[en] Full text: Metallic ferromagnetism of Ni has attracted much interest for a long time. The electron correlation effects in the Ni 3d bands play an important role for the narrowing of the 3d bands and the appearance of the 6 eV satellite. In order to study manybody effects directly, we have done high-resolution, angle-resolved photoemission (PE) study of Ni(110) on the undulator beamline of a compact electron storage ring (HiSOR), Hiroshima University. By the quantitative analyses of the PE spectra of Ni, we evaluated the self-energy in which all of the manybody effects are included. We discuss spin- and wave-number dependence of the real and imaginary part of the self-energy

[en] Full text: Layer-structured nitride β-HfNCl is the parent insulator of high-T_c superconductors: when electrons are doped by intercalation of alkali metals between layers, the compound shows superconductivity at T_c 25.5 K for Li_0.48(THF)_yHfNCl. We have revealed the valence-band dispersion of β-HfNCl by angle-resolved photoemission (ARPES) spectroscopy using synchrotron radiation. As shown the second derivatives of the ARPES spectra well outline the valence-band dispersion. Some of the characteristic features of the obtained dispersion are consistent with the result of the FLAPW calculation. The valence-band maxima are located at Γ and K points at binding energies ∼ 3.2 eV and ∼ 3.6 eV below the Fermi level, respectively. On the other hand, the electrical conductivity experiments have suggested that the Fermi level is at donor impurity states formed about 0.18 eV below the bottom of the conduction band. Then, the magnitude of the band gap is suggested to be ∼ 3.4 eV and ∼ 3.8 eV for indirect and direct transitions, respectively. The present result demonstrated that the ARPES is also applicable to layer-structured nitrides and implies the possibilities for observation of the superconducting electronic state of this system

[en] The electronic band structures of 1T-TiTe₂ and Fe_0.25TiTe₂ have been clarified by angle-resolved resonant-photoemission spectroscopy. Upon Fe intercalation, while most of the band structure is unchanged, two new flat bands appear at the Fermi level and at E∼-0.5 eV. Constant-initial-state spectra indicate that these states are derived from both Ti 3d and Fe 3d states. While the Fermi wave number of the electron pocket around M point is reduced by Fe intercalation, those of hole pockets around Γ point are almost unchanged, suggesting that the additional electrons are accommodated by the new flat band at -0.5 eV

[en] Full text: The orthorhombic CeRhAs, known as a Kondo semiconductor, has attracted much interest for its unusual energy-gap formation associated with the successive 1st order phase transitions. In order to elucidate the mechanism of the energy- gap formation, we have done high-resolution temperature-dependent photoemission spectroscopy on the undulator beamlines of a compact electron-storage ring, HiSOR, at Hiroshima University. We have observed directly the energy-gap formation in the Ce 4f states and in the conduction bands. Comparing with the isostructural Kondo semimetal CeRhSb, we discuss the energy gap formation in CeRhAs

[en] Full text: have performed Ce 4d-4f resonant photoemission measurements (hv 122 eV) at temperatures from 5.6 to 200 K in order to investigate the relation between the Ce 4f electronic structure and the peculiar magnetic structure of CeSi. The photoemission measurements were done at BL-1 of HiSOR. Temperature dependent resonant photoemission spectra of CeSi are shown. The spectral intensity of Ce 4f_5/2¹ final state observed at 25 meV gradually increases as the temperature goes form 200 to 30 K. However, that at 5.6 K decreases in comparison with that at 30 K. This temperature dependence is consistent with that of the electric resistivity data. Furthermore, the photoemission spectra measured with He I (hv= 21.2 eV), in which the electronic structure of conduction electrons like Si 3p is dominantly observed, have also shown a spectral change similar to the result mentioned above. These suggest that the hybridization between the Ce 4f and conduction electrons may play an important role for the formation of magnetic structure

