arXiv subjects

Kazuhiko Kuroki

Publications and source records attributed to Kazuhiko Kuroki.

At least 55 records · Page 3Linked to original sources

arXiv2021

Tunable spin-valley coupling in layered polar Dirac metals

In non-centrosymmetric metals, spin-orbit coupling (SOC) induces momentum-dependent spin polarization at the Fermi surfaces. This is exemplified by the valley-contrasting spin polarization in monolayer transition metal dichalcogenides (TMDCs) with in-plane inversion asymmetry. However, the valley configuration of massive Dirac fermions in TMDCs is fixed by the graphene-like structure, which limits the variety of spin-valley coupling. Here, we show that the layered polar metal BaMn$X_2$ ($X =$Bi, Sb) hosts tunable spin-valley-coupled Dirac fermions, which originate from the distorted $X$ square net with in-plane lattice polarization. We found that in spite of the larger SOC, BaMnBi$_2$ has approximately one-tenth the lattice distortion of BaMnSb$_2$, from which a different configuration of spin-polarized Dirac valleys is theoretically predicted. This was experimentally observed as a clear difference in the Shubnikov-de Haas oscillation at high fields between the two materials. The chemically tunable spin-valley coupling in BaMn$X_2$ makes it a promising material for various spin-valleytronic devices.

cond-mat.str-el↗

arXiv2021

Enhancing thermopower and Nernst signal of high-mobility Dirac carriers by Fermi level tuning in the layered magnet EuMnBi$_2$

Dirac/Weyl semimetals hosting linearly-dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder effects from the substituted ions. Here it is shown that the Fermi energy is tunable across the Dirac point for layered magnet EuMnBi$_2$ by partially substituting Gd$^{3+}$ for Eu$^{2+}$ in the insulating block layer, which dopes electrons into the Dirac fermion layer without degrading the mobility. Clear quantum oscillation observed even in the doped samples allows us to quantitatively estimate the Fermi energy shift and optimize the power factor (exceeding 100 $μ$W/K$^2$cm at low temperatures) in combination with the first-principles calculation. Furthermore, it is shown that Nernst signal steeply increases with decreasing carrier density beyond a simple theoretical prediction, which likely originates from the field-induced gap reduction of the Dirac band due to the exchange interaction with the Eu moments. Thus, the magnetic block layer provides high controllability for the Dirac fermions in EuMnBi$_2$, which would make this series of materials an appealing platform for novel transport phenomena.

cond-mat.mtrl-sci↗

arXiv2020

Possible pairing mechanism switching driven by structural symmetry breaking in BiS2-based layered superconductors

Investigation of isotope effects on superconducting transition temperature (Tc) is one of the useful methods to examine whether electron-phonon interaction is essential for pairing mechanisms. The layered BiCh2-based (Ch: S, Se) superconductor family is a candidate for unconventional superconductors, because unconventional isotope effects have previously been observed in La(O,F)BiSSe and Bi4O4S3. In this study, we investigated the isotope effects of 32S and 34S in the high-pressure phase of (Sr,La)FBiS2, which has a monoclinic crystal structure and a higher Tc of 10 K under high pressures, and observed conventional-type isotope shifts in Tc. The conventional-type isotope effects in the monoclinic phase of (Sr,La)FBiS2 are different from the unconventional isotope effects observed in La(O,F)BiSSe and Bi4O4S3, which have a tetragonal structure. The obtained results suggest that the pairing mechanisms of BiCh2-based superconductors could be switched by a structural-symmetry change in the superconducting layers induced by pressure effects.

