Kensei Terashima

Okayama University · Research Institute for Interdisciplinary Science

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distort...

The BiS2-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the EuFBiS2 compound, showing a superconducting transition temperature Tc∼0.3 K at ambient pressure. Upon increasing external pressure, Tc goes through a large amplification, accompanied...

Stoichiometric EuFBiS2 with mixed valent Eu shows self-doped superconductivity at low temperatures due to the Eu-to-Bi charge transfer. The metallic/nonmetallic bistability of the BiS2 layer can couple with the valence fluctuation of Eu and provide a highly susceptible electronic state. Here, we report space-resolved photoemission measurements on E...

Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS2−xSex thermoelectric material. It is found that Se substitution effectively suppresses local distortion that can be responsible for the increased carrier mobili...

Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carri...

The local structures of 122-type paradium arsenides, namely BaPd2As2 and SrPd2As2, are examined by As K-edge extended x-ray absorption fine structure measurements to find a possible correlation between the variation of their superconducting transition temperature and the local structure. The local atomic distances are found to be consistent with av...

We present multilayer formation via spinodal decomposition in rutile TiO2-VO2 (TVO) epitaxial films on sapphire substrates. (001)- and (101)-oriented TVO solid-solution films are grown epitaxially on TiO2/Al2O3 using a pulsed laser deposition technique and annealed inside the spinodal region. X-ray diffraction measurements and scanning transmission...

Recently CeOBiS2 system without any fluorine doping is found to show superconductivity posing question on its origin. Using space resolved ARPES we have found a metallic phase embedded in the morphological defects and at the sample edges of stoichiometric CeOBiS2. While bulk of the sample is semiconducting, the embedded metallic phase is characteri...

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessit...

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal increase of electron carriers in the system with x (i.e. the excess Sn concentration) for 0 ≤ x...

We have studied the local structure of layered Nax(NH3)yMoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature ( ) of Mo-Mo dist...

Electronic structure has been studied in lightly electron doped correlated spin-orbit insulator Sr$_2$IrO$_4$ by angle-resolved photoelectron spectroscopy. We have observed coexistence of the lower Hubbard band and the in-gap band, the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-g...

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO2-VO2 system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as-prepared polycrystalline samples were annealed at 673 k inside the spinodal. For undoped samples, phase decomposition occurred along the [001]...

We have studied the effect of Se substitution on Eu valence in a layered Eu0.5La0.5FBiS2−xSex superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu L3-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting Eu2+ and Eu3+. The avera...

We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce 4f electronic states of a new BiS2 layered superconductor CeO1−xFxBiS2, for polycrystalline and single-crystal samples. The Ce 3d spectrum of the single crystal of nominal composition x=0.7 has no f0 component and the spectral shape closely resembles the ones observed for Ce tri...

We have investigated the electronic states and spin polarization of half-metallic ferromagnet CrO2 (100) epitaxial films by bulk-sensitive spin-resolved photoemission spectroscopy with a focus on non-quasiparticle (NQP) states derived from electron-magnon interactions. We found that the averaged values of the spin polarization are approximately 100...

We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estim...

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (Tc B 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L3 edge EXAFS reveals wide Bi–Te distance distribution for both compounds ind...

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key featu...

We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique, and can be represented as '(NH3)yMxMoSe2'. The Tcs of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)yNaxMoSe2 w...

Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Li x (NH 3) y Fe 2 Se 2 superconductor with T c ~ 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using...

BiS2-based layered superconductors are new superconductors, having been paid special attention due to the layered crystal structure and the characteristic electronic structure that can lead to exotic mechanism of superconductivity as well as cooperation of superconductivity and ordered phases. In this article, we report valence band dispersions and...

Bulk electronic structure of novel layered superconductor Nd(O,F)BiS2 was studied by using soft x-ray angle-resolved photoelectron spectroscopy (ARPES). Electron-like Fermi surface centered at the X(R) point was observed, consistent with earlier ARPES reports on surface-sensitive VUV light source. Based on the comparison of the electronic structure...

We performed soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements at Fe L2,3-edges and Ce M4,5-edges on Ce2Fe14B at 300 K. The valence and total atomic moment of Ce in Ce2Fe14B were determined to be ∼+3.12 and ∼0.100 μ B, respectively, which mainly stem from a large portion of the well-localized 4f 1 co...

