3-delta thin film with indium-tin-oxide electrodes, we observed that the shape of the conducting area between two electrodes is reversibly varied with applied electric voltage pulses and that causes a repeatable change in the electrical resistance between the electrodes. These results can be explained by the oxygen-ion migration with applied electric field, which induces a variation in the distribution of the Fe4+ ions in the thin film.

The magnetodielectric and magnetoelastic properties of spinel Mn(3)O(4) with the orbital degree of freedom in the e(g) state of the Mn(3+) site were investigated. We found that strain (Delta L/L) shows anomalies at the magnetic phase transition temperatures and is anisotropic against the crystallographic axis. We also found that this compound exhibits large magnetic field (H) dependences of Delta L/L, details of which are different at three different magnetic phases. Similar anomalies and H dependence are observed in dielectric measurements. These behaviors can be explained by the inverse effect of single-ion spin anisotropy.

The spinel oxide AlV2O4 exhibits a structural phase transition from cubic to rhombohedral lattice at T-c = 700 K, accompanied by V valence skipping. We have investigated the microscopic magnetic properties by the V-51 and Al-27 NAIR measurements across T-c. in a temperature range from 2 K to 850 K. The Knight shifts at the 51V and 27Al sites selectively monitor the local spin susceptibilities with different temperature dependence. We reveal that there are two V sites having a nonmagnetic ground state and a frustrated paramagnetic spin. The results are consistent with the spin-singlet heptamer formation consisting of seven vanadium atoms with 18 electrons. This transition uniquely releases the spin and charge frustration on the original V pyrochlore lattice with an aid of the orbital order and the structural distortion. The spin frustration is also highlighted in the anomalous magnetic correlations detected by the nuclear spin-lattice relaxation rate at the V and Al sites.

We report V-51 and Al-27 NMR studies of the spin-frustrated spinel oxide AlV2O4 in a temperature range from 850 to 2 K across a structural phase transition at 700 K. Knight shifts and nuclear spin-lattice relaxation rates at the V and Al sites show that the local spin susceptibility at two V sites diminishes at low temperatures in the insulating state, giving direct evidence for the spin-singlet formation. In addition to the singlet spins, one of the two Al sites probes paramagnetic spins with the Curie-Weiss behavior and the spin-glass transition. The results demonstrate coexistence of the spin-singlet cluster and the nearly free spins, anticipated from a theoretical model and the bulk magnetic-susceptibility measurements.

We studied the dielectric properties and strain of spinel Mn3O4 with applied magnetic field. It was found that the dielectric constant (epsilon(1)) exhibits a large magnetic-field (H) dependence, which is anisotropic both against the direction of the magnetic field and the electric field in the magnetically ordered phases below T-N=43 K. It was also found that the epsilon(1)(H) exhibits distinctively different behaviors at three different magnetic phases. These results can be explained by the change of the orbital state of Mn3+ with the applied magnetic field due to the inverse process of single-ion spin anisotropy.

FeV2O4 with orbital degrees of freedom both at the Fe2+ and V3+ ions exhibits successive structural phase transitions. We studied this structural and magnetic properties as well as the correlation between the two. We found that the crystalline domains in the tetragonal or orthorhombic phases can be aligned by the applied magnetic field, inducing a large magnetostriction (similar to 1%). The competition and cooperation of Fe2+ and V3+ orbitals is the possible origin of these intriguing properties in FeV2O4.

We studied perovskite SrTiO(3) into which magnetic impurities (Cr or V) and itinerant carriers (electrons in the Ti 3d band) are introduced. We found that the itinerancy of the electrons on the magnetic impurities and magnetic properties of the compounds depend on the species of the transition metal as impurities. Anomalous Hall resistivity was observed in Cr-doped SrTiO(3), and it was found that its coefficient is inversely proportional to the number of itinerant carriers.

Using neutron scattering techniques, we have investigated spin wave excitations in noncollinear ferrimagnetic spinels Mn B2 O4 (B=Mn,V) with eg and t2g orbital degeneracies, respectively, that lead to tetragonal distortions along opposite directions. Linear spin wave analysis of the excitations yields spatially inhomogeneous nearest neighbor interactions in both tetragonal spinels. We find the ratio Jc Jab ≈-0.06 (4) for Mn3 O4 (c>a=b) and ≈0.3 (1) for Mn V2 O4 (c<a=b). Resulting exchange couplings of Mn3 O4 can be qualitatively explained in terms of possible overlaps of t2g and eg electrons of Mn2+ and Mn3+ ions. On the other hand, those of Mn V2 O4, in particular, the strong Jc, seem to contradict with the antiferro-orbital state of V3+ (t 2g 2) ions that was proposed by a recent synchrotron x-ray study. Theoretical implications to the orbital physics are also discussed. © 2008 The American Physical Society.

The authors found that capacitance of polycrystalline spinel FeV2O4 exhibits a magnetic-field dependence with hysteresis and takes two values at zero field depending on the direction of a small magnetic field (similar to 1000 G) applied prior to measurement. This behavior can be potentially used as a nonvolatile memory device in which the data are stored as a difference of capacitance through the change of magnetic-field directions. (C) 2007 American Institute of Physics.

We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (T-s,T-N=57 K). The structural phase transition was induced also by magnetic field above T-s. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below T-s. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t(2g) orbitals.

Perovskite SrTiO3 is a wide-gap band insulator. Because of the stability of its crystal structure, various nonmagnetic and magnetic atoms can be doped into the A-site and the B-site, respectively. A-site substitution, namely La3+ for Sr2+, introduces free electrons in the conduction band, while B-site substitution, namely Cr3+ for Ti4+, introduces not only localized spins but also free holes in the valence band. Therefore, we can independently introduce x localized spins and y free electrons (or holes) in SrTiO3 by co-doping La and Cr, by the composition Sri(-x-y)La(x+y)Ti(1-x)Cr(x)O(3) In this paper, we present an ab initio calculation of the electronic structure of Sr1-x,La--y(x),Ti1-x,CrxO3 by the KKR-CPA-LDA method, changing the doping concentrations x and y systematically within 0 &lt; x &lt; 0.2 and -0.2 &lt; y &lt; 0.2. We found that the Cr t(2g)-states are well localized, and due to the large exchange splitting, only the majority spin states are occupied. However, the energy of the occupied Cr t(2g)-states are considerably shallow compared to the photoemission experiment, suggesting the importance of the electron correlation for these localized states. The total magnetic moment is almost proportional to the Cr concentration as expected; however, it considerably changes with La concentration. (c) 2006 Elsevier B.V. All rights reserved.

Using photoemission (PES) spectroscopy and a first-principles band-structure calculation, we have investigated the electronic structure of perovskite-type Ti-Cr oxides Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O(3), in which the charge and spin density can be controlled. It was found that the near-Fermi level (E(F)) electronic structure consisted of a Cr 3d localized state and a Ti 3d itinerant state that were well separated in energy. The spectral weight of the Cr 3d state was proportional to x, demonstrating a controllable spin density. In contrast, the near-E(F) spectral weight exhibited different behavior. The band theory has reproduced the essential features of the Cr and Ti 3d states as well as the O 2p ones, but failed to predict the location of the Cr 3d and O 2p states. Implications of this discrepancy have been discussed in connection with calculated magnetic moments. (c) 2006 Elsevier B.V. All rights reserved.

