arXiv subjects

Kazuhiko Kuroki

Publications and source records attributed to Kazuhiko Kuroki.

At least 73 records · Page 4Linked to original sources

arXiv2019

Hidden kagome-lattice picture and origin of high conductivity in delafossite PtCoO$_2$

We study the electronic structure of delafossite PtCoO$_2$ to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that it is formed mainly by Pt $d$ orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structure: one originating from Pt $s+p_x/p_y$ orbitals, and the other from Pt $d_{3z^2-r^2}+d_{xy}/d_{x^2-y^2}$ orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt $s+p_x/p_y$ bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt $s+p_x/p_y$ band. More importantly, the kagome-like electronic structure gives rise to "orbital-momentum locking" on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in PtCoO$_2$.

cond-mat.mtrl-sci↗

arXiv2018

Spin Hall effect in 2D metallic delafossite PtCoO$_2$ and vicinity topology

The two-dimensional (2D) metal PtCoO$_2$ is renowned for the lowest room temperature resistivity among all oxides, close to that of the top two materials Ag and Cu. In addition, we theoretically predict a strong intrinsic spin Hall effect. This originates from six strongly-tilted Dirac cones that we find in the electronic structure near the Fermi surface, where a gap is opened by large spin-orbit coupling (SOC). This is underpinned by rich topological properties; in particular, the phenomenology of a mirror Chern metal is realized not exactly, but very accurately, on account of an approximate crystalline symmetry. We expect that such 'vicinity topology' to be a feature of relevance well beyond this material. Our Wilson loop analysis indicates further elaborate features such as fragile topology. These findings highlight PtCoO$_2$ as a promising material for spintronic applications as well as a platform to study the interplay of symmetry and topology.

cond-mat.mes-hall↗

arXiv2018

Pressure-induced Insulator to Metal Transition of Mixed Valence Compound Ce(O,F)SbS$_{2}$

Transport properties of Ce$_{0.85}$F0.15SbS$_{2}$ and undoped CeOSbS$_{2}$ under high pressure were investigated experimentally and theoretically. Electrical resistivity measurements of the Ce$_{0.85}$F0.15SbS$_{2}$ single crystals were performed under various high pressures using a diamond anvil cell with boron-doped diamond electrodes. The samples showed the insulator to metal transition by applying high pressure up to 30-40 GPa. On the other hand, the undoped CeOSbS$_{2}$ showed almost same transport property with the F-doped sample under high pressure. The valence state analysis using X-ray photoelectron spectroscopy revealed a simple valence state of Ce3+ in Ce$_{0.85}$F0.15SbS$_{2}$ and mixed valence state between Ce3+ and Ce4+ in undoped CeOSbS$_{2}$. The valence fluctuation in Ce carried out the comparable transport nature in the both samples. A band calculation suggests that the undoped CeOSbS$_{2}$ could be metallic under high pressure of 30 GPa in accordance with the experimental results. A superior thermoelectric property of power factor in CeOSbS$_{2}$ was estimated under high pressure around 20 GPa in comparison with that of ambient pressure.

cond-mat.mtrl-sci↗

arXiv2018

Maximally-localized Wannier orbitals and the extended Hubbard model for the twisted bilayer graphene

We develop an effective extended Hubbard model to describe the low-energy electronic properties of the twisted bilayer graphene. By using the Bloch states in the effective continuum model and with the aid of the maximally localized algorithm, we construct the Wannier orbitals and obtain an effective tight-binding model on the emergent honeycomb lattice. We found the Wannier state takes a peculiar three-peak form in which the amplitude maxima are located at the triangle corners surrounding the center. We estimate the direct Coulomb interaction and the exchange interaction between the Wannier states. At the filling of two electrons per super cell, in particular, we find an unexpected coincidence in the direct Coulomb energy between a charge-ordered state and a homogeneous state, which would possibly lead to an unconventional many-body state.

cond-mat.mes-hall↗

arXiv2018

Possible correlated insulating states in magic-angle twisted bilayer graphene under strongly competing interactions