TaO2 thin films were prepared using rutile-type NbO2 template layers on Al2O3(0001) substrates by a pulsed laser deposition technique. The structural and electronic properties of the films were characterized by performing four-circle X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. Four-circle XRD measurements reveal...

We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spect...

The electronic structure of nearly optimally-doped novel superconductor LaO$_{1-x}$F$_x$BiS$_2$ (${\it x}$ = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (${\it E}_{\rm F}$), which are well reproduced by first principles ca...

We show that the strongly correlated 4f-orbital symmetry of the ground state is revealed by linear dichroism in core-level photoemission spectra as we have discovered for YbRh2Si2 and YbCu2Si2. Theoretical analysis tells us that the linear dichroism reflects the anisotropic charge distributions resulting from crystalline electric field. We have suc...

A powder sample of body-centered tetragonal Ni2N in a single phase was prepared by a high rate-type triode dc reactive sputtering and the magnetic properties have been reinvestigated. It is confirmed that Ni2N is not a ferromagnet. However, the material is not a simple paramagnet either and shows a weak spin-glass-like behavior at low temperatures...

The bulk electronic structures of an iron-based superconductor FeSe have been investigated by photoelectron and photoabsorption spectroscopies (PES and XAS) with use of high-energy photons (hν ≊ 0.7-0.9 and 8 keV). The core-level PES and XAS combined with the configuration interaction cluster model calculation reveal the highly coherent nature of t...

We have succeeded in developing a method for photoemission electron microscopy (PEEM) on fully magnetized ferromagnetic bulk samples and have applied this technique to Dy-doped Nd-Fe-B permanent magnets. Remanence magnetization of the sample was approximately 1.2 T, and its dimension was 3 $ times$ 3 $times$ 3 mm $^{3}$ . By utilizing a yoke as an...

We report results of angle-resolved photoemission spectroscopy on a heavily electron-doped BaFe2−xCoxAs2. By using synchrotron-radiation light, we have investigated the band dispersion near the Fermi level (EF) of BaFe2−xCoxAs2 (x = 0.3) for different wave vectors perpendicular to the crystal surface (kz). We find that observed holelike band does n...

We have performed high-resolution angle-resolved photoemission spectroscopy of an electron-doped iron-based superconductor BaFe1.85Co0.15As2, and experimentally determined the Fermi-surface, the band structure and the superconducting gap. We found an isotropic s-wave-like superconducting gap on a hole pocket centered at the Γ point and on electron...

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy of (Bi,Pb)2Sr2CuO6 by using a newly developed xenon-plasma light source to clarify the origin of the pseudogap (PG). We determined the comprehensive momentum and temperature dependences of the superconducting (SC) gap and the PG, and revealed a smooth evolution of the...

We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the...

We report the fabrication of single-crystalline La-doped EuO thin films with a Curie temperature (T_C) of about 200 K, the highest among rare-earth compounds without transition metals. From first-principle band calculation and x-ray diffraction measurement, the observed increase in T_C cannot be explained only by the increas...

We have measured the temperature-dependent X-ray diffraction pattern of a single-crystalline EuO (100) thin film to investigate the change of the lattice constant due to the ferromagnetic phase transition. No structural transition was observed and the contracting rate of the lattice constant was dramatically increases below the ferromagnetic phase...

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy study of slightly-overdoped (Bi,Pb)2Sr2CuO6 to elucidate the origin of pseudogap. By using a newly developed xenon-plasma light source, we determined the comprehensive momentum and temperature dependences of the superconducting gap and the pseudogap. We found that t...

We report on the single-crystal growth of EuO thin films [fcc 1× 1 (100)] and their magnetic, optical, and photoelectrical properties to examine the consistency with the bulk materials. EuO thin films were fabricated on a BaO buffer layer created on a SrTiO3 substrate. The obtained EuO thin film was found to be a ferromagnetic semiconductor with a...

The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface(FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hol...

We have performed high-resolution angle-resolved photoemission spectroscopy of YBa2Cu3O7-delta with various hole concentrations from under-to-optimal doping. We have determined the momentum dependence of bulk superconducting gap as a function of hole concentration and found the enhancement of antinodal gap, the deviation from simple dx2-y2 wave, an...

Much excitement has surrounded the recent discovery of the doped iron pnictides which exhibit high temperature superconductivity. These new materials are generally grouped into either 1111 or 122 compounds by the stoichiometric formula of the parent compounds. Understanding how doping with in-plane elements with different valence affects their supe...