We measured the temperature dependence of dielectric constant (epsilon(1)) and lattice striction (Delta L/L) for MnV2O4, Mn3O4, and MnCr2O4. For MnV2O4 and Mn3O4, which have the orbital degree of freedom in the t(2g) or e(g) orbitals of the transition metal on the octahedral site, a critical decrease of epsilon(1) and Delta L/L at the ferrimagnetic transition temperature (T-N) was observed. On the other hand, MnCr2O4 which has no orbital degree of freedom, anomalies in epsilon(1) and Delta L/L at T-N are barely observed. These results indicate that the orbital degree of freedom is a key factor for the correlation between magnetic, dielectric properties and crystal structure in the series of MnT2O4. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated microstructures related to anomalous ferroelectric propel-ties such as the magnetocapacitance effect in magnetic ferroelectric compounds YMn1-xTixO3 and (Y,A)Mn0.825Ti0.175O3 (A = La, Ca, Zr) mainly by the transmission electron microscopy. It is found that the size of the ferroelectric domains in YMn1-xTixO3 depends strongly on the Ti concentration (x) and the size is estimated to be about 10-20 nm in the x = 0.175 compound. In addition, structural change from the ferroelectric P6(3) cm phase to the rhombohedral one was found in the x = 0.40 compound. On the other hand, characteristic diffuse scattering elongating along both the [001] and [110] direction was found in (Y,Zr)Mn0.825Ti0.175O3, which is similar to that found in the x = 0.175 compound. It is revealed that the partial substitution in YMnO3 breaks the long-ranged FE ordering and structural fluctuation is induced, which should be responsible for the anomalous FE behaviors in these compounds.

The charge ordered (CO) state in spinel A1V₂O₄, which appears below 700 K, was examined by electron and synchrotron x-ray diffraction experiments. It was found that the CO structure is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. In addition, it was revealed that the Cr substitution for V in A1V₂O₄ suppressed the formation of the long-ranged CO structure and induced the charge disordered (CD) state. As a result, the coexisting state between the CO and CD states with the 20-30 nm size was stabilized in A1V_2-xCr_xO₄ with x=0.125.

We studied the dynamics of two-phase coexistence in perovskite manganite RMnO3 (R=rare earth) by means of dynamical specific heat measurement. We found that specific heat exhibits a Debye-type frequency dependence around the transition temperature between two different magnetic phases, which can be attributed to the relaxational domain motion between the two phases. We also found that relaxation time (tau) systematically varies with the change of R as well as with an applied magnetic field, and dispersion frequency (1/tau) divergently increases as the system approaches the critical point of the magnetic phases.

We studied the transport, magnetic, and structural properties of spinel MnTi2O4 and V-doped samples, MnTi2-xVxO4, to clarify the relation between charge, spin, and orbital degrees of freedom of 3d electrons in this series of compounds. A structural phase transition occurs in MnTi2O4 at 180 K, which is presumably dominated by the ordering of t(2g) orbitals, but it can be easily suppressed by the V doping to the Ti site. Associated with this suppression of the structural anomaly, a ferrimagnetic and electrically conducting state appears in MnTi2-xVxO4 with finite values of x. Fairly large magnetoresistance was observed in this state, whose sign changes from positive to negative with an increase in the V concentration x. With approaching the other end of this series, MnV2O4, the electrical resistivity increases again and another type of structural phase transition occurs at 53 K. Large magnetostriction was observed in MnV2O4, the only compound in this series to have both a ferrimagnetic state and tetragonal lattice distortion.

We demonstrate that switching of crystal structure as well as domain rotation occur in spinel MnV2O4 with applied magnetic field. Such large magnetoelastic effects arise from a strong coupling between spin and orbital degrees of freedom for the V ions in this compound. (c) 2006 Elsevier B.V. All rights reserved.

Optical conductivity spectra of ferromagnetic pyrochlore vanadate Yb2V2O7 were investigated under a variation of both temperature and magnetic field. It was found that the spectrum of a Mott-gap excitation, a d-d transition between neighboring V sites, exhibits spectral weight transfer to lower energies with ferromagnetic ordering as well as an applied magnetic field. This result indicates that charge dynamics is influenced by the spin state in this prototypical orbital-spin-coupled system.

We show that polarity-dependent, nonvolatile resistance switching by electric field occurs in the thin film of various transition-metal oxides in almost the same manner. This result indicates that, contrary to the general acceptance, perovskite manganite is by no means a special compound for this phenomenon. It is also suggested that the resistance switching is not dominated by a detailed electronic structure of each sample, but dominated by a more general origin, e.g., crystalline defect. (c) 2006 American Institute of Physics.

We investigated the charge-ordered (CO) state in spinel AlV2O4 by electron diffraction, synchrotron x-ray diffraction, and magnetic measurements. It was found that the CO structure appearing below 700 K is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. Theoretical consideration indicates that this unique molecularlike V heptamer is stabilized by a strong bonding of vanadium t(2g) orbitals.

We present the electronic structure of Sr1-(x+y)Lax+yTi1-xCrxO3 investigated by high-resolution photoemission spectroscopy. In the vicinity of the Fermi level, it was found that the electronic structure was composed of a Cr 3d local state with the t(2g)(3) configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.

We measured the temperature dependence of dielectric constant and striction for spinel MnT2O4 (T = V, Cr, Mn) under magnetic field. We found critical changes of the dielectric constant and striction with ferrimagnetic ordering as well as applied magnetic field in MnV2O4 and Mn3O4, which have orbital degree of freedom in the T3+ ion. This result indicates the importance of the orbital degree of freedom for the coupling between dielectric, magnetic properties and crystal structure in these spinel compounds.

We have investigated microstructures giving rise to magnetocapacitance effect in Ti-doped YMnO3 (YMn1-xTixO3) by both electron diffraction and real-space imaging techniques. Diffuse scattering elongating along the [110] direction due to the instability of the Mn trimerization were found in the x=0.175 compound, which exhibits the enhanced magnetocapacitance effect. With increasing Ti concentration, the centrosymmetric (paraelectric) R3̄c phase grows up at the expense of the ferroelectric P63cm phase around x=0.20 and the R3̄c phase is dominant above x=0.30. Our present results indicate that the Mn trimers with the short-range correlation inside the ferroelectric nanodomains with the 10-20nm size should be responsible for the enhancement of the magnetocapacitance and, on the other hand, the appearance of the R3̄c phase suppressed the magnetocapacitance effect. © 2005 The American Physical Society.

