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We have performed systematic ultrahigh-resolution photoemission spectroscopy on the iron-based superconductor LaFeAsO 1- x F x ( 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 contrast to conventional phonon-mediated superconductors. We observed a pseudogap in the DOS above the superconducting transition temperature ( T c ) which opens/closes at a temperature ( T * ) well above T c . The energy scale of pseudogap and the T * value are gradually reduced upon fluorine substitution. The origin of pseudogap is discussed in terms of the partial nesting of Fermi surfaces due to the spin-density-wave formation or the short-range spin correlations.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation on layered transition-metal dichalcogenide 1 T -VSe 2 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 wave vector normal to the layer ( k z ) despite the apparently two-dimensional crystal structure. ARPES spectra below the CDW transition temperature show a signature of pseudogap opening at a specific k z -region on the Fermi surface, demonstrating that the CDW transition of 1 T -VSe 2 is driven by three-dimensional Fermi-surface nesting.

We have performed high-resolution photoemission spectroscopy on newly discovered novel boride compounds, MgB 2 , La-doped CaB 6 , and DyB 2 C 2 . In MgB 2 , we observed the superconducting gap of about 4.5 meV with s -wave symmetry. In La-doped CaB 6 , we found a 1 eV energy gap at the X point in the Brillouin zone, in contrast with the LDA band calculation but in good agreement with the GW calculation. This requests reexamination of the excitonic-insulator model based on an overlap of an electron and a hole band at the X point. In DyB 2 C 2 , we found two (one electron-like and one hole-like) Fermi surfaces, which are composed of the mixed states of the B2 p , C2 p and Dy5 d orbitals, below the antiferro-quadrupolar-ordering temperature.

We studied the effect of strain, interface, and electron doping on the superconductivity in thin FeSe film on SrTiO3 by angle-resolved photoemission spectroscopy (ARPES). We observed the superconductivity with Tc as high as 43 K in a lithium (Li) deposited heavily electron-doped multilayer FeSe film. We found a significant relaxation of tensile strain at the surface in contrast to the case of potassium (K) deposition, although the high-Tc superconductivity is commonly observed in both cases. We discuss the interplay among carrier doping, tensile strain, and high-Tc superconductivity by comparing ARPES results on Li- and K-deposited FeSe films.

We present a comprehensive characterization of the electronic structure of anisotropic layered semiconductor In4Se3 by means of high-resolution angle-resolved photoemission spectroscopy. Our study reveals the quasi-one-dimensionality in the electronic structure of In4Se3 and provides a direct estimate of its band gap (0.86 ± 0.05 eV). The identification of the quasi-one-dimensional electronic structure gives new insights into the electronic contribution to the high thermoelectric figure of merit of In4Se3.

We have performed high-resolution angle-resolved photoemission spectroscopy on Rb-intercalated bilayer graphene, which is regarded as the thinnest limit of Rb-intercalated graphite (Rb-GIC). We have clearly observed a velocity renormalization of the π* bands near the Fermi level, and electron doping from Rb atoms to the graphene layers. We attribute the renormalization to strong coupling of electrons with graphene’s in-plane optical phonon mode. The electron–phonon coupling constant has been measured, and found to exhibit a strong momentum anisotropy similar to that of bulk GIC superconductors. This result provides evidence for universal electron–phonon coupling in intercalated multi-layer graphene and bulk GICs.

We have performed angle-resolved photoemission spectroscopy of quasi-free-standing bilayer graphene epitaxially grown on silicon carbide. Prepared bilayer graphene shows a semiconducting behavior with a finite energy gap at the Fermi level in contrast to the theoretical prediction. We found that potassium-deposition on the sample leads to the semiconductor-to-metal transition together with the enhancement of the gap energy between the π and π * bands. The observed controllable tuning of the Fermi-level position and the gap energy provides an important step toward the band-gap engineering with bilayer graphene.

We have performed low-energy high-resolution photoemission spectroscopy of a superconducting cement 12CaO·7Al 2 O 3 (C12A7). By using the bulk-sensitive xenon-plasma light, we clearly observed a small but well-defined peak at the Fermi level ( E F ). Comparison with the band calculations suggests that the observed near- E F peak is ascribed to the conduction electrons distributed over the crystal cages responsible for the metallic nature and superconductivity of C12A7.

We have performed high-resolution photoemission spectroscopy of a rare-earth inter-metallic compound SmNiC 2 which undergoes a charge-density-wave (CDW) transition at T CDW = 148 K and a ferromagnetic transition at T c = 17.5 K. We have determined the valence-band and near-E F (Fermi level) electronic states, and observed a marked suppression of the spectral density of states (DOS) within 60-70 meV with respect to E F below T CDW , indicative of a partial gap (pseudogap) opening due to the CDW formation. Finite DOS at E F in the CDW state suggests an imperfect nature of the Fermi-surface nesting.

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 5 p 3/2 and Ce 5 d 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 antiferro-magnetic 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 report high-resolution photoemission spectroscopy (PES) of quasi one-dimensional (1D) metal NaV 2 O 4 . We found that the valence band consists of well-separated two main bands; one originates in the V 3 d states located around the Fermi level ( E F ) and the other is due to the O 2 p states situated far away from E F in qualitatively good agreement with the band calculation. The PES spectrum near E F shows a power-law behavior characteristic of quasi-1D materials. The anomalous exponent α shows a considerable temperature dependence, in particular around the magnetic transition temperature. The temperature dependence of α is also consistent with the variation of the transport anisotropy with temperature. These results suggest a close relationship between the electronic structure near E F and the magnetic order and/or the dimensional crossover.

We have performed high-resolution angle-resolved photoemission spectroscopy on high- T c superconductors La 1.85 Sr 0.15 CuO 4 (LSCO) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) to study the universality of many-body interaction in hole-doped high- T c superconductors. We found that the emergence of kink in the dispersion in the off-nodal region at the superconducting state is a common feature, while the energy scale of kink is remarkably different between LSCO and Bi2212. We conclude that the coupling of electrons with spin excitations at the off-nodal region is universal in hole-doped high- T c cuprates.