Kazuhiko Kuroki

Osaka University, Suika, Ōsaka, Japan

Are you Kazuhiko Kuroki?

Claim your profile

Publications (254)603.21 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite decades of progress, an understanding of unconventional superconductivity still remains elusive. An important open question is about the material dependence of the superconducting properties. Using the quasiparticle self-consistent GW method, we re-examine the electronic structure of copper oxide high-Tc materials. We show that QSGW captures several important features, distinctive from the conventional LDA results. The energy level splitting between dx2-y2 and d3z2-r2 is signi?cantly enlarged and the van Hove singularity point is lowered. The calculated results compare better than LDA with recent experimental results from resonant inelastic xray scattering and angle resolved photoemission experiments. This agreement with the experiments supports the previously suggested two-band theory for the material dependence of the superconducting transition temperature, Tc.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Motivated by recent experiments on isovalent-doped 1111 iron-based superconductors ${\text{LaFeAs}}_{1$-${}x}{\text{P}}_{x}{\text{O}}_{1$-${}y}{\text{F}}_{y}$ and the theoretical study that followed, we investigate, within the five-orbital model, the correlation between spin fluctuations and the superconducting transition temperature, which exhibits a double-dome feature upon varying the Fe-As-Fe bond angle. Around the first dome with higher ${T}_{c}$, the low-energy spin fluctuation and ${T}_{c}$ are not tightly correlated because the finite-energy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the low-energy spin fluctuation originating from the ${d}_{xz/yz}$ orbital is correlated with ${T}_{c}$ in the second dome with lower ${T}_{c}$. These calculation results are consistent with a recent NMR study, and hence strongly suggest that the pairing in iron-based superconductors is predominantly caused by multiorbital spin fluctuations.
    Physical Review B 04/2015; 91(13). DOI:10.1103/PhysRevB.91.134511 · 3.66 Impact Factor
  • Hideo Hosono, Kazuhiko Kuroki
    [Show abstract] [Hide abstract]
    ABSTRACT: Since the discovery of high Tc iron-based superconductors in early 2008, more than 15,000 papers have been published as a result of intensive research. This paper describes the current status of iron-based superconductors (IBSC) covering most up-to-date research progress along with the some background research, focusing on materials (bulk and thin film) and pairing mechanism.
    Physica C Superconductivity 03/2015; DOI:10.1016/j.physc.2015.02.020 · 1.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Motivated by recent experiments on isovalent-doped 1111 iron-based superconductors LaFeAs$_{1-{x}}$P$_{x}$O$_{1-{y}}$F$_{y}$ and the theoretical study that followed, we investigate, within the five orbital model, the correlation between the spin fluctuation and the superconducting transition temperature, which exhibits a double dome feature upon varying the Fe-As-Fe bond angle. Around the first dome with higher $T_c$, the low energy spin fluctuation and $T_c$ are not tightly correlated because the finite energy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the low-energy spin fluctuation originating from the $d_{xz/yz}$ orbital is correlated with $T_c$ in the second dome with lower $T_c$. These calculation results are consistent with recent NMR study, and hence strongly suggest that the pairing in the iron-based superconductors is predominantly caused by the multi-orbital spin fluctuation.
  • Source
    Hidetomo Usui, Katsuhiro Suzuki, Kazuhiko Kuroki
    [Show abstract] [Hide abstract]
    ABSTRACT: Motivated by recent experimental investigations of the isovalent doping iron-based superconductors LaFe(AsxP1-x)O1-yFy and NdFe(AsxP1-x)O1-yFy we theoretically study the correlation between the local lattice structure, the Fermi surface, the spin fluctuation-mediated superconductivity, and the composition ratio. In the phosphides, the dXZ and dYZ orbitals barely hybridize around the Gamma point to give rise to two intersecting ellipse shape Fermi surfaces. As the arsenic content increases and the Fe-As-Fe bond angle is reduced, the hybridization increases, so that the two bands are mixed to result in concentric inner and outer Fermi surfaces, and the orbital character gradually changes to dxz and dyz, where x-y axes are rotated by 45 degrees from X-Y. This makes the orbital matching between the electron and hole Fermi surfaces better and enhances the spin fluctuation within the dxz/yz orbitals. On the other hand, the hybridization splits the two bands, resulting in a more dispersive inner band. Hence, there is a trade-off between the density of states and the orbital matching, thereby locally maximizing the dxz/yz spin fluctuation and superconductivity in the intermediate regime of As/P ratio. The consistency with the experiment strongly indicate the importance of the spin fluctuation played in this series of superconductors.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors.
    Physica C Superconductivity 12/2014; DOI:10.1016/j.physc.2015.02.022 · 1.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near ${\it E}_{\rm F}$ shows a square-like distribution around the $\Gamma$(Z) point in addition to electronlike Fermi surface (FS) sheets around the X(R) point, indicating that FS of LaO$_{0.54}$F$_{0.46}$BiS$_2$ is in close proximity to the theoretically-predicted topological change.