We have investigated microstructures giving rise to magnetocapacitance effect in Ti-doped YMnO3 (YMn1-xTixO3) by both electron diffraction and real-space imaging techniques. Diffuse scattering elongating along the [110] direction due to the instability of the Mn trimerization were found in the x=0.175 compound, which exhibits the enhanced magnetocapacitance effect. With increasing Ti concentration, the centrosymmetric (paraelectric) R3c phase grows up at the expense of the ferroelectric P6(3)cm phase around x=0.20 and the R3c phase is dominant above x=0.30. Our present results indicate that the Mn trimers with the short-range correlation inside the ferroelectric nanodomains with the 10-20 nm size should be responsible for the enhancement of the magnetocapacitance and, on the other hand, the appearance of the R3c phase suppressed the magnetocapacitance effect.

We studied the magnetic and structural properties of spinel MnV2O4, which has S=5/2 spin with no orbital degrees of freedom on the Mn2+ site and S=1 spin and three orbital degrees of freedom on the V3+ site. We found that the ferrimagnetic ordering at T-N=56.5 K and the structural phase transition at T-s=53.5 K are closely correlated in this compound and found a switching of crystal structure between cubic and tetragonal phases by the magnetic field. This phenomenon can be explained by the coupling between orbital and spin degrees of freedom in the t(2g) states of the V site.

We investigated changes in domain structures induced by a partial substitution of Ti for Mn in multiferroic YMnO3 by transmission electron microscopy (TEM). In the ferroelectric phase of YMnO3 at room temperature, there exist 180-degree ferroelectric (FE) domain structures of a micrometer size. On substituting Ti for Mn, the size of FE domains becomes fragile and, at approximately x = 0.175 in YMn1-xTixO3, the size can be estimated to be 10-20nm. On further substitution, it is found that a structural phase transition from a hexagonal P6(3) cm structure to a new rhombohedral R3c one takes place at approximately x = 0.20. In YMn1-xTixO3 with 0.20 &lt; x &lt; 0.40, these two distinct structures (the hexagonal P6(3) cm and rhombohedral R3c structures) coexist and this coexistence should have some strong effect on the ferroelectric properties such as the magnetocapacitance effect found in Ti-substituted YMnO3.

We studied SrTiO3 into which spins and carriers are doped by Cr substitution for Ti and La substitution for Sr, respectively. It was found that Weiss temperatures vary from negative to positive values with carrier doping, indicating the appearance of a ferromagnetic interaction between Cr spins via the conduction carriers. We also found negative magnetoresistance in these compounds, whose magnitude amounts to 70% at low temperatures. These results demonstrate that magnetic semiconductors with large magnetoresistance can be obtained based on conventional oxides, like SrTiO3, with independent doping of spins and carriers.

The interplay between charge and spin degrees of freedom is a key ingredient of the colossal magnetoresistance in perovskite manganites. However, our knowledge of such an interplay was limited to its static aspect, and very little was known regarding its dynamics. Here, we show that the ac (optical) conductivity of perovskite manganite Pr0.6Sr0.4MnO3 (T-C=297 K) under magnetic field does not show such a universal M-2 scaling (M is the magnetization) as observed in the dc conductivity, but shows a marginal parallel to M parallel to(1) dependence in the critical regime. This result indicates a unique interplay between fluctuating spin and charge in this series of compounds.

We have performed the soft X-ray photoemission and absorption studies of CaB6. The crystal field splitting in the Ca 3d levels is estimated as 0.65 eV from the analysis of the Ca 2p X-ray absorption spectrum. It is found that the surface Ca 2p contribution is seen at the lower binding energy side of the bulk Ca 2p peak in the core-level spectra. The Ca 2p-3d resonance photoemission shows the clear enhancement of the spectral weight in the vicinity of the Fermi level. (c) 2005 Elsevier B.V. All rights reserved.

We have studied the crystal structure, magnetic properties, and dielectric properties of Ti-doped hexagonal YMnO3 composed of a Mn triangular lattice with trimerization. It was found that a structural phase transition occurring in YMn1-xTixO3 around x=0.2, which is correlated with a suppression of Mn trimerization by Ti doping, has a substantial effect on the magnetic and dielectric properties of this series of compounds. This correlation between the crystal structure and magnetic and dielectric properties indicates that Mn trimerization plays an important role for the large magnetodielectric coupling unique to hexagonal RMnO3.

We report the measurement of dielectric constant in ZnFe2O4 as a function of temperature and magnetic field. The behavior of dielectric constant for single crystals, which is dominated by the pair correlation of the neighboring Fe spins, indicates that ferromagnetic correlation is enhanced below 30 K, but is suppressed below 10 K, where positive magnetocapacitance is enhanced. It is also found that the magnetocapacitance of polycrystalline samples is much larger (similar to 10% at 14T) than that of single crystals, because of the additional dielectric relaxation coupled with a uniform magnetization.

Effect of the Cr doping on the stable charge ordering (CO) in the spinal compound AlV2O4 was investigated by transmission electron microscopy. It is found that the stable CO is destabilized drastically by substituting Cr ions for V ones and the long-ranged CO in AlV2O4 becomes short ranged. The size of the CO microdomains in AlV2-xCrxO4 with x=0.125 was estimated to be about 10-20 nm. Furthermore, high-resolution lattice images revealed fine mixture of the CO and charge disordered microdomains in AlV2-xCrxO4 at room temperature. In addition, we found the presence of the characteristic diffuse scattering due to local lattice distortion induced by the Cr substitution in AlV2-xCrxO4. Our results suggested that doped Cr 31 ions should serve as a quenched "charge" impurity in the CO structure of AlV2O4, which is characterized as the charge frustration system.

The interplay between charge and spin degrees of freedom is a key ingredient of the colossal magnetoresistance in perovskite manganites. However, our knowledge of such an interplay was limited to its static aspect, and very little was known regarding its dynamics. Here, we show that the ac (optical) conductivity of perovskite manganite Pr0.6Sr0.4MnO3 (TC=297K) under magnetic field does not show such a universal M2 scaling (M is the magnetization) as observed in the dc conductivity, but shows a marginal £ M £1 dependence in the critical regime. This result indicates a unique interplay between fluctuating spin and charge in this series of compounds. © 2005 The American Physical Society.

In order to clarify the microstructure related to the ferroelectric properties in multi-ferroic YMnO3, we carefully examined the ferroelectric-domain configuration by transmission electron microscopy (TEM). It was found that ferroelectric domains in YMnO3 consist of six anti-phase domains at room temperature. On warming from room temperature, the ferroelectric-domain configuration changes into one consisting of three anti-phase domains around 738 K. In addition, the ferroelectric domains in YMnO3 change drastically into microdomains with a size of 10 similar to 20 nm by substitution of Ti ions for some of the Mn ones.

We report the reflectivity and the resistivity measurement of Ca 2-xNaxCuO2Cl2 (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical conductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Γ(ω) with doping. It was also found that Γ(ω) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.

We report the reflectivity and the resistivity measurement of Ca2-xNaxCuO2Cl2 (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical conductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Gamma(omega) with doping. It was also found that Gamma(omega) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.

