Kazuhiko Kuroki

Osaka University, Suika, Ōsaka, Japan

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Publications (234)571.85 Total impact

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    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.
    08/2014;
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    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
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    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). · 1.64 Impact Factor
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    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.
    03/2014;
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    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.
    01/2014;
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    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 $T_c$.
    11/2013;
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    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). · 3.66 Impact Factor
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    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). · 3.77 Impact Factor
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    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). · 3.77 Impact Factor
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    ABSTRACT: Motivated by a recent observation of good thermoelectric properties in %a %pyrite material PtSb$_2$, we theoretically study related pyrites %materials in an attempt to %find a %better thermoelectric material. %Our aim is to seek for a material which overcomes the suppression of the Seebeck coefficient at high temperatures.% in PtSb$_2$. We find that PtAs$_2$ and PtP$_2$ are good candidates, where a larger band gap than in PtSb$_2$ combined with the overall flatness of the band top gives rise to a monotonically increasing Seebeck coefficient up to high temperatures. This expectation has been confirmed quite recently for hole doped PtAs$_2$, where a very large power factor of $\sim$ 65$\mu$W/cmK$^2$ at T=440 K is observed.
    08/2013;
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    ABSTRACT: We investigate the doping dependence of the magnetic excitations in two-superconducting-dome-system LaFeAsO1-xDx. Using inelastic neutron scattering, spin fluctuations at different wavenumbers were observed under both superconducting domes around x = 0.1 and 0.4, but vanished at x = 0.2 corresponding to the Tc valley. Theoretical calculations indicate that the characteristic doping dependence of spin fluctuations is rationally explained as a consequence of the switching of the two intra-orbital nestings within Fe-3dYZ, ZX and 3dX2-Y2 by electron doping. The present results imply that the multi-orbital nature plays an important role in the doping and / or material dependence of the Tc of the iron pnictide superconductors.
    Physical Review B 07/2013; 88(6). · 3.66 Impact Factor
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    ABSTRACT: Recently, a very rich phase diagram has been obtained for an iron-based superconductor Ca4Al2O6Fe2(As1−xPx)2. It has been revealed that nodeless (x∼0) and nodal (x=1) superconductivity are separated by an antiferromagnetic phase. Here we study the origin of this peculiar phase diagram using a five orbital model constructed from first-principles band calculation, and applying the fluctuation exchange approximation assuming spin-fluctuation-mediated pairing. At x=1, there are three hole Fermi surfaces, but the most inner one around the wave vector (0,0) has strong dX2−Y2 orbital character, unlike in LaFeAsO, where the most inner Fermi surface has dXZ/YZ character. Since the Fermi surfaces around (0,0), (π,0), and (π,π) all have dX2−Y2 orbital character, the repulsive pairing interaction mediated by the spin fluctuations gives rise to a frustration in momentum space, thereby degrading superconductivity despite the bond angle being close to the regular tetrahedron angle. As x decreases and the bond angle is reduced, the inner hole Fermi surface disappears, but the frustration effect still remains because the top of the band with dX2−Y2 character lies close to the Fermi level. On the other hand, the loss of the Fermi surface itself gives rise to a very good nesting of the Fermi surface because the number of electron and hole Fermi surfaces are now the same. The pairing interaction frustration and the good nesting combined favors antiferromagnetism over superconductivity. Finally for x close to 0, the band sinks far below the Fermi level, reducing the frustration effect, so that superconductivity is enhanced. There, the Fermi surface nesting is also lost to some extent, once again favoring superconductivity over antiferromagnetism. To see whether the present theoretical scenario is consistent with the actual nature of the competition between superconductivity and antiferromagnetism, we also perform hydrostatic pressure experiment for Ca4Al2O6Fe2(As1−xPx)2. In the intermediate x regime where antiferromagnetism occurs at ambient pressure, applying hydrostatic pressure smears out the antiferromagnetic transition, but superconductivity does not take place. This supports our scenario that superconductivity is suppressed by the momentum space frustration in the intermediate x regime, apart from the presence of the antiferromangnetism.
    Physical review. B, Condensed matter 05/2013; 87(17). · 3.77 Impact Factor
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    ABSTRACT: Using the dx2-y2+dz2 two orbital model of the high Tc cuprates obtained from the first-principle calculation, we show that the material dependence of the Fermi surface shape can be understood by the degree of the mixture between the dx2-y2$ and the dz2 orbitals. We explain, through investigating the tightbinding hopping integrals, why some cuprates have square shaped Fermi surface, while others have more rounded ones. From this viewpoint, we explain the experimentally observed correlation between the curvature of the Fermi surface and Tc.
    Journal of Physics Conference Series 05/2013; 400(2).
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    ABSTRACT: We theoretically study the spin fluctuation and superconductivity in La1111 and Sm1111 iron-based superconductors for a wide range of electron doping. When we take into account the band structure variation by electron doping, the hole Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust against electron doping, and this gives rise to large spin fluctuations and consequently $s\pm$ pairing even in the heavily doped regime. The stable hole Fermi surface is larger for Sm1111 than for La1111, which can be considered as the origin of the apparent difference in the phase diagram.
    Journal of the Physical Society of Japan 04/2013; 82:083702. · 2.09 Impact Factor