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2. We find that the two hole Fermi surface pockets at the Brillouin zone center observed in the hole-doped superconducting Ba0.6K0.4Fe2As2 are absent or very small in this compound, while the two electron pock...

We have performed high-resolution angle-resolved photoemission spectroscopy on hole-doped high-Tc cuprate superconductors (HTSCs) to study many-body interaction and the universality of low-energy excitation gap. In Bi2Sr2CaCu2O8 (Bi2212), we observed a kink in the dispersion in the off-nodal region in the superconducting state, which remarkably wea...

We have performed systematic ultrahigh-resolution photoemission spectroscopy on the iron-based superconductor LaFeAsO1-xFx (x = 0-14%) to elucidate the evolution of electronic states with doping. We found that the density of states (DOS) around the Fermi level of the superconducting sample is markedly smaller than that of the undoped sample in cont...

We have performed ultrahigh energy- and momentum-resolved angle-resolved photoemission spectroscopy of impurity-doped Bi2Sr2CaCu2O8. We found that the point node of the superconducting gap near the (π/2,π/2) point in a pristine sample gradually evolves into a small gapless Fermi arc upon Zn or Ni doping. The shape of impurity-induced density of sta...

We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O0.93F0.07)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s-or axial p-wave case, while it is 4.1 meV in the polar p-or d-...

High-resolution angle-resolved photoemission spectroscopy has been performed on Zn-substituted La1.85Sr0.15CuO4 to study the impurity effect on the electronic structure near the Fermi level. Upon Zn substitution, the kink in the band dispersion at 20 meV and the coherence peak in the off-nodal region in the superconducting state are significantly w...

We have performed high-resolution angle-resolved photoemission spectroscopy on high-Tc superconductors La1.85Sr0.15CuO4 (LSCO) and Bi2Sr2CaCu2O8+delta (Bi2212) to study the universality of many-body interaction in hole-doped high-Tc superconductors. We found that the emergence of kink in the dispersion in the off-nodal region at the superconducting...

We report angle-resolved photoemission spectroscopy on optimally doped high-Tc superconductor La1.85Sr0.15CuO4 (Tc = 37 K) to elucidate the character of low-energy excitations in the superconducting state. We find a clear evidence for the emergence of a sharp superconducting quasiparticle peak below Tc together with opening of a superconducting gap...

We report angle-resolved photoemission results on high-Tc superconductor La1.85Sr0.15CuO4. A sharp Bogoliubov quasiparticle peak and a dispersion kink at 20 meV are observed in the superconducting state at off-nodal region. Present experimental result suggests a coupling between electrons and the incommensurate magnetic resonance.

To study the superconducting-gap symmetry in electron-doped HTSC, we performed a high-resolution ARPES on Pr0.89LaCe0.11CuO4 (optimally-doped, Tc=26K). Obtained momentum dependence of the superconducting gap is basically consistent with the dx2-y2-wave symmetry, but obviously deviates from the monotonic form of the dx2-y2 gap function. The maximum...

We studied the impurity effect on the electronic structure near the Fermi level in Bi2Sr2CaCu2O8+delta by high-resolution angle-resolved photoemission spectroscopy to clarify the origin of dispersion kink in high-T-c superconductors. We found that the dispersion kink at off-nodal region is significantly suppressed by Zn or Ni impurity while that at...

We performed high-resolution angle-resolved photoemission spectroscopy on La1.85Sr0.15CuO4 to study the nature of the single-particle excitation gap. We found that there is a well-defined superconducting coherence peak in the off-nodal region while it is strongly suppressed around the antinode. The momentum dependence of the single-particle excitat...

We find that peaks in the autocorrelation of angle-resolved photoemission spectroscopy data of Bi2Sr2CaCu2O8+δ in the superconducting state show dispersive behavior for binding energies smaller than the maximum superconducting energy gap. For higher energies, though, a striking anomalous dispersion is observed that is a consequence of the interacti...

We find that peaks in the autocorrelation of angle-resolved photoemission spectroscopy data of Bi2Sr2CaCu2O8+δ in the superconducting state show dispersive behavior for binding energies smaller than the maximum superconducting energy gap. For higher energies, though, a striking anomalous dispersion is observed that is a consequence of the interacti...

The doping dependence of electronic states in an electron-doped high-temperature superconductor (HTSC) Nd2-xCexCuO4 was studied by high-resolution angle-resolved photoemission spectroscopy. We observed that the high-energy pseudogap around the hot spot shows an abrupt filling-in at the magnet-superconductor phase boundary, resulting in the unusual...