    Physical Review B 12/2014; 90(22). DOI:10.1103/PhysRevB.90.220512 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the Seebeck effect in the $\tau$-type organic conductors, $\tau$-(EDO-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $\tau$-(P-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$, where EDO-$S$,$S$-DMEDT-TTF and P-$S$,$S$-DMEDT-TTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform first-principles band calculation for the two materials and construct a two-orbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient $S$ is semi-quantitatively consistent with the experimental observation. In both materials, the absolute value of the Seebeck coefficient is maximum at a certain temperature, and this temperature is lower for NNSS than for OOSS. From a band structure viewpoint, we find that this can be traced back to the narrowness of the band gap between the upper and the lower pudding-mold type bands. On the other hand, the Seebeck coefficient of NNSS in the low temperature regime steeply increases with increasing temperature, which is due to the narrowness of the upper band. These differences in thermoelectric properties demonstrate the effectiveness of controlling the band structure through molecular modification.
    Journal of the Physical Society of Japan 08/2014; 83(10). DOI:10.7566/JPSJ.83.104705 · 1.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a theoretical understanding of the superconducting phase diagram of the electron-doped iron pnictides. We show that, besides the Fermi surface nesting, a peculiar motion of electrons, where the next nearest neighbor (diagonal) hoppings between iron sites dominate over the nearest neighbor ones, plays an important role in the enhancement of the spin fluctuation and thus superconductivity. In the highest T_{c} materials, the crossover between the Fermi surface nesting and this "prioritized diagonal motion" regime occurs smoothly with doping, while in relatively low T_{c} materials, the two regimes are separated and therefore results in a double dome T_{c} phase diagram.
    Physical Review Letters 07/2014; 113(2):027002. · 7.73 Impact Factor
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: We theoretically study the thermoelectric properties of electron-doped FeAs2 and hole-doped PtSe2 from a band-structure point of view using first-principles band calculations. The band structure of both materials has a peculiar band shape with a flat portion at the top (bottom) of the band, namely the pudding-mold-type band as found in Nax CoO2. The pudding-mold-type band has a quasi-one-dimensional nature in FeAs2 and a quasi-two-dimensional nature in PtSe2. We study the origins of the pudding-mold-type band and find that the $ d_{z^2} $ orbital in FeAs2, and not only the p z but also the p x and the p y orbitals in PtSe2 play an important role in making the pudding-mold-type band. We calculate the Seebeck coefficients by the Boltzmann equation approach using a tight-binding model constructed from first-principles band calculations, finding values close to experimental observations. The present study shows the general efficiency of the pudding-mold-type band. We suggest that an efficient route towards obtaining good thermoelectric materials is to realize ideal pudding-mold-type bands by modification of lattice structures.
    Journal of Electronic Materials 06/2014; 43(6). DOI:10.1007/s11664-013-2823-5 · 1.68 Impact Factor
  • Journal of the Physical Society of Japan 03/2014; 83(3):033703. DOI:10.7566/JPSJ.83.033703 · 1.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first principles, we have obtained a systematic correlation between the Fermi surface warping and the evaluated $T_c$ for various bilayer as well as single-layer cuprates. This reveals that smaller mixture of the $d_{z^2}$ orbital component on the Fermi surface leads to both of larger Fermi surface warping and higher $T_c$. The theoretical correlation strikingly resembles a systematic plot for the experimentally observed $T_c$ against the Fermi surface warping due to Pavarini {\it et al.} [Phys. Rev. Lett. {\bf 87}, 047003 (2001)], and the present result unambiguously indicates that the $d_{z^2}$ mixture is a key factor that determines $T_c$ in the cuprates.
    Physical Review B 03/2014; 89(22). DOI:10.1103/PhysRevB.89.224505 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use core level and valence band soft x-ray photoemission spectroscopy (SXPES) to investigate electronic structure of new BiS$_{2}$ layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$. Core level spectra of doped samples show a new spectral feature at the lower binding energy side of the Bi 4${f}$ main peak, which may be explained by core-hole screening with metallic states near the Fermi level ($E_{\rm F}$). Experimental electronic structure and its ${x}$ dependence (higher binding energy shift of the valence band as well as appearance of new states near $E_{\rm F}$ having dominant Bi 6${p}$ character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near $E_{\rm F}$ from that of calculations might give insight into the interesting physical properties. These results provide first experimental electronic structure of the new BiS$_{2}$ layered superconductors.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Application of physical pressure on the cuprate superconductors often results in an enhancement of Tc. Motivated by this fact, we study the chemical pressure effect on the single-layered La2CuO4 and HgBa2CuO4 starting from the two-orbital Hubbard model deduced from a first-principles calculation. It is shown that the chemical pressure effects induced by La-site substitution in La2CuO4 or Hg-site substitution in HgBa2CuO4 are not expected to be effective for raising Tc.
    Physics Procedia 01/2014; 58:34–37. DOI:10.1016/j.phpro.2014.09.009