We investigate correlated insulating states in magic-angle twisted bilayer graphene (TBG) by the exact diagonalization method applied to the extended Hubbard model with interaction parameters recently evaluated in the realistic effective model. Our model can handle the competing interactions among Wannier orbitals owing to their significant overlap, which is a crucial but overlooked aspect of the magic-angle TBG. We propose two candidates for the correlated insulating states: spin- and valley-ferromagnetic band insulator and the Dirac semimetallic state for two flavors with peculiar renormalization, where a flavor denotes a combined degree of freedom with spin and valley. One of the important consequences for the latter candidate is that it allows van Hove singularity near half-filling of the whole band structure (i.e. near the Dirac points) to play some role in superconductivity. The consistency between the two flavor degrees of freedom for the Dirac semimetallic state and the two-fold degeneracy of the Landau level observed in the experiment is also noteworthy.

cond-mat.str-el↗

arXiv2018

Thermoelectric performance of materials with Cu$Ch_4$ ($Ch=$ S, Se) tetrahedra: Similarities and differences among their low-dimensional electronic structure from first principles

In this study, we perform a comparative theoretical study on the thermoelectric performance of materials with Cu$Ch_4$ ($Ch=$ S, Se) tetrahedra, including famous thermoelectric materials BiCuSeO and tetrahedrite Cu$_{12}$Sb$_4$S$_{13}$, by means of first-principles calculations. By comparing these electronic band structures, we find that many of these materials possess a Cu-$t_{2g}$ band structure consisting of quasi-one-dimensional band dispersions and the isotropic (two-dimensional for layered compounds) band dispersion near the valence-band edge. Therefore, the key factors for the thermoelectric performance are the anisotropy of the former band dispersion and the degeneracy of these two kinds of band dispersions. We also find that a large extension of the chalcogen orbitals often improves their thermoelectric performance by improving these two factors or by going beyond such a basic band structure through a large alternation of its shape. Such a large extension of the chalcogen orbitals might partially originate from the anisotropic Cu-$Ch$ bond geometry of a tetrahedron. Our study reveals interesting similarities and differences of materials with Cu$Ch_4$, which provides important knowledge for a future search of high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Electronic Structure Calculation and Superconductivity in $λ$-(BETS)$_{2}$GaCl$_{4}$

Quasi-two-dimensional molecular conductor $λ$-(BETS)$_2$GaCl$_4$ shows superconductivity (SC) below 5.5K, neighboring the dimer-type Mott insulating phase. To elucidate the origin of SC and its gap function, we carry out first-principles band calculation and derive a four-band tight-binding model from the maximally localized Wannier orbitals. Considering the spin-fluctuation-mediated mechanism by adding the Hubbard $U$-term to the model, we analyze the SC gap function by applying the random phase approximation. We show that the SC gap changes its sign four times along the Fermi surface (FS) in the unfolded Brillouin zone, suggestive of a $d$-wave-like SC gap, which only has two-fold symmetry because of the low symmetry of the crystal structure. Decomposing the SC gap into the pairing functions along the crystal axes, we compare the result to similar analysis of the well-studied $κ$-type molecular conductors and to the experiments.

cond-mat.supr-con↗

arXiv2018

Microscopic theory of a superconducting gap in the quasi-one-dimensional organic conductor (TMTSF)$_2$ClO$_4$: Model derivation and two-particle self-consistent analysis

We present a first-principles band calculation for the quasi-one-dimensional (Q1D) organic superconductor (TMTSF)$_2$ClO$_4$. An effective tight-binding model with the TMTSF molecule to be regarded as the site is derived from a calculation based on maximally localized Wannier orbitals. We apply a two-particle self-consistent (TPSC) analysis by using a four-site Hubbard model, which is composed of the tight-binding model and an on-site (intramolecular) repulsive interaction, which serves as a variable parameter. We assume that the pairing mechanism is mediated by the spin fluctuation, and the sign of the superconducting gap changes between the inner and outer Fermi surfaces, which correspond to a d-wave gap function in a simplified Q1D model. With the parameters we adopt, the critical temperature for superconductivity estimated by the TPSC approach is approximately 1K which is consistent with experiment.

cond-mat.supr-con↗

arXiv2017

Prediction of the High Thermoelectric Performance of Pnictogen Dichalcogenide Layered Compounds with Quasi-One-Dimensional Gapped Dirac-like Band Dispersion