cond-mat.supr-con↗

arXiv2020

Epitaxial stabilization of SrCu$_3$O$_4$ with infinite Cu$_{3/2}$O$_2$ layers

We report the epitaxial thin film synthesis of SrCu$_3$O$_4$ with infinitely stacked Cu$_3$O$_4$ layers composed of edge-sharing CuO$_4$ square-planes, using molecular beam epitaxy. Experimental and theoretical characterizations showed that this material is a metastable phase that can exist by applying tensile biaxial strain from the (001)-SrTiO$_3$ substrate. SrCu$_3$O$_4$ shows an insulating electrical resistivity in accordance with the Cu$^{2+}$ valence state revealed X-ray photoelectron spectroscopy. First-principles calculations also indicated that the unoccupied $d_{3z^2-r^2}$ band becomes substantially stabilized owing to the absence of apical anions, in contrast to $A_2$Cu$_3$O$_4$Cl$_2$ ($A = $Sr, Ba) with an $A_2$Cl$_2$ block layer and therefore a trans-CuO$_4$Cl$_2$ octahedron. These results suggest that SrCu$_3$O$_4$ is a suitable parent material for electron-doped superconductivity based on the Cu$_3$O$_4$ plane.

cond-mat.mtrl-sci↗

arXiv2020

Hidden Ladder in SrMoO$_3$/SrTiO$_3$ Superlattices: Experiments and Theoretical Calculations

A double-layered perovskite oxide Sr$_3$Mo$_2$O$_7$ is considered a "hidden ladder" system with wide and narrow bands near the Fermi level, for which high-$T_{\rm c}$ superconductivity is expected. However, the difficulty in synthesis, especially in the preparation of samples without oxygen deficiency, can hinder the observation of superconductivity. In this study, we constructed a double-layer SrMoO$_3$ block through artificial superlattices with the insulating SrTiO$_3$ block, (SrMoO$_3$)$_m$/(SrTiO$_3$)$_t$ ($m = 2, 4$; $t = 4$). First-principles calculations for bilayered SrMoO$_3$ ($m = 2$) exhibit a wide-narrow band structure near the Fermi level, which bears a close resemblance to Sr$_3$Mo$_2$O$_7$. The dispersion along the $k_z$ direction is strongly suppressed by increasing the number of the SrTiO$_3$ layers, $t$. However, no superconductivity is observed down to 0.1 K. We discuss the absence of the superconductivity for the present films on the basis of results of scanning transmission electron microscopy and band structure calculations.

cond-mat.str-el↗

arXiv2020

Designing nickelate superconductors with $d^8$ configuration exploiting mixed-anion strategy

Inspired by a recently proposed superconducting mechanism for a new cuprate superconductor Ba$_2$CuO$_{3+δ}$, we theoretically design an unconventional nickelate superconductor with $d^8$ electron configuration. Our strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so that the $3d$ bands other than $d_{x^-y^2}$ lie just below the Fermi level for the $d^8$ configuration, acting as incipient bands that enhance superconductivity. We also discuss a possible relevance of the present proposal to the recently discovered superconductor (Nd,Sr)NiO$_2$.

cond-mat.supr-con↗

arXiv2020

Quantifying the stability of the anion ordering in SrVO$_2$H

We investigate a text-book mixed-anion compound SrVO$_2$H using first-principles calculation, to theoretically pin down the factors that stabilize its anion ordering. We find that the $trans$ preference by the characteristic crystal field in the VO$_4$H$_2$ octahedron in addition to a coherent shrinkage along the V-H-V direction, taking place when such direction is consistent among neighboring hydrogens, stabilize the anion ordering observed in experiment. Our study gives an important clue for controlling the anion ordering in transition metal oxyhydrides.

cond-mat.mtrl-sci↗

arXiv2020

Superconducting mechanism for the cuprate Ba$_2$CuO$_{3+δ}$ based on a multiorbital Lieb lattice model