Raman scattering spectra have been investigated for the correlated 4d-electron system, R2Mo2O7 (Tf=Nd, Sm, Gd, Tb,Dy,Nd1-xDyx), which undergoes a metal-insulator transition with changing the rare-earth ion R, or equivalently the one-electron bandwidth. It is found that several phonon peaks modulating the Mo-O-Mo bond angle appear in the metallic phase (R=Nd, Sm, Gd, Nd1-xJDy x), whereas they are remarkably suppressed in intensity in the insulating phase (R=Tb,Dy). This result indicates that the phonon modes of R2Mo2O7 are coupled with the electron-hole excitation across the Fermi level, thus probing sensitively the low-energy charge dynamics in the vicinity of the bandwidth-control Mott transition.

Raman scattering spectra have been investigated for the correlated 4d-electron system, R2Mo2O7 (R=Nd, Sm, Gd, Tb,Dy,Nd1-xDyx), which undergoes a metal-insulator transition with changing the rare-earth ion R, or equivalently the one-electron bandwidth. It is found that several phonon peaks modulating the Mo-O-Mo bond angle appear in the metallic phase (R=Nd, Sm, Gd, Nd1-xDyx), whereas they are remarkably suppressed in intensity in the insulating phase (R=Tb,Dy). This result indicates that the phonon modes of R2Mo2O7 are coupled with the electron-hole excitation across the Fermi level, thus probing sensitively the low-energy charge dynamics in the vicinity of the bandwidth-control Mott transition.

We demonstrate a nonvolatile, reversible change of infrared reflectance from the thin film of perovskite manganite (Pr1-xCaxMnO3) by applying electric pulse. The result provides a possibility to use the electric-pulse-induced phenomena of this compound in optical devices. (C) 2004 American Institute of Physics.

We have found that various Mn oxides have the universal resistivity and thermopower in the high temperature limit. The resistivities and thermopowers of all the samples go toward constant values of 7+/-1 mOmega cm and -79+/-3 muV/K, which are independent of carrier density and crystal structures. We propose that the electric conduction occurs in a highly localized way in the high temperature limit, where the exchange of entropy and charge occurs in the neighboring Mn3+ and Mn4+ ions. (C) 2004 American Institute of Physics.

Magnetocapacitance has been measured for the geometrically frustrated magnets R2Ti2O7 (R=rare earth). The temperature dependence of magnetocapacitance, which largely deviates from the square of magnetization, indicates the evolution of strong ferromagnetic (R=Ho) or antiferromagnetic (R=Gd) fluctuation at low temperatures. This result demonstrates that the measurement of mangetocapacitance is a powerful technique to detect the magnetic fluctuation of exotic magnets.

We used inelastic neutron scattering to show that well below its Néel temperature, TN, the two-dimensional (2D) XY nearly triangular antiferromagnet YMnO3 has a prominent central peak associated with 2D antiferromagnetic fluctuations with a characteristic lifetime of 0.55(5) ps, coexisting with the conventional long-lived spin waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.

We have observed the charge and the magnetic ordering in La2-xSrxNiO4(0.28less than or equal toxless than or equal to 0.46) by neutron scattering. In the c-plane, holes and spins order as a two-dimensional system, where holes form stripes and they are inserted into the antiferromagnetic ordering. Along the c-axis, the two-dimensional ordering stacks. However, the coherence along the c-axis is much weaker than that in the c-plane. We discuss how two-dimensional ordering stacks and why the coherence along the c-axis becomes weak. (C) 2003 Elsevier Science B.V. All rights reserved.

It was found that the charge-ordered phase of hte AlV2O4 spinel was rapidly suppressed and a charge-disorder phase grows up instead with Cr doping into the V site, yielding a two-phase-coexisting state in modestly Cr-doped samples. The magnetic ground state is a spin-glass state dominated by geometrical frustrtion low temperatures in the charge-ordered phase. Such a difference in magnetic properties can be attributed to the difference in dimensionality as a spin system etween the charge-ordered phase and the charge-disordered phase.

The relationship between charge and spin degrees of freedom in a geometrically frustrated system, AlV2-xCrxO4 spinel, is investigated. Upon Cr doping, the charge-ordered phase of AlV2O4 is rapidly suppressed and a charge-disordered phase grows up instead. It is found that the magnetic ground state is a spin-glass state dominated by geometrical frustration for both phases, but larger spin entropy remains down to low temperatures in the charge-ordered phase, possibly owing to its two-dimensional character.

We have investigated the charge-spin-ordering transition in La1.5Sr0.5NiO4 by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below ∼400 K can be probed by activated phonon (A 1g and E.,) and two-magnon (B 1g) modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another B2g Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation.

We have investigated the charge-spin-ordering transition in (formula presented) by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below (formula presented) can be probed by activated phonon (formula presented) and (formula presented) and two-magnon (formula presented) modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another (formula presented) Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation. © 2003 The American Physical Society.

The relationship between charge and spin degrees of freedom in a geometrically frustrated system, [Formula presented] spinel, is investigated. Upon Cr doping, the charge-ordered phase of [Formula presented] is rapidly suppressed and a charge-disordered phase grows up instead. It is found that the magnetic ground state is a spin-glass state dominated by geometrical frustration for both phases, but larger spin entropy remains down to low temperatures in the charge-ordered phase, possibly owing to its two-dimensional character. © 2003 The American Physical Society.

We used inelastic neutron scattering to show that well below its Néel temperature, TN, the two-dimensional (2D) XY nearly triangular antiferromagnet YMnO3 has a prominent central peak associated with 2D antiferromagnetic fluctuations with a characteristic lifetime of 0.55(5) ps, coexisting with the conventional long-lived spin waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system. © 2003 The American Physical Society.

We have investigated the charge-spin-ordering transition in La1.5Sr0.5NiO4 by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below similar to400 K can be probed by activated phonon (A(1g) and E-u) and two-magnon (B-1g) modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another B-2g Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation.

We succeeded in observing ferromagnetic (FM) microdomains in the phase-separated state found in Cr-doped Nd0.5Ca0.5MnO3 by low-temperature Lorentz microscopy. The presence of the FM microdomains with the size of 20-30 nm were found in Nd0.5Ca0.5Mn1-yCryO3 for y=0.03. In addition, it is clearly shown that the low-temperature phase characterized as the phase-separated state consists of a fine mixture of the two competing ground states, the FM metallic state and the charge/orbital ordered insulator one. Our experimental findings clearly demonstrate the occurrence of microscopic and static phase separation in the Cr-doped manganites.

Stripe order in La2-xSrxNiO4 with 0.289less than or similar toxless than or similar to1/2 was studied with the neutron-diffraction technique. The stripe order of holes and spins was observed at low temperatures in all samples. The incommensurability E of the stripe order is approximately linear with the hole concentration n(h) up to x = 1/2. We found that the stripe is most stable at n(h) = 1/3 with epsilon = 1/3 from the analyses of the onset temperatures of the charge and spin orderings and the n(h) dependence of the correlation length of the stripe order. The stability of an 8 = 1/3 stripe induces a systematic deviation of E from the epsilon = n(h) law for samples with n(h) not equal 1/3. This deviation affects the stripe order on both sides of the hole concentration with n(h)&lt;1/3 as well as n(h)&gt;1/3.

A ferroelectricity-magnetism-coexisting system, hexagonal RMnO3 (R = Y, Lu, and Sc), has been investigated by synchrotron x-ray and neutron powder diffraction measurements. It is found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of MnO5 polyhedra and the buckling of R layers, which persists up to 1000 K. Neutron diffraction measurements have revealed the reduction of ordered moments from the expected value, as well as strong magnetic diffuse scattering existing far above TN, both of which are caused by geometrical frustration of the triangular lattice of Mn ions. We have also investigated the effects of Zr doping into the R site and have found that Zr doping drastically suppresses both ferroelectric distortion and magnetic ordering.

A ferroelectricity-magnetism-coexisting system, hexagonal RMnO3 (R=Y, Lu, and Sc), has been investigated by synchrotron x-ray and neutron powder diffraction measurements. It is found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of MnO5 polyhedra and the buckling of R layers, which persists up to 1000 K. Neutron diffraction measurements have revealed the reduction of ordered moments from the expected value, as well as strong magnetic diffuse scattering existing far above T-N, both of which are caused by geometrical frustration of the triangular lattice of Mn ions. We have also investigated the effects of Zr doping into the R site and have found that Zr doping drastically suppresses both ferroelectric distortion and magnetic ordering.

Ferromagnetism was recently reported in La-doped alkaline-earth hexaborides A1-xLaxB6 (A = Ca, Sr, and Ba). We have performed reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (∼ 10 μm in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is not intrinsic.

Ferromagnetism was recently reported in La-doped alkaline-earth hexaborides A(1-x)La(x)B(6) (A=Ca, Sr, and Ba). We have performed reflectivity, Hall resistivity, and magnetization measurements of A(1-x)La(x)B(6). The results indicate that A(1-x)La(x)B(6) can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (similar to10 mum in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A(1-x)La(x)B(6) reported so far is not intrinsic.

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

Impurity effect on the charge and orbital ordered (CO/OO) state in doped manganites has been studied by the transmission electron microscopy (TEM), in addition to the electric and magnetic measurements. In the Cr-doped manganites (Nd0.5Ca0.5Mn1-yCryO3), the CO/OO state characterized by the incommensurate wave vector is still realized as the microdomains with the size of 10-30 nm even at lower temperature of 20 K. On the other hand, the Sc-doped manganites (Nd0.5Ca0.5Mn1-yScyO3) clearly show an incommensurate to commensurate structural phase transition accompanying the formation of the lon-ranged CO/OO state around 60 K. From our experimental findings, it is suggested that the substituted Cr3+ and Sc3+ ions serve as the spin impurity and hole vacancy, respectively.

Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications. (C) 2002 Optical Society of America.

B-site substitution effects on the structural and magnetic properties are investigated in Nd1/2Ca1/2(Mn0.97M0.03) O3 (M = Al, Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below ∼200 K, these compounds transform into a two-phase state. With further decrease of temperature below ∼100 K, the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the e g band.

B-site substitution effects on the structural and magnetic properties are investigated in Nd1/2Ca1/2(Mn0.97M0.03)O-3 (M = Al, Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below similar to200 K, these compounds transform into a two-phase state. With further decrease of temperature below similar to100 K, the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the e(g) band.

Ferromagnetism was recently reported in La-doped alkaline-earth hexaborides (formula presented) (formula presented) Sr, and Ba). We have performed reflectivity, Hall resistivity, and magnetization measurements of (formula presented) The results indicate that (formula presented) can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (formula presented) in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of (formula presented) reported so far is not intrinsic. © 2002 The American Physical Society.

(formula presented) -site substitution effects on the structural and magnetic properties are investigated in (formula presented) (formula presented) (formula presented) (formula presented) Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below (formula presented) these compounds transform into a two-phase state. With further decrease of temperature below (formula presented) the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the (formula presented) band. © 2002 The American Physical Society.

We investigated an influence of substituting Sc for Mn sites on the charge and orbital ordered (CO/OO) state in Nd0.5Ca0.5MnO3. In the 3% Sc-doped sample, it is found that a magnetic susceptibility curve shows an anomalous cusp around 60 K on cooling. Electron diffraction measurement revealed that an incommensurate (IC) to commensurate (C) structural phase transition takes place around 60 K on cooling and it has a large thermal hysteresis. Furthermore, real space image in the IC phase by electron microscopy shows the presence of the discommensurastions with a phase slip of π. © 2002 Published by Elsevier Science Ltd.

The far-infrared c-axis reflectance of Sr2RuO4 has been measured above and below the 2.5 K superconducting transition temperature of the sample. A plasma edge develops near 70 cm(-1) as the temperature is lowered, which corresponds to a Drude peak in the real optical conductivity associated with the onset of the coherent motion of the carriers. A gaplike suppression of the frequency-dependent scattering rate at low temperatures indicates that the coherent transport is related to a loss of scattering below 60 cm(-1). A strong resonance near 9 meV is responsible for the scattering that destroys coherence.

Ordering process of stripe order in La2-xSrxNiO4 with x being around 1/3 was investigated by neutron-diffraction experiments. When the stripe order is formed at high temperature, incommensurability ε of the stripe order has a tendency to show the value close to 1/3 for the samples with x at both sides of 1/3. With decreasing temperature, however, ε becomes close to the value determined by the linear relation of ε=nh, where nh is a hole concentration. This variation of the ε strongly affects the character of the stripe order through the change of the carrier densities in stripes and antiferromagnetic domains.

Ordering process of stripe order in La2-xSrxNiO4 with x being around 1/3 was investigated by neutron-diffraction experiments. When the stripe order is formed at high temperature, incommensurability epsilon of the stripe order has a tendency to show the value close to 1/3 for the samples with x at both sides of 1/3. With decreasing temperature, however, epsilon becomes close to the value determined by the linear relation of epsilon = n(h) where n(h) is a hole concentration. This variation of the epsilon strongly affects the character of the stripe order through the change of the carrier densities in stripes and anti ferromagnetic domains.

Single crystals of hexagonal RMnO3 (R = Y, Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to T-N is similar to 10 in RMnO3, indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below T-N, indicating that a part of the spins are still fluctuating at T much less thanT(N). It is also found that the dielectric anomaly at T-N is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.

The dielectric constant of quantum paraelectric EuTiO3, which contains Eu2+ with S=7/2 spin and Ti4+, has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial change under a magnetic field (by ∼7% with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency.

The dielectric constant of quantum paraelectric EuTiO3, which contains Eu2+ with S=7/2 spin and Ti4+, has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial chance under a magnetic field (by similar to7% with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency.

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.

We have found that spinel AlV2O4 shows a phase transition at similar to 700 K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V2.5+ ions are separated into three V2.5-delta ions and one V2.5+3 delta loll, This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of "change frustration." thus can be understood in terms of "change frustration."

Structural parameters are systematically investigated for Mo pyrochlore R2Mo2O7 (R = Dy, Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius r(R) of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with r(R) from 129.7 degrees for Dy2Mo2O7 (SG) to 131.5 degrees for Nd2Mo2O7 (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states of this compound. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3.

