arXiv subjects

Kazuhiko Kuroki

Publications and source records attributed to Kazuhiko Kuroki.

At least 91 records · Page 5Linked to original sources

arXiv2016

Current-induced giant diamagnetism in the Mott insulator Ca2RuO4

Mott insulators have surprised us many times by hosting new and diverse quantum phenomena when the frozen electrons are perturbed by various stimuli. Superconductivity, metal-insulator transition, and colossal magnetoresistance induced by element substitution, pressure, and magnetic field are prominent examples. Here we report a novel phenomenon, namely giant diamagnetism, in the Mott insulator Ca2RuO4 induced by electric current. With application of 1 A/cm2 current, the strongest diamagnetism among all nonsuperconducting materials is induced as the system is tuned to a semimetallic state. The origin lies in the emergence of indirect Dirac cones in the many-body spectrum and associated monopole-like anomaly in the momentum dependent susceptibility. This record-breaking and switchable diamagnetism is a new class of non-equilibrium quantum phenomena on the verge of Mott insulating states.

cond-mat.str-el↗

arXiv2016

Direct theoretical evidence for weaker correlations in electron-doped and Hg-based hole-doped cuprates

Many important questions for high-$T_c$ cuprates are closely related to the insulating nature of parent compounds. While there has been intensive discussion on this issue, all arguments rely strongly on, or are closely related to, the correlation strength of the materials. Clear understanding has been seriously hampered by the absence of a direct measure of this interaction, traditionally denoted by $U$. Here, we report a first-principles estimation of $U$ for several different types of cuprates. The $U$ values clearly increase as a function of the inverse bond distance between apical oxygen and copper. Our results show that the electron-doped cuprates are less correlated than their hole-doped counterparts, which supports the Slater picture rather than the Mott picture. Further, the $U$ values significantly vary even among the hole-doped families. The correlation strengths of the Hg-cuprates are noticeably weaker than that of La$_2$CuO$_4$. Our results suggest that the strong correlation enough to induce Mott gap may not be a prerequisite for the high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2016

Strong bilayer coupling induced by the symmetry breaking in the monoclinic phase of BiS$_2$-based superconductors

We perform first-principles band structure calculations for the tetragonal and monoclinic structures of LaO$_{0.5}$F$_{0.5}$BiS$_2$. We find that the Bi $6p_{x,y}$ bands on two BiS$_2$ layers exhibit a sizable splitting at the X = ($π$, 0, 0) and several other \textbf{k}-points for the monoclinic structure. We show that this feature originates from the inter-BiS$_2$ layer coupling strongly enhanced by the symmetry breaking of the crystal structure. The Fermi surface also shows a large splitting and becomes anisotropic with respect to the $k_x$ and $k_y$ directions in the monoclinic structure, whereas it remains almost flat with respect to the $k_z$ direction.

cond-mat.supr-con↗

arXiv2016

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2016

Theoretical aspects of the study on the new bismuth chalcogenide based superconductors

We review theoretical aspects of the studies on the recently discovered BiCh2 (Ch=chalcogen) based superconductors. We first focus on the electronic band structure, the Fermi surface and their correlation with the lattice structure. We then discuss the phonon features and the issue of the lattice instability. Finally, we survey the phonon-mediated as well as the unconventional pairing mechanisms that have been proposed so far.

cond-mat.supr-con↗

arXiv2015

Asymmetry of Superconductivity in Hole- and Electron-doped Cuprates: Explanation within Two-Particle Self-Consistent Analysis for the Three-Band Model

In the hole-doped cuprate superconductors, the superconducting transition temperature $T_c$ exhibits a dome-like feature against the doping rate. By contrast, recent experiments reveal that $T_c$ in the electron-doped systems monotonically increases as the doping is reduced, at least up to a very small doping rate. Here we show that this asymmetry is reproduced by performing a two-particle self-consistent analysis for the three-band model of the CuO$_2$ plane. This is explained as a combined effect of the intrinsic electron-hole asymmetry in systems comprising Cu3$d$ and O2$p$ orbitals and the band-filling-dependent vertex correction.