Thermoelectric power generation has been recognized as one of the most important technologies, and high-performance thermoelectric materials have long been pursued. However, because of the large number of candidate materials, this quest is extremely challenging, and it has become clear that a firm theoretical concept from the viewpoint of band-structure engineering is needed. In this study, we theoretically demonstrate that pnictogen-dichalcogenide layered compounds, which originally attracted attention as a family of superconductors and have recently been investigated as thermoelectric materials, can exhibit very high thermoelectric performance with elemental substitution. In particular, we clarify a promising guiding principle for materials design and find that LaOAsSe$_2$, a material that has yet to be synthesized, has a powerfactor that is six times as large as that of the known compound LaOBiS$_2$ and can exhibit a very large $ZT$ under some plausible assumptions. This large enhancement of the thermoelectric performance originates from the quasi-one-dimensional gapped Dirac-like band dispersion, which is realized by the square-lattice network. Our study offers one ideal limit of the band structure for thermoelectric materials. Because our target materials have high controllability of constituent elements and feasibility of carrier doping, experimental studies along this line are strongly awaited.

cond-mat.mtrl-sci↗

arXiv2017

Possible high-$T_c$ superconductivity due to incipient narrow bands originating from hidden ladders in Ruddlesden-Popper compounds

We introduce a concept of hidden ladders for the bilayer Ruddlesden-Popper type compounds: While the crystal structure is bilayer, $d_{xz}$ ($d_{yz}$) orbitals in the relevant $t_{2g}$ sector of the transition metal form a two-leg ladder along $x$ ($y$), since the $d_{xz}$ ($d_{yz}$) electrons primarily hop along the leg direction $x$ ($y$) on top of the rung $z$ direction. We predict that Sr$_3$Mo$_2$O$_7$ and Sr$_3$Cr$_2$O$_7$ are candidates for the hidden-ladder material with a right position of $E_F$, from a first-principles band calculation. Based on the analysis of Eliashberg equation for these systems, we propose a possible occurrence of high temperature superconductivity in these materials arising from the interband pair scattering processes between the wide and "incipient" narrow bands on the ladder.

cond-mat.supr-con↗

arXiv2017

Wide applicability of high $T_c$ pairing originating from coexisting wide and incipient narrow bands in quasi one dimension

We study superconductivity in the Hubbard model on various quasi-one-dimensional lattices with coexisting wide and narrow bands originating from multiple sites within a unit cell, where each site corresponds to a single orbital. The systems studied are the 2-leg and 3-leg ladders, the diamond chain, and the criss-cross ladder. These one-dimensional lattices are weakly coupled to form two-dimensional (quasi-one-dimensional) ones, and the fluctuation exchange approximation is adopted to study spin-fluctuation-mediated superconductivity. When one of the bands is perfectly flat, and the Fermi level, intersecting the wide band, is placed in the vicinity of the flat band, superconductivity arising from the interband scattering processes is found to be strongly enhanced owing to the combination of the light electron mass of the wide band and the strong pairing interaction due to the large density of states of the flat band. Even when the narrow band has finite band width, the pairing mechanism still works since the edge of the narrow band, due to its large density of states, plays the role of the flat band. The results indicate the generality of the high $T_c$ pairing mechanism due to coexisting wide and "incipient" narrow bands in quasi-one-dimensional systems.

cond-mat.supr-con↗

arXiv2017

DMFT study on the electron-hole asymmetry of the electron correlation strength in the high Tc cuprates

Recent experiments revealed a striking asymmetry in the phase diagram of the high temperature cuprate superconductors. The correlation effect seems strong in the hole-doped systems and weak in the electron-doped systems. On the other hand, a recent theoretical study shows that the interaction strengths (the Hubbard U) are comparable in these systems. Therefore, it is difficult to explain this asymmetry by their interaction strengths. Given this background, we analyze the one-particle spectrum of a single band model of a cuprate superconductor near the Fermi level using the dynamical mean field theory. We find the difference in the "visibility" of the strong correlation effect between the hole- and electron-doped systems. This can explain the electron-hole asymmetry of the correlation strength without introducing the difference in the interaction strength.

cond-mat.supr-con↗

arXiv2017

Enhanced power factor and reduced Lorenz number in the Wiedemann--Franz law due to pudding mold type band structures