We investigated thoroughly change in the microstructure related to the charge ordered (CO) state in half-doped manganites such as Pr0.5Ca0.5MnO3 and Nd0.5Ca0.5MnO3 by the transmission electron microscopy. We found the incommensurate (IC) to commensurate (C) structural phase transition in Pr0.5Ca0.5MnO3 and Nd0.5Ca0.5MnO3. The magnitude of the wave vector characterizing the CO state changes with respect to the temperature. Real space images taken by using the superlattice spots due to the CO state revealed that the IC structure is characterized as the regular arrangement of the discommensurations with the phase slip of pi along the [100] direction. Furthermore. the IC CO state is present as the microdomains with the size of 20-30nm.

Structural parameters are systematically investigated for Mo pyrochlore R2Mo2O7 (R = Dy, Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius rR of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with rR from 129.7° for Dy2Mo2O7 (SG) to 131.5° for Nd2Mo2O7 (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

Single crystals of hexagonal (formula presented) (formula presented) Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to (formula presented) is (formula presented) in (formula presented) indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below (formula presented) indicating that a part of the spins are still fluctuating at (formula presented) It is also found that the dielectric anomaly at (formula presented) is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds. © 2001 The American Physical Society.

The dielectric constant of quantum paraelectric (formula presented) which contains (formula presented) with (formula presented) spin and (formula presented) has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial change under a magnetic field (by (formula presented) with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency. © 2001 The American Physical Society.

Destabilization of the charge-ordered (CO) state in half-doped manganites by chromium (Cr) doping in the: manganese (Mn) sites was investigated. The Cr doping: in the half-doped manganites makes the CO state short-ranged and induces new ferromagnetic metallic (FMM) phase at low temperature. Real space images taken in the CO state by electron microscope clearly show coexistence of the short-ranged CO and FMM states, which is characterized as a microscopic scale phase separation. Furthermore, the CO state in the Cr-doped manganites has a superlattice structure with the incommensurate wave vector q = (1/2 - epsilon )a(o)(*); where epsilon depends on the Cr concentration. These results suggest that the Cr doping suppresses the incommensurate to commensurate structural transition found in the half-doped manganites and produces new FMM phase.

The electronic structure of bilayered manganite (La2-2xSr1+2xMn2O7) has been investigated by optical measurements. In the paramagnetic phase, the in-plane and out-of-plane optical spectra show a doping dependence reflecting the change in the orbital character of conduction electrons. A drastic electronic structural change has been observed with the evolution of in-plane magnetic ordering irrespective of the ferromagnetic (e.g., x=0.4) or antiferromagnetic (x=0.3) interbilayer magnetic coupling. The optical conductivity spectra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Drude component followed by a dominant broad peak centered around 0.4 eV. Anomalies in the phonon spectra are found in the antiferromagnetic state of the x=0.45 and 0.5 compounds, indicating a different structural change accompanied by the A-type antiferromagnetic (and perhaps x2-y2-type orbital) ordering. The relevant charge ordering and lattice distortion may be responsible for the nonmetallic nature of the A-type antiferromagnetic phase.

The electronic structure of bilayered manganite (La2-2xSr1+2xMn2O7) has been investigated by optical measurements. In the paramagnetic phase, the in-plane and out-of-plane optical spectra show a doping dependence reflecting the change in the orbital character of conduction electrons. A drastic electronic structural change has been observed with the evolution of in-plane magnetic ordering irrespective of the ferromagnetic (e.g., x = 0.4) or antiferromagnetic (x = 0.3) interbilayer magnetic coupling. The optical conductivity spec tra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Dr de component followed by a dominant broad peak centered around 0.4 eV. Anomalies in the phonon spectra are found in the antiferromagnetic state of the x=0.45 and 0.5 compounds, indicating a different structural change accompanied by the A-type antiferromagnetic (and perhaps x(2)-y(2)-type orbital) ordering. The relevant charge ordering and lattice distortion may be responsible for the nonmetallic nature of the A-type antiferromagnetic phase.

The optical spectra of CeTiO3.04, which undergoes a phase transition from a paramagnetic metal to an antiferromagnetic metal, are investigated with changing temperature (T). In the paramagnetic phase, optical conductivity shows a T-dependent feature up to 1 eV, which can be attributed to the T-dependent scattering rate. In the antiferromagnetic phase, on the other hand, optical conductivity reduces its low-energy weight below 0.4 eV, but no gaplike structure appears. A two-component model, where the spectrum is a sum of a metallic and an insulating component, gives a simple explanation for the change of the optical spectra in the antiferromagnetic metallic phase.

The optical spectra of CeTiO3.04. which undergoes a phase transition from a paramagnetic metal to an antiferromagnetic metal, are investigated with changing temperature (T). In the paramagnetic phase, optical conductivity shows a T-dependent feature up to 1 eV, which can be attributed to the T-dependent scattering rate. In the antiferromagnetic phase, on the other hand, optical conductivity reduces its low-energy weight below 0.4 eV, but no gaplike structure appears. A two-component model, where the spectrum is a sum of a metallic and an insulating component, gives a simple explanation for the change of the optical spectra in the antiferromagnetic metallic phase.

The lattice and electronic response in the course of concomitant charge and orbital-ordering transition has been investigated for single crystals of layered manganites, single-layered La0.5Sr1.5MnO4, and bilayered LaSr2Mn2O7, by Raman-scattering spectroscopy. In the charge-orbital-ordered state (T&lt;217 K for La0.5Sr1.5MnO4 and similar to 100 K&lt;T&lt;210 K for LaSr2Mn2O7), four phonon modes are activated in the MnO2 plane, indicating the existence of both Jahn-Teller type and breathing mode type lattice distortions. The diffuse scattering response with B-1g symmetry is suppressed in the charge-orbital-ordered state, both for La0.5Sr1.5MnO4 and LaSr2Mn2O7, signaling that the fluctuation caused by the dynamical charge-orbital correlation is suppressed in the charge-orbital-ordered state. These features reveal the unique nature of the charge-orbital-ordering transition realized by the complex interplay of charge and orbital degrees of freedom.

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.

The magnetic and transport properties of pyrochlore R2Mo2O7 have been studied with variation of the rare earth (R). The change of the mean ionic radius of R, which induces change of the lattice structure, determines the magnetic ground state (ferromagnetic or spin glass), and the magnetic phase boundary is correlated with the metal-insulator crossover. Furthermore, we found enhanced magnetoresistance and unusual residual anomalous Hall effect at low temperatures near the phase boundary, which can be attributed to the coexistence of both phases.

The stripe order in (Formula presented) with (Formula presented) was studied with neutron-scattering technique. At low temperatures, all samples exhibit hole stripe order. Incommensurability (Formula presented) of the stripe order is approximately linear in the hole concentration (Formula presented) up to (Formula presented) where (Formula presented) denotes the off stoichiometry of oxygen atoms. The charge and spin ordering temperatures exhibit maxima at (Formula presented) and both decrease beyond (Formula presented) For (Formula presented) the stripe ordering consists of the mixture of the (Formula presented) stripe order and the (Formula presented) charge/spin order. © 2000 The American Physical Society.