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    ABSTRACT: To show an optimization method of element substitution for cuprate superconductors, we investigate Fermi surface shape of TlR2A2Cu3O9 with R=La, Y, and A=Li, Na, K, Rb, Cs. We adopt the generalized gradient approximation in the density-functional theory (DFT-GGA) for the study of over-doped phases of these unknown cuprates. The electronic structures of crystals optimized by DFT-GGA show systematic element dependence in a Fermi surface shape controlling parameter, r, of Cu dx2-y2 bands, where nearly absent dz2 component at the Fermi level and smaller r keeping t1 suggest enhancement of the superconducting transition temperature within the spin-fluctuation mechanism. For TlYRb2Cu3O9, smaller r by a reduction factor larger than 10% compared to a reference system of TlBa2Ca2Cu3O9 (TBCCO) appears in the outer CuO2 plane, but with 12 % reduction in t1. For TlR2Li2Cu3O9, (R=Y, La), smaller r by a factor larger than 1% appears keeping t1 as large as TBCCO in the inner plane. Our method may be used to predict an optimized material structure referencing known cuprate superconductors.
    04/2013;
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    ABSTRACT: In order to explore why the multi-layered cuprates have such high Tc's, we have examined various inter-layer processes. Since the inter-layer one-electron hopping has little effects on the band structure, we turn to the inter-layer pair hopping. The superconductivity in a double-layer Hubbard model with and without the inter-layer pair hopping, as studied by solving the Eliashberg equation with the fluctuation exchange approximation, reveals that the inter-layer pair hopping acts to increase the pairing interaction and the self-energy simultaneously, but that the former effect supersedes the latter and enhances the superconductivity. The inter-layer pair hopping considered here is for off-site pairs, for which we discuss the effect of retaining SU(2) symmetry, along with how the the sign of the pair hopping determines the relative configuration of d-waves between the adjacent layers.
    Physical review. B, Condensed matter 12/2012; 88(1). · 3.77 Impact Factor
  • Hidetomo Usui, Kazuhiko Kuroki
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    ABSTRACT: In the present study, we investigate the effect of the lattice structure in the iron pnictides within the spin fluctuation pairing theory based on the effective five band models of several hypothetical lattice structures of LaFeAsI We show that the presence and absence of Fermi surface pockets is sensitive to the Fe-As-Fe bond angle due to the multiorbital nature of the system, which is reflected to the nodeless/nodal form of the superconducting gap and Tc. Superconductivity is maximized within the maximum hole Fermi surface multiplicity regime where the arsenic atoms form a regular tetrahedron. Superconductivity has an overall tendency of increasing upon increasing Fe-As bond length. We provide a guideline of higher Tc from the lattice structure point of view.
    Journal of Physics Conference Series 12/2012; 400(2):2129-.
  • K. Suzuki, H. Usui, K. Kuroki
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    ABSTRACT: We obtain an approximate five orbital model for the 122 iron pnictides, and study its spin fluctuations within the random phase approximation. In particular, we focus on the peak position of the spin fluctuations, and study the origin of its electron-hole asymmetry. The origin of the incommensurate spin fluctuations in the heavily doped materials is found to be essentially the same as that driving the commensurate spin fluctuations in the less doped materials, and the electron-hole asymmetry can be understood from the multiorbital nature of the Fermi surface.
    Journal of Physics Conference Series 12/2012; 400(2):2117-.
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    Katsuhiro Suzuki, Hidetomo Usui, Kazuhiko Kuroki
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    ABSTRACT: We perform first principles band calculation of the newly discovered superconductor LaO$_{1-x}$F$_x$BiS$_2$, and study the lattice structure and the fluorine doping dependence of the gap between the valence and conduction bands. We find that the distance between La and S as well as the fluorine doping significantly affects the band gap. On the other hand, the four orbital model of the BiS$_2$ layer shows that the lattice structure does not affect this portion of the band. Still, the band gap can affect the carrier concentration in the case of light electron doping, which in turn should affect the transport properties.
    Physics Procedia. 11/2012; 45.
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    ABSTRACT: We perform a first-principles band calculation for a group of quasi-two-dimensional organic conductors β-(BDA-TTP)2MF6 (M = P, As, Sb and Ta). The ab-initio calculation shows that the density of states is correlated with the bandwidth of the singly occupied (highest) molecular orbital, while it is not necessarily correlated with the unit-cell volume. The direction of the major axis of the cross section of the Fermi surface lies in the Γ–B-direction, which differs from that obtained by the extended Hückel calculation. Then, we construct a tight-binding model which accurately reproduces the ab-initio band structure. The obtained transfer energies give a smaller dimerization than in the extended Hückel band. As to the difference in the anisotropy of the Fermi surface, the transfer energies along the inter-stacking direction are smaller than those obtained in the extended Hückel calculation. Assuming spin-fluctuation-mediated superconductivity, we apply random phase approximation to a two-band Hubbard model. This two-band Hubbard model is composed of the tight-binding model derived from the first-principles band structure and an on-site (intra-molecule) repulsive interaction taken as a variable parameter. The obtained superconducting gap changes sign four times along the Fermi surface like in a d-wave gap, and the nodal direction is different from that obtained in the extended Hückel model. Anion dependence of Tc is qualitatively consistent with the experimental observation.
    New Journal of Physics 11/2012; 14(11):113045. · 4.06 Impact Factor

Publication Stats

3k Citations
571.85 Total Impact Points

Institutions

  • 2012–2014
    • Osaka University
      • Department of Physics
      Suika, Ōsaka, 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
  • 2007
    • RIKEN
      Вако, Saitama, Japan
  • 2006
    • Hokkaido University
      • Division of Applied Physics
      Sapporo-shi, Hokkaido, Japan
  • 1990–2004
    • The University of Tokyo
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 2003
    • Nagoya University
      • Department of Quantum Engineering
      Nagoya, Aichi, Japan
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan