We found that a surface state induced by hydrogenation on the surface of SrTiO3(001) (STO) did not obey the rigid-band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconductor behavior in the temperature dependence of electronic conductivity is explained by the thermal activation of carriers. The HDS and the conduction band are nonrigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 60 meV below the conduction-band minimum. The sheet conductivity, accordingly, also saturates at ∼1.0μS□ with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.

Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials.

We have studied the electronic band structure of the hydrogen-terminated Si(110)-(1 x 1) [H:Si(110)-(1 x 1)] surface using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations in the framework of density functional theory with local density approximation (LDA). The bulk-truncated H: Si(110)-(1 x 1) surface is a good template to investigate the electronic band structure of the intrinsic Si(110). In the ARPES spectra, seven bulk states and one surface state due to the H-H interaction are observed clearly. The four bulk states consisting of Si 3p(xy) orbitals exhibit anisotropic band dispersions along the high symmetric direction of (Gamma) over bar-(X) over bar and (Gamma) over bar-(X) over bar' directions, where one state shows one-dimensional character. The calculated band structures show a good agreement with the experimental results except the surface state. We discuss the exact nature of electronic band structures and the applicability of LDA. We have estimated the anisotropic effective masses of electrons and holes of Si(110) for device application.

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angleresolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 x 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 +/- 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of similar to 100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future topotronics devices.

<p>We have fabricated alkali metal (Li, Rb, Cs) and alkaline-earth metal (Ca) intercalated bilayer graphene on SiC substrate, and characterized them by low-energy electron diffraction, angle-resolved photoemission spectroscopy, and 4-point-probe measurements. We observed a free-electron-like state in the center of the Brillouin zone, called &ldquo;interlayer state&rdquo;, as well as the folded &pi;/&pi;^* bands in Rb-, Cs-, and Ca-intercalated graphene, while it was absent in Li counterpart. Ca-intercalated bilayer graphene shows the zero-resistance below 4K, indicative of the two-dimensional superconductivity. These results suggest that the interlayer state plays an important role for the superconductivity in intercalated bilayer graphene.</p>

The topology of pure Bi is controversial because of its very small (similar to 10 meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14-202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to similar to 10 meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach.

We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C6CaC6, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point probe method in ultrahigh vacuum under zero- or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C6LiC6) and C6CaC6 fabricated on SiC substrate. We observed that the zero-resistance state occurs in C6CaC6 with the onset temperature (T-c(onset)) of 4 K, while the T-c(onset) is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. On the other hand, both pristine bilayer graphene and C6LiC6 exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.

<p>A surface reconstruction consisting of one monolayer of Tl and one-third monolayer of Pb on a Si(111) was found to exhibit a large Rashba-type spin splitting in the metallic surface-state bands together with two-dimensional superconductivity. Temperature dependent angle-resolved photoelectron spectroscopy revealed a strong electron-phonon interaction in one of the bands. In situ four-point-probe resistivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transited into the superconducting state at 2.3 K. The (Tl, Pb)/Si(111) is a prototypical system for prospective studies of intriguing properties of the space-inversion-symmetry-broken superconductor.</p>

A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having root 3 x root 3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.

We report high-resolution spin- and angle-resolved photoemission spectroscopy of bismuth thin film on Si(1 1 1) to discuss the spin structure of surface states. We found that, unlike conventional picture of the Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric between two hole pockets across the Brillouin-zone center. In addition, these pockets exhibit a giant out-of-plane spin polarization as large as the in-plane counterpart, which changes the sign across the (Gamma) over bar (M) over bar line. We also revealed that the spin polarization of the Rashba surface states near the Brillouin-zone boundary (the (M) over bar point) is gradually reduced on decreasing the film thickness. Our result provides a new pathway to generate and control spin-polarized electrons in the Rashba system. (C) 2014 Elsevier B.V. All rights reserved.