For the recently discovered cuprate superconductor $\mathrm{Ba_{2}CuO_{3+δ}}$, we propose a lattice structure which resembles the model considered by Lieb to represent the vastly oxygen-deficient material. We first investigate the stability of the Lieb-lattice structure, and then construct a multiorbital Hubbard model based on first-principles calculation. By applying the fluctuation-exchange approximation to the model and solving the linearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings closely compete with each other, and, more interestingly, that the intra-orbital and inter-orbital pairings coexist. We further show that, if the energy of the $d_{3z^2-r^2}$ band is raised to make it "incipient" with the lower edge of the band close to the Fermi level within a realistic band filling regime, $s\pm$-wave superconductivity is strongly enhanced. We reveal an intriguing relation between the Lieb model and the two-orbital model for the usual K$_2$NiF$_4$ structure where a close competition between $s-$ and $d-$wave pairings is known to occur. The enhanced superconductivity in the present model is further shown to be related to an enhancement found previously in the bilayer Hubbard model with an incipient band.

cond-mat.supr-con↗

arXiv2020

First principles study on the thermoelectric performance of CaAl$_2$Si$_2$-type Zintl phase compounds

We investigate the thermoelectric properties of CaAl$_2$Si$_2$-type Zintl phase compounds $AB_2X_2$ ($A$ = Mg, Ca, Sr, Ba, $B$ = Mg, Zn, Cd, and $X$ = P, As, Sb) using first principles band calculations within the Boltzmann transport theory assuming the constant relaxation time approximation. We introduce the effective degree of valley degeneracy $n_{TE}$ to focus on the relationship between the thermoelectric properties and the multivalley character of the electronic band structure around the Fermi level. We also introduce a quantity $γ_{TE}$, which takes into account $n_{TE}$ and anisotropy of the valley structure, and it is found that $γ_{TE}$ enables us to well understand the overall trend of the material dependence of the power factor. We finally suggest promising thermoelectric materials, e.g. BaMg$_2$P$_2$ for PF $\sim 20μ$W/cmK$^2$ and $ZT > 0.2$ at 300K and SrZn$_2$As$_2$ for PF $\sim 35μ$W/cmK$^2$ and $ZT > 0.35$ at 300K assuming a relaxation time of 10 fs and a lattice thermal conductivity value of 2 W/mK.

cond-mat.mtrl-sci↗

arXiv2020

Possible counterintuitive enhancement of superconductivity in ladder-type cuprates by longitudinal compression

We theoretically study the effect of uniaxial deformation of ladder-type cuprate superconductors. Model construction based on first principles calculation shows that the rung-to-leg ratio of the nearest neighbor hoppings counterintuitively increases when the lattice is compressed in the longitudinal (leg) direction. This leads to an enhancement of the superconducting transition temperature, which intuitively is expected when compressed in the rung direction. Such a trend is traced back to the on-site hybridization between Cu$4s$ and Cu$d_{x^2-y^2}$ orbitals, which varies and changes sign upon lattice deformation.

cond-mat.supr-con↗

arXiv2019

Many-variable variational Monte-Carlo study of superconductivity in two-band Hubbard models with an incipient band

We study superconductivity in two-band models where one of the bands does or does not intersect the Fermi level depending on the parameter values. Applying a many-variable variational Monte-Carlo method for a Hubbard model on two-leg ladder and bilayer square lattices, we show that superconductivity can be enhanced in a parameter regime where the edge of one of the bands is near the Fermi energy, that is, when the band is incipient. The resemblence of the present results to those obtained by a weak coupling method in a recent study suggests that, even in the large $U$ regime, the suppression of the near-zero-energy spin fluctuations and the development of finite energy spin fluctuations are the key factors for the enhancement of superconductivity by an incipient band.

cond-mat.supr-con↗

arXiv2019

Strongly enhanced superconductivity due to finite energy spin fluctuations induced by an incipient band : a FLEX study on the bilayer Hubbard model with vertical and diagonal interlayer hoppings

We study the spin-fluctuation-mediated $s\pm$-wave superconductivity in the bilayer Hubbard model with vertical and diagonal interlayer hoppings. As in the two-leg ladder model with diagonal hoppings, studied previously by the present authors, superconductivity is strongly enhanced when one of the bands lies just below (or touches) the Fermi level, that is, when the band is incipient. The strong enhancement of superconductivity is because large weight of the spin fluctuations lies in an appropriate energy range, whereas the low energy, pair-breaking spin fluctuations are suppressed. The optimized eigenvalue of the linearized Eliashberg equation, a measure for the strength of superconductivity, is not strongly affected by the bare width of the incipient band, but the parameter regime where superconductivity is optimized is wide when the incipient band is narrow, and in this sense, the coexistence of narrow and wide bands is favorable for superconductivity.