[en] The temperature-induced paramagnetism in LaCoO₃ around 100 K has long been known as a characteristic phenomenon of this compound, but its interpretation is not settled yet. One reason is that the low-spin (LS) ground state and other intermediate-spin (IS) or high-spin (HS) states cannot be resolved completely because such states are populated by thermal excitation. Here we present a first observation of a distinct change in the electronic structure due to a pure LS-IS transition of a Co oxide; Pr_0.5Ca_0.5CoO₃ exhibits a simultaneous LS-IS and insulator-metal first-order phase transition around 90 K with increasing temperature. Because of the first- order nature of the transition, the IS phase is not populated by thermal excitation, which enables us to investigate the electronic structure of the LS- and IS-Co 3d states, independently. Figure 1 shows temperature-dependent photoemission spec- tra of Pr_0.5Ca_0.5CoO₃. The leading peak A, which is Co 3d t 2g states, is rapidly suppressed from 70 K to 100 K. Compared with a theoretical calculation, this change should be representing the LS to IS spin-state transition. The observed change between the 'pure' LS and IS spectra will exclude the simple LS-HS scenario in LaCoO₃ and hence demonstrates the importance of the IS state in both excited states and the carrier-doped region

[en] The temperature dependences of the Yb 4f and 3d states of Kondo semiconductor YbB₁₂ single crystal have been studied by high-resolution photoemission spectroscopy taken at hν=100 and 5951 eV. The Yb²⁺4f_7/2 state at the Fermi level is enhanced on cooling. With improved energy resolution, we discover an additional quasi-particle peak at 15 meV that develops below 60 K. From the Yb 3d spectrum at 22 K, the Yb valence is estimated to be∼2.88

[en] A rotatable high-resolution angle-resolved photoemission spectroscopy (ARPES) system that utilizes tunable linear-polarization geometries on the linear undulator beamline BL-1 at Hiroshima Synchrotron Radiation Center is described. A rotatable high-resolution angle-resolved photoemission spectroscopy (ARPES) system has been developed to utilize tunable linear-polarization geometries on the linear undulator beamline (BL-1) at Hiroshima Synchrotron Radiation Center. By rotating the whole ARPES measurement system, the photoelectron detection plane can be continuously changed from parallel to normal against the electric field vector of linearly polarized undulator radiation. This polarization tunability enables us to identify the symmetry of the initial electronic states with respect to the mirror planes, and to selectively observe the electronic states based on the dipole selection rule in the photoemission process. Specifications of the rotatable high-resolution ARPES system are described, as well as its capabilities with some representative experimental results.

[en] Full text: Recent angle-resolved photoemission (ARPES) studies have revealed the abrupt change in nodal-band electron velocity at ∼ 70 meV below Fermi level (E_F), i.e., so-called iokinkle structure for all high-Tc cuprates studied so far including T-phase cuprate La_2-xSrxCuO₄, except for T'-phase cuprate Ne_2-xCe_xCuO₄. We have investigated the electronic structure of T*-phase cuprate SmLa_1-xSr_xCuO₄, whose crystal structure is the alternate stack of the T- and T'-phase structures. The kink structure has been observed in the nodal band dispersion as in other hole-doped cuprates. The coupling constant λ of the band renormalization appears to decrease with hole doping from x = 0.15 to x = 0.2. In the anti-nodal direction, a broad ( π, 0)-band structure observed around ∼ 200 meV for x = 0.15 is rapidly shifted toward E_F with hole doping. When the ( π, 0)-band crosses E_F for x 0.2, an antinodal kink clearly appears at ∼ 60 meV, as show. The present structure is almost unchanged with increasing temperature above T_c = 20 K, suggesting that this anti-nodal kink has the same origin as nodal kink

[en] We report the first observation of a distinct change in the electronic structure due to a 'pure' low-spin-to-intermediate-spin (LS-IS) transition accompanied by an insulator-metal transition in Pr_0.5Ca_0.5CoO₃ by high-resolution photoemission spectroscopy. With increase in T, the distinct Co 3d LS peak just below the Fermi level (E _F) rapidly collapses at ∼80 K, whose spectral weight is transferred into the very near-E _F region, forming small coherent spectral weight. In connection with a cluster-model analysis, this change can be interpreted as an LS-to-IS spin-state transition accompanied by a metal-insulator transition