It is well known that a topologically protected gapless state appears at an interface between a topological insulator and an ordinary insulator; however, the physics of the interface between a topological insulator and a metal has largely been left unexplored. Here we report a novel phenomenon termed topological proximity effect, which occurs between a metallic ultrathin film and a three-dimensional topological insulator. We study one bilayer of bismuth metal grown on the three-dimensional topological insulator material TlBiSe2, and by using spin-and angle-resolved photoemission spectroscopy, we found evidence that the topological Dirac-cone state migrates from the surface of TlBiSe2 to the attached one-bilayer Bi. We show that such a migration of the topological state occurs as a result of strong spin-dependent hybridization of the wave functions at the interface, which is also supported by our first-principles calculations. This discovery points to a new route to manipulating the topological properties of materials.

To realize a one-dimensional (1D) system with strong spin-orbit coupling is a big challenge in modern physics, since the electrons in such a system are predicted to exhibit exotic properties unexpected from the 2D or 3D counterparts, while it was difficult to realize genuine physical properties inherent to the 1D system. We demonstrate the first experimental result that directly determines the purely 1D band structure by performing spin-resolved angle-resolved photoemission spectroscopy of Bi islands on a silicon surface that contains a metallic 1D edge structure with unexpectedly large Rashba-type spin-orbit coupling suggestive of the nontopological nature. We have also found a sizable out-of-plane spin polarization of the 1D edge state, consistent with our first-principles band calculations. Our result provides a new platform to realize exotic quantum phenomena at the 1D edge of the strong spin-orbit-coupling systems.

We have performed spin- and angle-resolved photoemission spectroscopy of antimony (Sb) and bismuth (Bi) thin films grown on Si(111) to elucidate the nature of the Rashba effect in the spin-split surface bands. In Sb, we revealed spin polarization with the in-plane vortical texture on an elongated hole-like Fermi surface. The spin polarization is strongly momentum-dependent, almost vanishing at the region away from the Brillouin zone center. Such unusual suppression of the spin polarization is not observed in Bi, pointing to a strong influence from the quantized bulk bands on the surface spin polarization in Sb. The present result strongly suggests that bulk-surface interband scattering should be properly taken into account to understand the Rashba surface states in group-V semimetals.

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)2Te4 and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)2Te4, Bi2Te3, Bi2Se3, and TlBiSe2] and elucidated that the out-of-plane spin polarization Pz is systematically correlated with the warping strength. However, the magnitude of Pz is found to be only half that expected from the k·p theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators. © 2014 American Physical Society.

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)(2)Te-4 and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)(2)Te-4, Bi2Te3, Bi2Se3, and TlBiSe2] and elucidated that the out-of-plane spin polarization P-z is systematically correlated with the warping strength. However, the magnitude of P-z is found to be only half that expected from the k . p theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.

We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S1-xSex)(2) which undergoes a topological phase transition at x similar to 0.5. In TlBiSe2 (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of similar to 0.8. The spin polarization still survives in the gapped surface states at x &gt; 0: 5, although it gradually weakens upon approaching x = 0.5 and vanishes in the nontopological phase. No evidence for the out-of-plane spin polarization was found, irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions.

The authors have developed an ultrahigh-resolution spin-resolved photoemission spectrometer equipped with a highly efficient mini Mott detector and a high-intensity xenon plasma discharge lamp. An electron deflector situated between the hemispherical electron-energy analyzer and the Mott detector enables the determination of the electron's spin-polarization in three independent directions and the spectrometer achieves an energy resolution of 0.9 and 8 meV for nonspin-resolved and spin-resolved modes, respectively. By using this spectrometer, we have performed spin-and angle-resolved photoemission spectroscopy of bismuth thin films on Si(111) to investigate the spin structure of surface states. Unlike conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric across the zone center between the two elongated surface hole pockets and there is a giant out-of-plane spin polarization. The authors discuss these unusual spin textures in terms of a possible time-reversal symmetry breaking. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4731467]

We performed a spin- and angle-resolved photoemission spectroscopy of bismuth ultrathin film on Si(111) with various film thickness d. We found that the spin polarization of spin-split Rashba surface states near the Brillouin-zone boundary, which is high (0.7) at d = 40 BL (bilayers), is gradually reduced on decreasing d and almost vanishes at d = 8 BL. This finding provides a novel method to generate spin-polarized electrons with tunable spin-polarization.

