Kozo Okazaki

In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional ga

We have made a detailed temperature-dependent photoemission study of ${\mathrm{VO}}_{2}/{\mathrm{TiO}}_{2}(001)$ thin films, which show a metal-insulator transition at \ensuremath{\sim}300 K. Clean surfaces were obtained by annealing the films in an oxygen atmosphere. Spectral weight transfer between the coherent and incoherent parts accompanying the metal-insulator transition was clearly observed. We also observed a hysteretic behavior of the spectra for heating-cooling cycles. We have derived

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta₂Ni(Se_1-xS_x)<sub>5</su

We have investigated the charge dynamics of ${\mathrm{VO}}_{2}$ by optical reflectivity measurements. Optical conductivity clearly shows a metal-insulator transition. In the metallic phase, a broad Drude-like structure is observed. On the other hand, in the insulating phase, a broad peak structure around $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is observed. It is found that this broad structure observed in the insulating phase shows a temperature dependence. We attribute this to the electron-

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC

We have measured Raman-scattering spectra of single-crystalline FeTe${}_{0.6}$Se${}_{0.4}$ (${T}_{c}~$ 14.5 K) and its parent compound Fe${}_{1.074}$Te at various temperatures. In the parent compound Fe${}_{1.074}$Te, ${A}_{1g}$ and ${B}_{1g}$ modes have been observed at 158 and 202 cm${}^{\ensuremath{-}1}$, respectively, at 5 K. These frequencies agree qualitatively with the calculated results. Two-magnon excitation has been observed around 2300 cm${}^{\ensuremath{-}1}$ for both compounds. A te

We study the superconducting-gap anisotropy of the Γ-centered hole Fermi surface in optimally doped FeTe(0.6)Se(0.4) (T(c)=14.5 K), using laser-excited angle-resolved photoemission spectroscopy. We observe sharp superconducting (SC) coherence peaks at T=2.5 K. In contrast to earlier angle-resolved photoemission spectroscopy studies but consistent with thermodynamic results, the momentum dependence shows a cos(4φ) modulation of the SC-gap anisotropy. The observed SC-gap anisotropy strongly indica

${\mathrm{TlGaTe}}_{2}$ has a quasi-one-dimensional crystal structure consisting of Tl and linked ${\mathrm{GaTe}}_{4}$ tetrahedral chains. We have studied the electronic band structure of ${\mathrm{TlGaTe}}_{2}$ by means of photoemission spectroscopy and band-structure calculation. Comparison between both results is rather favorable and energy bands show strong dispersion perpendicular to the chain direction. We have measured the temperature dependence of the spectra and attributed the observed

We have studied the electronic structure of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Na}}_{0.33}{\mathrm{V}}_{2}{\mathrm{O}}_{5},$ which becomes a superconductor under pressure, by angle-resolved photoemission spectroscopy. Clear band dispersions are observed only along the chain direction, indicating the quasi-one-dimensional (1D) electronic structure. The spectra of the V $3d$ band are dominated by a Gaussian-like broad feature at $\ensuremath{\sim}1\mathrm{eV}$ below the Fermi level ${(E}_{F

We have studied the temperature-dependent electronic structure of VO 2 in the insulating phase. The V 3 d and O 2 p bands become broader and their band edges are shifted toward the Fermi level ( E F ) with increasing temperature. The V 2 p and O 1 s core-level spectra also show a similar temperature dependence. These observations combined with optical spectra indicate that the position of E F relative to the conduction-band minimum is fixed and that the band gap changes mainly below E F . We als

Time- and angle-resolved photoemission spectroscopy has played an important role in revealing the non-equilibrium electronic structures of solid-state materials. The implementation of high harmonic generation to obtain a higher photon energy also allows us to investigate the wide Brillouin zone on a time scale below 100 fs. In this article, we review our recent studies using high-harmonic-generation-laser-based time- and angle-resolved photoemission spectroscopy to study a variety of quantum mat

We have measured the photoemission spectra of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sm}}_{x}{\mathrm{NiO}}_{3},$ where the metal-insulator transition and the N\'eel ordering occur at the same temperature for $x\ensuremath{\lesssim}0.4$ and the metal-insulator transition temperature ${(T}_{\mathrm{MI}})$ is higher than the N\'eel temperature for $x\ensuremath{\gtrsim}0.4.$ For $x&lt;~0.4,$ the spectral intensity at the Fermi level is high in the metallic phase above ${T}_{\mathrm{MI}}$ and gr

We investigate the transient electronic structure of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, a parent compound of iron-based superconductors, by time- and angle-resolved photoemission spectroscopy. In order to probe the entire Brillouin zone, we utilize extreme ultraviolet photons and observe photoemission intensity oscillation with the frequency of the ${A}_{1g}$ phonon which is antiphase between the zone-centered hole Fermi surfaces (FSs) and zone-cornered electron FSs. We attribute the antiph