We find that peaks in the autocorrelation of angle resolved photoemission spectroscopy data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ in the superconducting state show dispersive behavior for binding energies smaller than the maximum superconducting energy gap. For higher energies, though, a striking anomalous dispersion is observed that is a consequen...

We have performed high-resolution angle-resolved photoemission spectroscopy on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-Tc cuprates. We unambiguously identified surface-bil...

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6+δ to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2×2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperatur...

The autocorrelation of angle resolved photoemission data from the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) shows distinct peaks in momentum space which disperse with binding energy in the superconducting state, but not in the pseudogap phase. Although it is tempting to attribute a nondispersive behavior in momentum space to charg...

Systematic angle-resolved photoemission spectroscopy (ARPES) has been performed on Bi2Sr2Can−1CunO2n+4 (n=1–3). For n=2 and 3, the dispersion kink becomes pronounced on approaching (π, 0) below Tc, while a kink appears only around the nodal direction above Tc. This indicates that the coupling of electrons with magnetic resonance mode plays a domina...

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on Co-, Ni-, and Zn-substituted Bi2Sr2CaCu2O8+δ (Bi2212) to study the impurity effect on the electronic structure. The shape of Fermi surface and the symmetry of superconducting gap are unchanged by impurities, while the superconducting coherent peak around (π, 0) i...

We report high-resolution angle-resolved photoemission spectroscopy on electron-doped cuprate Nd2−xCexCuO4 to study the quasiparticle (QP) state in the vicinity of the Fermi level. Measurements of momentum dependence have revealed that the mass-renormalized QP state around (π, 0) continuously evolves into the high-energy correlation gap, which open...

We studied the impurity effect on the electronic structure near the Fermi level in Bi2Sr2CaCu2O8+δ by high-resolution angle-resolved photoemission spectroscopy to clarify the origin of dispersion kink in high-Tc superconductors. We found that the dispersion kink at the off-nodal region is significantly suppressed by Zn or Ni impurity while that at...

Despite years of intensive research on copper oxide superconductors with high transition temperatures (Tc), the driving force of superconductivity has not yet been clarified. Angle-resolved photoemission spectroscopy (ARPES) 1-3 measurements have uncovered an important contribution of lattice vibrations (phonons) to superconductivity, sparking a fi...

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2- wave symmetry, but obviously deviates from the monotonic dx2-y2- gap function. The maximum ga...

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on Nb1−xTixXc2 (Xc = S, Se, Te) to elucidate the change in the band structure as a function of x and Xc. We found that spectral intensity near EF of S- and Se-based compounds is remarkably suppressed, suggesting the strong scattering of electrons by the substitutio...

We have performed systematic and comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on Bi-system high-Tc superconductors to study the quasi-particles related to and/or responsible for the occurrence of the superconductivity. We have experimentally determined the full energy dispersion and the coherence factors of the Bo...

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi,0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to...

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation on layered transition-metal dichalcogenide 1T-VSe2 to study the mechanism of the three-dimensional charge-density-wave (CDW) transition. We found that the Fermi surface around the M(L) and K(H) points shows considerable wiggling along the...

We have performed systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) on layered transition-metal dichalcogenides Nb1−xTixXc2 (Xc = S, Se, Te) to study the mechanism of metal–semiconductor transition as a function of x and Xc. We found a pseudogap near EF in the S- and Se-based compounds while a clear Fermi edge is observed...

We have performed a high-resolution angle-resolved photoemission spectroscopy (ARPES) on high-Tc superconductors (HTSCs) Bi2Sr2Can-1CunO2n+4 (n=1–3) to study the systematics of the electronic structure and the origin of many-body interactions responsible for superconductivity. We find that a large hole-like Fermi surface and d-wave superconducting...

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of Co-, Ni-, and Zn-substituted Bi2Sr2CaCu2O8+δ (Bi2212) to study the impurity effect on the electronic structure. In all compounds, we observed a remarkable suppression of the intensity of coherent peak around (π, 0) point at the superconducting state, consistent w...

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on a layered transition-metal dichalcogenide (TMDC) 1T-VSe2 to study the (4×4) charge-density wave (CDW) mechanism. We observed a partial Fermi-surface (FS) nesting on the electronlike FS centered at the M (L) point. The spectral weight near EF is considerably supp...