cond-mat.supr-con↗

arXiv2019

Pressure-induced superconductivity in layered pnictogen diselenide NdO$_{0.8}$F$_{0.2}$Sb$_{1-x}$Bi$_x$Se$_2$ (x = 0.3 and 0.7)

Polycrystalline samples of layered pnictogen diselenide NdO0.8F0.2Sb1-xBixSe2 (x = 0 to 0.8) were successfully synthesized by solid-state reactions. Electrical resistivity in the synthesized samples was systematically decreased with an increase in Bi content x. Crystal structure analysis using synchrotron X-ray diffraction suggests that insulator to metal transition upon Bi doping correlates with anomalous change in c-axis length and/or corrugation in conducting layer. The emergence of superconductivity under high pressure is demonstrated using diamond anvil cell (DAC) with boron-doped diamond electrodes, for x = 0.3 and 0.7 as the representative samples. For Sb-rich one (x = 0.3), we observed a superconducting transition with Tconset = 5.3 K at 50 GPa, which is the first-ever report of the superconductivity in layered SbCh2-based (Ch: chalcogen) compounds. The Tconset of x = 0.3 increased with increasing pressure and reached 7.9 K at 70.8 GPa, followed by the gradual decrease in Tc up to 90 GPa. For Bi-rich one (x = 0.7), a superconducting transition with Tconset = 5.9 K was observed at 43.5 GPa, which is the almost comparable to that of x = 0.3; besides, upper critical field (Hc2) is evaluated to be ~10 T for x = 0.7, which is higher than that of x = 0.3 (Hc2 = 6.7 T at 50 GPa).

cond-mat.supr-con↗

arXiv2019

Comparative first-principles study of antiperovskite oxides and nitrides as thermoelectric material: multiple Dirac cones, low-dimensional band dispersion, and high valley degeneracy

We perform a comparative study on thermoelectric performance of antiperovskite oxides $Ae_3Tt$O and nitrides $Ae_3Pn$N ($Ae=$ Ca, Sr, Ba; $Tt=$ Ge, Sn, Pb; $Pn=$ As, Sb, Bi) by means of first-principles calculation. As for the oxides with the cubic structure, Ca$_3$GeO with a sizable band gap exhibits high thermoelectric performance at high temperatures, while Ba$_3$PbO with Dirac cones without the gap is favorable at low temperatures. The latter high performance owes to high valley degeneracy including the multiple Dirac cones and the valleys near the $Γ$ and R points. For the nitrides with the cubic structure, insulator with strong quasi-one-dimensionality exhibits high thermoelectric performance. We also find that the orthorhombic structural distortion sometimes sizably enhances thermoelectric performance, especially for Ba$_3$GeO and Sr$_3$AsN where the high valley degeneracy is realized in the $Pnma$ phase. Our calculation reveals that antioerpvskites offer a fertile playground of various kinds of characteristic electronic structure, which enhance the thermoelectric performance, and provides promising candidates of high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Model construction and a possibility of cuprate-like pairing in a new d9 nickelate superconductor (Nd,Sr)NiO2

Effective models are constructed for a newly discovered superconductor (Nd,Sr)NiO2, which has been considered as a possible nickelate analogue of the cuprates owing to the d9 electron configuration. Estimation of the effective interaction, which turns out to require a multiorbital model that takes account of all the orbitals involved on the Fermi surface, shows that the effective interactions are significantly larger than in the cuprates. A fluctuation exchange study for the model indicates that dx2-y2-wave superconductivity is likely to occur as in the cuprates, where the transition temperature in the nickelate can be lower from the cuprates due to the larger interaction and narrower bandwidth.

cond-mat.supr-con↗

arXiv2019

First-principles study of LaOPbBiS$_3$ and its analogous compounds as thermoelectric materials

LaOBiPbS$_3$ is a kind of pnictogen-dichalcogenide layered compounds, which have recently been experimentally investigated as thermoelectric materials owing to their low thermal conductivity and high controllability of constituent elements. However, thermoelectric performance of LaOBiPbS$_3$ is at present not very high and that of its analogous compounds remains to be unknown. In this study, we theoretically investigate thermoelectric properties of 24 possible variations of the constituent elements in LaOBiPbS$_3$ from the viewpoint of the electronic structure. We find that some compounds can have much better thermoelectric performance than LaOBiPbS$_3$; in particular, LaOSbPbSe$_3$ is predicted to have a power factor five times as large as that of LaOBiPbS$_3$. Here, the choice of the pnictogen atom (As, Sb, and Bi), of which the low-energy conduction bands mainly consist, correlates with the calculated power factor and the dimensionless figure of merit, $ZT$. Such correlation comes from the fact that the low-dimensionality of the electronic structure, which enhances the density of states near the band edge, strongly depends on the pnictogen atom through, e.g., the strength of the spin-orbit coupling. Moreover, hybridization of the wave functions in the pnictogen-dichalcogenide layer and those in the rock-salt layer plays a key role in gap opening, and thus is important for achieving high thermoelectric performance. In LaOSbPbSe$_3$, such hybridization also pushes up the conduction band bottom, which enhances the density of states near the band edge and thus the power factor.

cond-mat.mtrl-sci↗

arXiv2019

Hidden robust presence of a hole Fermi surface in a heavily electron doped iron based superconductor LaFe$_2$As$_2$

We investigate the electronic structure of a recently discovered, heavily electron-doped iron based superconductor LaFe$_2$As$_2$. Although first principles calculation shows apparent absence of hole Fermi surfaces around the $Γ$ point, we reveal, by hypothetically removing the La $d$ orbital contribution, that a hole Fermi surface around the $Γ$ point is essentially present. In the collapsed phase of LaFe$_2$As$_2$, which is non-superconducting, the hole Fermi surface is found to be absent, and the difference from the uncollapsed superconducting phase can be naturally understood within the spin-fluctuation mediated pairing scenario.

cond-mat.supr-con↗

arXiv2019

Effective interaction for vanadium oxyhydrides Sr$_{n+1}$V$_n$O$_{2n+1}$H$_n$ ($n=1$ and $n\to\infty$): A constrained-RPA study

Transition metal oxides have been one of the central objects in the studies of electron correlation effects because of their rich variety of physical properties mainly depending on the transition metal element. On the other hand, exploiting the anion degrees of freedom is less popular but can be another promising way to control properties of strongly correlated materials. In particular, oxyhydrides offer a unique playground of strongly correlated low-dimensional electronic structure, where the $s$ orbitals of hydrogen breaks a chemical bond between the cation $t_{2g}$ orbitals. In this study, we evaluate the effective interaction, i.e., the screened Coulomb interaction parameters in low-energy effective models, for vanadium oxyhydrides Sr$_{n+1}$V$_n$O$_{2n+1}$H$_n$ ($n=1,\infty$) using the constrained random-phase approximation (cRPA). We find that the effective interaction in the $t_{2g}$ model, where only the $t_{2g}$ orbitals are explicitly considered, is strongly screened by the $e_g$ bands compared with that for oxides, because the $e_g$ bands are much entangled with the $t_{2g}$ bands in the oxyhydrides. On the other hand, the effective interaction is rather strong in the $d$ model, where all the vanadium $d$ orbitals are explicitly considered, owing to a large energy separation between the V-$d$ bands and the anion bands (O-$p$ and H-$s$), because the O-$p$ states are stabilized by the existence of the hydrogen atoms. These findings suggest that non-trivial and unique correlation effects can take place in vanadium oxyhydrides.

cond-mat.str-el↗