cond-mat.supr-con↗

arXiv2015

First-principle study of antimony doping effects on the iron-based superconductor CaFe(Sb$_{x}$As$_{1-x}$)$_{2}$

We study antimony doping effects in the iron-based superconductor CaFe(Sb$_{x}$As$_{1-x}$)$_{2}$ by using the first-principle calculation. The calculations reveal that the substitution of the doped antimony atom into As of the chainlike As layers is more stable than that in FeAs layers. This prediction can be checked by experiments. Our results suggest that doping homologous elements into the chainlike As layers existing only in novel 112 system is responsible for rising up the critical temperature. We discuss antimony doping effects on the electronic structure. It is found that the calculated band structures with and without the antimony doping are similar to each other within our framework.

cond-mat.supr-con↗

arXiv2015

Enhancement of electron correlation due to the molecular dimerization in organic superconductors $β$-(BDA-TTP)$_{2}X$ ($X$=I$_3$, SbF$_6$)

We perform a first principles band calculation for quasi-two-dimensional organic superconductors $β$-(BDA-TTP)$_{2}$I$_{3}$ and $β$-(BDA-TTP)$_{2}$SbF$_{6}$. The first principles band structures between the I$_{3}$ and SbF$_{6}$ salts are apparently different. We construct a tight-binding model for each material which accurately reproduces the first principles band structure. The obtained transfer energies give the differences such as (i) larger dimerization in the I$_{3}$ salt than the SbF$_{6}$ salt, and (ii) different signs and directions of the inter-stacking transfer energies. To decompose the origin of the difference into the dimerization and the inter-stacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and extract the difference caused by the inter-stacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originated from the effect of the electron correlation such as the spin susceptibility applying two particle self-consistent (TPSC) method. We find that the maximum value of the spin susceptibility of the I$_{3}$ salt is larger than that of the SbF$_{6}$ salt. Hypothetically decreasing the dimerization within the model of the I$_{3}$ salt, the spin susceptibility takes almost the same value as that of the SbF$_6$ salt for the same magnitude of the dimerization. We expect that the different ground state between the I$_{3}$ and SbF$_{6}$ salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular $k$-path.

cond-mat.str-el↗

arXiv2015

Iron-Based Superconductors: current status of materials and pairing mechanism

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.

cond-mat.supr-con↗

arXiv2015

Quasiparticle self-consistent GW study of cuprates: electronic structure, model parameters, and the two-band theory for Tc

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 significantly 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.

cond-mat.str-el↗

arXiv2015

Theoretical study on the correlation between the spin fluctuation and $T_c$ in the isovalent-doped 1111 iron-based superconductors

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.

cond-mat.supr-con↗

arXiv2015

Origin of the non-monotonic variance of Tc in the 1111 iron based superconductors with isovalent doping

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.

cond-mat.supr-con↗

arXiv2014

Unconventional Superconductivity in electron-doped layered metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I)

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.

cond-mat.supr-con↗

arXiv2014

Molecular Dependence of the Large Seebeck Effect in τ-type Organic Conductors

We study the Seebeck effect in the $τ$-type organic conductors, $τ$-(EDO-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $τ$-(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.

cond-mat.mtrl-sci↗

arXiv2014

Soft x-ray photoemission study of new BiS$_{2}$-layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$

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.

cond-mat.supr-con↗

arXiv2014

Model of the Electronic Structure of Electron-Doped Iron-Based Superconductors: Evidence for Enhanced Spin Fluctuations by Diagonal Electron Hopping

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.

cond-mat.supr-con↗

arXiv2014

Orbital mixture effect on the Fermi surface-$T_c$ correlation in the cuprate superconductors --- bilayer vs single layer

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.

cond-mat.supr-con↗

arXiv2013

Theoretical expectation of large Seebeck effect in PtAs$_2$ and PtP$_2$

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$μ$W/cmK$^2$ at T=440 K is observed.

cond-mat.mtrl-sci↗