Microstructure related to the charge ordered (CO) state and metal-insulator (MI) transition in Nd0.5Sr0.5MnO3 was investigated by transmission electron microscopy. An in situ observation on heating from the CO state revealed a drastic change in the microstructure: the nucleation and growth of the ferromagnetic (FM) charge disordered regions proceed in the vicinity of the antiphase boundaries in the CO regions, which are closely related to the MI transition around 160 K. These transition processes suggest that an abrupt increase in the electronic conductivity during the MI transition is mainly due to the percolation mechanism. Further more, the FM metallic phase is found to be characterized by a spatially fine mixture of incommensurate charge ordered microdomains in the FM charge disordered phase. This result clearly demonstrates the occurrence of microscopic-scale electronic phase separation in the manganese oxides. [S0163-1829(99)02541-2].

We report the transport and magnetic properties of a series of ferromagnetic metallic compounds, (Formula presented) where the maximum (Formula presented) is 8 K at (Formula presented) Magnetic interaction between Eu (Formula presented) spins mediated by itinerant Ti (Formula presented) electrons is essential for the ferromagnetism of this compound, and a clear correlation between transport properties and magnetism exists in this series. Possible roles of the electron-correlation effect in Ti (Formula presented) electrons are also discussed. © 1999 The American Physical Society.

Raman spectra have been investigated for a single crystal of La1.2Sr1.8Mn2O7 that shows colossal magnetoresistance around the Curie temperature Tc = 126K. Several broad phonon peaks grow with decrease of temperature down to TC but disappear below TC, which can be clearly ascribed to a change in the charge/orbital correlation by comparison with the spectra for the charge/orbital ordered states of the layered manganites (La0.5Sr1.5MnO4 and LaSr2Mn2O7). Such a real-space charge density fluctuation is also observed to cause the low-energy diffuse Raman response, the B1g component of which is critically suppressed below TC as well.

Frequency- and temperature-dependent conductivity has been studied by optical spectroscopy for perovskite titanates, which are typical three-dimensional correlated metals. Extended Drude analysis of the data indicates that only one parameter, the (Formula presented) coefficient of the scattering rate, dominates the temperature dependence of conductivity over a wide energy range (from dc to (Formula presented) This result is consistent with the Fermi-liquid theory in the low-frequency limit, and can be regarded as a prototypical behavior of correlated metals. © 1999 The American Physical Society.

Low-energy optical spectra and their temperature dependence have been investigated for single crystals of perovskite-type manganese oxides, R1-xSrxMnO3, at a fixed doping level (x=0.4) but with varying perovskite R species or one-electron bandwidth (W). Variation of the optical conductivity spectra with a change of R, such as a diminishment of the Drude peak and the emergence of a nearly ω-independent component, indicates a coherent-incoherent crossover of charge dynamics in the ferromagnetic metallic phase. This cannot be accounted for in terms of simple change in carrier mass or scattering rate, but indicates the importance of competitive instabilities triggered by decreasing W against the double-exchange interaction near the charge/orbital-ordering instability. © 1999 The American Physical Society.

Measurements of Hall coefficients, thermoelectric power, and optical conductivity spectra were done for single crystals of La2-xSrxNiO4 (x=0.27-0.50) showing prototypical charge ordering. We found that the character of the carriers in the charge-ordered state changes from electronlike to holelike when the hole concentration p crosses 1/3, and that the correlation gap in the optical spectrum for p=1/3 is collapsed when p is away from 1/3. These results indicate that the deviation of p from 1/3 can be regarded as electron or hole doping into the commensurate charge-ordered state at p=1/3. © 1999 The American Physical Society.

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.

Raman scattering spectra have been investigated for a single crystal of La1.67Sr0.33NiO4 which shows stripe ordering of charge (T-c = 230 K) and spin (T-s = 180 K). Upon the charge ordering, the activated phonon peaks grow with the evolution of the charge modulation amplitude, while the electronic Raman continuum of the metallic state abruptly disappears. The two broad peaks with the B-1g, symmetry arising from the two-magnon excitations emerge immediately below T-c,, indicating the evolution of the two-dimensional antiferromagnetic spin correlation in the charge-ordered state even above T-s,. [S0031-9007(98)05303-4].

Highly anisotropic optical conductivity spectra and their temperature variation have been investigated for a layered ferromagnetic (Formula presented)=121 K) crystal of La(Formula presented)Sr(Formula presented)Mn(Formula presented)O(Formula presented)=0.4). The in-plane spectra show conspicuous spectral-weight transfer over an energy region of 0-3 eV with evolution of spin polarization below (Formula presented) and in the ferromagnetic, highly conducting ground state form a prominent peak structure around 0.4 eV with minimal Drude weight. By contrast, the (Formula presented)-axis spectra show little change with temperature and remain insulatorlike. The results indicate highly incoherent dynamics of the fully quasi-two dimensional spin-polarized carriers. © 1998 The American Physical Society.

Transport and magnetic properties of (Formula presented) have been systematically investigated varying the one-electron bandwidth (Formula presented) and the band filling (Formula presented) which can be controlled by the (Formula presented)-dependent lattice distortion and by the Ca content (Formula presented) and/or oxygen offstoichiometry (Formula presented) respectively. The end compound (Formula presented) is a (Formula presented) Mott-Hubbard insulator and its charge-gap magnitude increases with decreasing ionic radius of (Formula presented) i.e., an increase of electron correlation (Formula presented) in proportion with (Formula presented) where (Formula presented) is the critical value for the (hypothetical) (Formula presented) Mott transition. Such a Mott insulator is transformed to a correlated metal by substitution of (Formula presented) with Ca (hole doping), and the nominal hole concentration required for the insulator-metal transition (Formula presented) increases in proportion with (Formula presented) Concerning magnetism, (Formula presented) with (Formula presented) Pr, Nd, and Sm, shows the antiferromagnetic ordering and its Néel temperature (Formula presented) decreases with smaller (Formula presented) also decreases with Ca doping, but remains finite up to the metal-insulator phase boundary. On the basis of these results, electronic phase diagrams are derived for a series of titanates as an electron-correlated system with changes of two parameters, i.e., the strength of electron correlation and band filling. Possible origins of the insulating state with finite hole doping are also discussed in terms of the kinetic energy of doped carriers in the Mott-Hubbard insulator. © 1997 The American Physical Society.

Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of (Formula presented)(Formula presented)(Formula presented)n(0⩽x⩽0.3). For the low-doped insulating crystal (x=0.1) which shows a ferromagnetic insulating state at low temperature, the spectral weight of the optical conductivity increases only in the inner-gap region around 0.5 eV, but no Drude part emerges due to carrier localization effect. For x⩾0.17, where the low-temperature ferromagnetic metallic state shows up, the optical conductivity spectrum above (Formula presented) is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below (Formula presented). The energy scale (up to ≈2 eV) of the spectral weight transfer is determined by the effective Hundșs-rule coupling energy. In the metallic phase, low-energy spectra arising from intraband excitations can be sorted into two parts: One is a nearly ω-independent broad structure (incoherent part), and the other a sharp coherent Drude peak with anomalously low spectral weight. This can hardly be reconciled with the simple double-exchange theory, but indicates that another degree of freedom (e.g., the orbital ordering and/or electron-lattice interactions) should be taken into account. © 1997 The American Physical Society.