We study the relationship between the shape of the electronic band structure and the thermoelectric properties. In order to study the band shape dependence of the thermoelectric properties generally, we first adopt models with band structures having the dispersion $E({\bf k}) \sim |{\bf k}|^n$ with $n = 2, 4$ and 6. We consider one, two- and three dimensional systems, and calculate the thermoelectric properties using the Boltzmann equation approach within the constant quasi-particle lifetime approximation. $n = 2$ corresponds to the usual parabolic band structure, while the band shape for $n = 4, 6$ has a flat portion at the band edge, so that the density of states diverges at the bottom of the band. We call this kind of band structure the "pudding mold type band". $n \ge 4$ belong to the pudding mold type band, but since the density of states diverges even for $n= 2$ in one dimensional system, this is also categorized as the pudding mold type. Due to the large density of states and the rapid change of the group velocity around the band edge, the spectral conductivity of the pudding mold type band structures becomes larger than that of the usual parabolic band structures. It is found that the pudding mold type band has a coexistence of large Seebeck coefficient and large electric conductivity, and small Lorenz number in the Wiedemann--Franz law due to the specific band shape. We also find that the low dimensionality of the band structure can contribute to large electronic conductivity and hence a small Lorenz number. We conclude that the pudding mold type band, especially in low dimensional systems, can enhance not only the power factor but also the dimensionless figure of merit due to stronger violation of the Wiedemann--Franz law.

cond-mat.mtrl-sci↗

arXiv2017

Finite energy spin fluctuation as a pairing glue in systems with coexisting electron and hole bands

We study, within the fluctuation exchange approximation, the spin-fluctuation-mediated superconductivity in Hubbard-type models possessing electron and hole bands, and compare them with a model on a square lattice with a large Fermi surface. In the square lattice model, superconductivity is more enhanced for better nesting for a fixed band filling. By contrast, in the models with electron and hole bands, superconductivity is optimized when the Fermi surface nesting is degraded to some extent, where finite energy spin fluctuation around the nesting vector develops. The difference lies in the robustness of the nesting vector, namely, in models with electron and hole bands, the wave vector at which the spin susceptibility is maximized is fixed even when the nesting is degraded, whereas when the Fermi surface is large, the nesting vector varies with the deformation of the Fermi surface. We also discuss the possibility of realizing in actual materials the bilayer Hubbard model, which is a simple model with electron and hole bands, and is expected to have a very high T_c.

cond-mat.supr-con↗

arXiv2017

Charge/quadrupole fluctuations and gap anisotropy in BiS$_2$-based superconductors

Recent angle-resolved spectroscopy in BiS$_2$-based superconductors has indicated that the superconducting gap amplitude possesses remarkable anisotropy and/or a sign change on a small Fermi pocket around $X$ point. It implies a possibility of an unconventional pairing state. Here we study the gap anisotropy in superconductivity mediated by inherent charge/quadrupole fluctuations in an extended Hubbard model, which includes inter-site interaction between Bi and S atoms. The first-principles downfolded band structure is composed of Bi $6p_x/p_y$ and S $3p_x/p_y$ orbitals on a BiS$_2$ single layer. Evaluating the linearized gap equation, we find that the ferroic charge/quadrupole fluctuation driven by the inter-site interaction leads to a fully-gapped $d_{x^2-y^2}$-wave pairing state, in which the gap amplitude has sizable anisotropy on the Fermi surface.

cond-mat.supr-con↗

arXiv2017

Temperature- and doping-dependent roles of valleys in thermoelectric performance of SnSe: a first-principles study

We theoretically investigate how each orbital and valley play a role for high thermoelectric performance of SnSe. In the hole-doped regime, two kinds of valence band valleys contribute to its transport properties: one is the valley near the U-Z line, mainly consisting of the Se-$p_z$ orbitals, and the other is the one along the $Γ$-Y line, mainly consisting of the Se-$p_y$ orbitals. Whereas the former valley plays a major role in determining the transport properties at room temperature, the latter one also offers comparable contribution and so the band structure exhibits multi-valley character by increasing the temperature. In the electron-doped regime, the conduction band valley around the $Γ$ point solely contributes to the thermoelectric performance, where the quasi-one-dimensional electronic structure along the $a$-axis is crucial. This study provides an important knowledge for the thermoelectric properties of SnSe, and will be useful for future search of high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2017

Intrinsic Phase Diagram of Superconductivity in the BiCh2-based System Without In-plane Disorder

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.

cond-mat.supr-con↗

arXiv2017

Model-mapped RPA for determining the effective Coulomb interaction

We present a new method to obtain interaction part of a model Hamiltonian from the result of the first-principles calculation. The effective interaction contained in the model is determined based on the random phase approximation (RPA). In contrast to previous methods such as projected RPA or constrained RPA, the new method takes into account the long-range part of the polarization effect when determining the interaction in the model. After we discuss problems in previous RPA methods, we will give the formulation of the new method, and show how it works for the single-band Hubbard model of HgBa$_2$CuO$_4$.

cond-mat.mtrl-sci↗