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We perform first principles band calculation of electron doped iron-based superconductors adopting the virtual crystal approximation. We find that when electrons are doped by element substitution in the blocking layer, the band structure near the Fermi level is affected due to the increase of the positive charge in the layer. On the other hand, when Fe in the conducting layer is substituted by Co, the band structure is barely affected. This difference should be a key factor in understanding the phase diagram of the heavily doped electron doped systems LnFeAsO1-xHx.
    Physics Procedia 01/2014; 58:38–41. DOI:10.1016/j.phpro.2014.09.010
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ever since the discovery of high $T_c$ iron pnictide superconductors, there has been interest in the origin of the spin fluctuation, which is likely to be the Cooper pairing glue. The presence of disconnected electron and hole Fermi surfaces having similar shapes and sizes in the lightly carrier doped systems naturally suggests that the Fermi surface nesting is its origin. However, recent experiments on \textit{Ln}FeAsO$_{1-x}$H$_{x}$ (\textit{Ln}= La, Ce, Sm, Gd), where $T_c$ exceeds 50K in the largely electron doped regime with degraded nesting, have brought about a renewed interest on the spin fluctuation origin. In the present study, we show that the spin fluctuation there is enhanced by a peculiar motion of electrons due to the tetrahedral coordination of pnictogens ; the next nearest neighbor (diagonal) hoppings between iron sites dominate over the nearest neighbor ones. We argue that this "prioritized" diagonal motion of electrons plays a key role in the occurrence of the high $T_c$.
    Physical Review Letters 11/2013; 113(2). DOI:10.1103/PhysRevLett.113.027002 · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present an ab initio GW calculation to study dynamical effects on an organic compound (TMTSF)_{2}PF_{6}. Calculated polarized reflectivities reproduce experimental plasma edges at around 0.2 eV for E∥b^{′} and 1.0 eV for E∥a. The low-energy plasmons come out from the low-energy narrow bands energetically isolated from other higher-energy bands, and affect the low-energy electronic structure via the GW-type self-energy. Because of the quasi-one-dimensional band structure, a distinct plasmaron state is observed along the Y-Γ line and a large plasmon-induced electron scattering is found in the low-energy occupied states along the X-M line.
    Physical Review B 09/2013; 88(12). DOI:10.1103/PhysRevB.88.125128 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A potential thermoelectric material CuAlO2 is theoretically studied. We first construct a model Hamiltonian of CuAlO2 based on the first principles band calculation, and calculate the Seebeck coefficient. Then, we compare the model with that of a well-known thermoelectric material NaxCoO2, and discuss the similarities and the differences. It is found that the two materials are similar from an electronic structure viewpoint in that they have a peculiar pudding-mold type band shape, which is advantageous for thermoelectric materials. There are, however, some differences, and we analyze the origin of the difference from a microscopic viewpoint. The band shape (a very flat band top but with an overall wide bandwidth) of CuAlO2 is found to be even more ideal than that of NaxCoO2, and we predict that once a significant amount of holes is doped in CuAlO2, thermoelectric properties (especially the power factor) even better than those of NaxCoO2 can be expected.
    Physical review. B, Condensed matter 08/2013; 88(7). DOI:10.1103/PhysRevB.88.075141 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the thermoelectric properties of the electron-doped FeAs2 both experimentally and theoretically. Electrons are doped by partially substituting Se for As, which leads to a metallic behavior in the resistivity. A Seebeck coefficient of about −200 μV/K is reached at 300 K for 1% doping, and about −120 μV/K even at 5% doping. The origin of this large Seebeck coefficient, despite the metallic conductivity, is analyzed from a band structure point of view. The first-principles band calculation reveals the presence of a pudding-mold-type band just above the band gap, somewhat similar to NaxCoO2, but with a quasi-one-dimensional nature. We calculate the Seebeck coefficient using a tight-binding model that correctly reproduces this band structure, and this gives results roughly in agreement with the experiments. Moreover, a consideration of electron correlations beyond the generalized gradient approximation by the fluctuation exchange method gives even better agreement. The origin of this peculiar band shape is also discussed. Combined with previous studies, we now have good thermoelectric materials with quasi-one-, two-, and three-dimensional band structures that have partially flat portions. The present study reinforces the general efficiency of this peculiar band shape in thermoelectric materials.
    Physical review. B, Condensed matter 08/2013; 88(7). DOI:10.1103/PhysRevB.88.075140 · 3.66 Impact Factor

Publication Stats

4k Citations
603.21 Total Impact Points

Institutions

  • 2012–2015
    • Osaka University
      • Department of Physics
      Suika, Ōsaka, Japan
  • 2013
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 1990–2013
    • The University of Tokyo
      • Department of Physics
      Tōkyō, Japan
  • 2001–2012
    • The University of Electro-Communications
      • • Department of Engineering Science
      • • Department of Applied Physics and Chemistry
      Edo, Tōkyō, Japan
  • 2009–2011
    • Osaka Electro-Communication University
      Edo, Tōkyō, Japan
    • Tokyo Electron
      Edo, Tōkyō, Japan
  • 2003
    • Okayama University
      Okayama, Okayama, Japan
    • Nagoya University
      • Department of Quantum Engineering
      Nagoya, Aichi, Japan