We report the Sr-concentration dependence of the spin and charge ordering in La2-xSrxNiO4. with 0.27 ≤ x ≤ 0.5. In this range of the Sr concentration x, the characteristic wave vector ε for the magnetic and charge order varies approximately linear in x, i.e. ε ∼ x. The transition temperature of spin order is always lower than that of stripe order. Two transition temperatures increase for x &lt
xc where xc = 1/3, and then tend to decrease as x further increases. © 1998 Elsevier Science B.V. All rights reserved.

Optical spectra of a single crystal of La1.67Sr0.33NiO4, which undergoes a charge-ordering transition at T(CO)similar to 240 K, have been investigated for wide ranges of photon energy (0.008-3 eV) and temperature (10-480 K). The opening of a charge gap as well as change of the gap magnitude (2 Delta) with temperature is clearly observed below T-CO, with an anomalously large ratio of 2 Delta(T=0)/k(B)T(CO) (similar to 13). Conspicuous spectral change with temperature is also observed above T-CO over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above T-CO.

Optical spectra of a single crystal of (Formula presented)(Formula presented)(Formula presented), which undergoes a charge-ordering transition at (Formula presented)∼240 K, have been investigated for wide ranges of photon energy (0.008-3 eV) and temperature (10-480 K). The opening of a charge gap as well as change of the gap magnitude (2Δ) with temperature is clearly observed below (Formula presented), with an anomalously large ratio of 2Δ(T=0)/(Formula presented)(Formula presented) (∼13). Conspicuous spectral change with temperature is also observed above (Formula presented) over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above (Formula presented). © 1996 The American Physical Society.

In-plane (ab-plane) and out-of-plane (c-axis) optical spectra were investigated st various temperatures for Sr2RuO4 which is isostructural with the cuprate superconductor La2-xSrxCuO4 and becomes superconducting below 1 K. The ratio of the in-plane to out-of-plane low-energy spectral weight of this compound is approximately 10(2), an anisotropy comparable to that of the cuprate. The temperature dependence of the c-axis spectrum suggests coherent motion of conduction carriers with strongly renormalized mass and scattering rate along the c axis at low temperatures.

Optical spectra have been systematically investigated for R(1-x)Ca(x)TiO(3) with various rare earth ions R (= La, Pr, Nd, Sm, and Y) as a typical three-dimensional correlated electron system, in which both the electon-correlation strength (U/W) and the band filling (n) can be varied. With a decrease of n from 1, the spectrum of the Mott-gap excitation collapses and the Drude part evolves. We found that the rate of the Drude-part evolution with the nominal hole-doping level delta (= 1 - n) is critically enhanced as U/W approaches the critical value (U/W)(c) for the bandwidth-controlled Mott transition.

Changes in electrical, magnetic, and optical properties have been investigated for the doping-induced insulator-metal (I-M) transition in hole-doped Mott insulators, La1-xSrxVO3. In the optical spectra, the I-M transition is characterized by doping-induced transfer of spectral weight into the in-gap region. Resistivity in the metallic phase near the critical magnetic phase boundary shows such a temperature dependence as expressed by the relation rho=rho(0)+AT(1.5) over a wide temperature range (2-200 K) with the enhanced coefficient A towards the phase boundary. The results are argued in terms of effect of the antiferromagnetic spin fluctuation on the charge transport.

Changes in electrical, magnetic, and optical properties have been investigated for the doping-induced insulator-metal (I-M) transition in hole-doped Mott insulators, La1-xSrxVO3. In the optical spectra, the I-M transition is characterized by doping-induced transfer of spectral weight into the in-gap region. Resistivity in the metallic phase near the critical magnetic phase boundary shows such a temperature dependence as expressed by the relation =0+AT1.5 over a wide temperature range (2200 K) with the enhanced coefficient A towards the phase boundary. The results are argued in terms of effect of the antiferromagnetic spin fluctuation on the charge transport. © 1995 The American Physical Society.

Electronic Raman scattering spectra for cuprate superconductors, La2-xSrxCuO4, and prototypical strongly correlated metals, Sr1-xLaxTiO3, were investigated and compared with each other. All the spectra can be reproduced by ''extended'' relaxational function with omega-dependent Gamma. Electron correlation effect leads to strong omega dependence of Gamma, which produces the spectral weight of chi''(omega) up to the high energy region. We also found anomalous symmetry dependence of the spectral intensity for La2-xSrxCuO4 in the lightly doped region.

Raman-scattering spectra were investigated for strongly correlated electron systems, La1-xSrxTiO3 and Y1-xCaxTiO3, which undergo a metal-to-Mott-insulator transition under variation of the band filling. The phonon spectra decrease in their intensity as the system approaches the metal-insulator phase boundary, accompanied by a critical enhancement of the value of the effective mass, and almost disappear in the insulating phase. We found that the phonon spectral intensity in the metallic region approximately scales with (n/m*)2, n and m* being the density and effective mass of conduction carriers.

Electronic Raman scattering has been investigated for metallic Sr1-xLaxTiO3 in which the 3d band filling (x) can be systematically changed from x = 0 (a band insulator) to x = 1 (a Mott-Hubbard insulator). The symmetry dependence of the scattering intensity can be accounted for in terms of the neutral carrier density fluctuation model. However, a systematic change of the spectral shape with the carrier density (congruent-to x) is observed. All the spectra can be reproduced by a relaxational function with an omega-dependent scattering rate (GAMMA). The omega dependence of GAMMA gets stronger with x, which is in accord with the x-dependent enhancement of the effective mass, due to the electron correlation effect.

Low-frequency clectronic Raman scattering in La2-xSrxCuO4 has been investigated over a wide compositional region, 0 less-than-or-equal-to x less-than-or-equal-to 0.34, covering the insulating, superconducting, and nonsuperconducting metallic compounds. We have found that the scattering intensity for the (xy) polarization predominates over that of the (x'y') polarization in the low-doping region below x less-than-or-equal-to 0.15, but vice versa at higher doping levels. The results indicate that anisotropy in the effective-mass tensor (or k-space dispersion) of the carriers changes systematically with hole doping, in contradiction with the simple band picture.

Crossover behavior of electronic structures from Mott insulator to Pauli metal has been investigated as a function of filling in titanium oxide compounds with perovskite-like structures. The role of the electron correlation manifests itself in filling-dependent enhancement of the density of states at the Fermi level, carrier scattering rate and spin susceptibility on the verge of the metal-insulator transition.

We have investigated effects of random substitution of proton sites with deuterons on the temperature- and pressure-induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of [C4O4]2- (SQ2-) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H1-xD(x))2SQ, the critical temperature T(c) for the dielectric phase transition was found to increase linearly with the deuterium concentration x, while the critical pressure P(c) increases nonlinearly with x at 300 K. Around the pressure approximately 20 kbar above P(c), the constituent SQ2- skeletons in the paraelectric phase show a symmetry change from the asymmetric (C1h) to centrosymmetric (C4h) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.