The authors have developed an ultrahigh-resolution spin-resolved photoemission spectrometer equipped with a highly efficient mini Mott detector and a high-intensity xenon plasma discharge lamp. An electron deflector situated between the hemispherical electron-energy analyzer and the Mott detector enables the determination of the electron's spin-polarization in three independent directions and the spectrometer achieves an energy resolution of 0.9 and 8 meV for nonspin-resolved and spin-resolved modes, respectively. By using this spectrometer, we have performed spin- and angle-resolved photoemission spectroscopy of bismuth thin films on Si(111) to investigate the spin structure of surface states. Unlike conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric across the zone center between the two elongated surface hole pockets and there is a giant out-of-plane spin polarization. The authors discuss these unusual spin textures in terms of a possible time-reversal symmetry breaking. © 2012 American Vacuum Society.

We performed a systematic, high-resolution, angle-resolved photoemission spectroscopy of Bi1-x Sb-x alloys to study the evolution of electronic states as a function of Sb concentration x. We revealed that the number of Fermi-level crossings is different between the topologically trivial (x = 0.023) and nontrivial (x = 0.17) phases: four and five times for the former and latter cases, respectively. We also found a systematic expansion of the bandwidth of spin-split surface bands at the (Gamma) over bar point upon substituting Bi with Sb, which is indicative of the monotonous reduction of the spin-orbit coupling strength.

We have performed spin-and angle-resolved photoemission spectroscopy of Bi2Te3 and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface of Bi2Te3 reaches maximally 25% of the in-plane counterpart, while such a sizable out-of-plane spin component does not exist in the more circular Fermi surface of TlBiSe2, indicating that the hexagonal deformation of the Fermi surface is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from the existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi2Te3.

We have performed high-resolution spin-and angle-resolved photoemission spectroscopy of bismuth thin film on Si(111) to investigate the spin structure of surface states. Unlike the conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric between the two elongated surface hole pockets across the zone center. Moreover, we uncovered a giant out-of-plane spin polarization as large as the in-plane counterpart which switches the sign across the (Gamma) over bar-(M) over bar line. We discuss the present finding in terms of the symmetry breaking and the many-body effects.

We have developed an ultrahigh-resolution spin-resolved photoemission spectrometer with a highly efficient mini Mott detector and an intense xenon plasma discharge lamp. The spectrometer achieves the energy resolutions of 0.9 and 8 meV for non-spin-resolved and spin-resolved modes, respectively. Three-dimensional spin-polarization is determined by using a 90 degrees electron deflector situated before the Mott detector. The performance of spectrometer is demonstrated by observation of a clear Rashba splitting of the Bi(111) surface states. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3480542]

We have performed low-energy ultrahigh-resolution angle-resolved photoemission spectroscopy on CeSb to elucidate the bulk electronic structure responsible for the magnetic phase transition. By using a newly developed xenon-plasma discharge lamp, we clearly observed the splitting of Sb 5p(3/2) and Cc 5d bands near the Fermi level as well as their strong hybridization below the Neel temperature (T-N = 16 K). We also revealed distinct differences in the band structure between the antiferropara- and antiferromagnetic phases. These experimental results are discussed in terms of the p-f/p-d mixing effect, the exchange splitting, and the magnetic band-folding. The present result also explains the emergence of fine structures in the optical conductivity in the magnetic phase.

We have performed a high-resolution angle- resolved photoelectron spectroscopy study on the newly discovered superconductor Ba(0.6)K(0.4)Fe(2)As(2) (T(c) = 37 K). We have observed two superconducting gaps with different values: a large gap (Delta similar to 12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (similar to 6meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (T(c)), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2 Delta/k(B)T(c) switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (pi, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets. Copyright (c) EPLA, 2008.