arXiv subjects

Hirofumi Sakakibara

Publications and source records attributed to Hirofumi Sakakibara.

28 records · Page 2Linked to original sources

arXiv2016

Coexistence of light and heavy surface states in a topological multi-band Kondo insulator

We analyze the impact of strong correlations on the surface states of a three-dimensional topological Kondo insulator. An important observation is that correlations are strongly increased at the surface, which leads to an energetical confinement of the surface $f$-electrons into a narrow window around the Fermi energy at low enough temperature. This correlation effect in combination with the nontrivial topology has two remarkable consequences: (i) coexistence of light and heavy surface states at low temperatures. While heavy surface states are formed directly in the surface layer, light surface states are formed in the next-nearest surface layer. Furthermore, (ii) with increasing the temperature, the heavy surface states become incoherent and only light surface states can be observed. The coexistence of light and heavy surface states is thereby a remarkable characteristic of the combination of strong correlations and nontrivial topology. We believe that these results are applicable to the candidate topological Kondo insulator SmB6.

cond-mat.str-el↗

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↗

arXiv2013

Two-orbital view on the origin of the material dependence of Tc in the single-layer cuprates

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.

cond-mat.supr-con↗

arXiv2013

Material Optimization for Fermi Surface Shape Control of Tl-based Cuprate Superconductors

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.

cond-mat.supr-con↗

arXiv2013

Pudding-mold type band in a potential thermoelectric material CuAlO$_2$ : comparison with Na$_x$CoO$_2$

A potential thermoelectric material CuAlO$_2$ is theoretically studied. We first construct a model Hamiltonian of CuAlO$_2$ 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 Na$_x$CoO$_2$, 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 as thermoelectric materials. There are however some differences, and we analyze the origin of the difference from a microscopic viewpoint. The band shape of CuAlO$_2$ is found to be even more ideal than that of Na$_x$CoO$_2$, and we predict that once a significant amount of holes is doped in CuAlO$_2$, thermoelectric properties (especially the power factor) even better than that of Na$_x$CoO$_2$ can be expected.

cond-mat.mtrl-sci↗

arXiv2012

First-principles band structure and FLEX approach to the pressure effect on $T_c$ of the cuprate superconductors

High-temperature cuprate superconductors have been known to exhibit significant pressure effects. In order to fathom the origin of why and how Tc is affected by pressure, we have recently studied the pressure effects on Tc adoptig a model that contains two cupper d-orbitals derived from first-principles band calculations, where the dz2 orbital is considere on top of the usually considered dx2-y2 orbital. In that paper, we have identified two origins for the Tc enhancement under hydrostatic pressure: (i) while at ambient pressure the smaller the hybridization of other orbital components the higher the Tc, an application of pressure acts to reduce the multiorbital mxing on the Fermi surface, which we call the orbital distillation effects, and (ii) the increase of the band width with pressure also contributes to the enhancement. In the present paper, we further elabolrate the two points. As for point (i), while the reduction of the apical oxygen height under pressure tends to increase the dz2 mixture, hence to lower Tc, here we show that this effect is strongly reduced in bi-layer materials due to the pyramidal coordination of oxygen atoms. As for point (ii), we show that the enhancement of Tc due to the increase in the band width is caused by the effect that the many-body renormalization arising from the self-energy is reduced.

cond-mat.supr-con↗

arXiv2012

Multiorbital analysis of the effects of uniaxial and hydrostatic pressure on $T_c$ in the single-layered cuprate superconductors

The origin of uniaxial and hydrostatic pressure effects on $T_c$ in the single-layered cuprate superconductors is theoretically explored. A two-orbital model, derived from first principles and analyzed with the fluctuation exchange approximation gives axial-dependent pressure coefficients, $\partial T_c/\partial P_a>0$, $\partial T_c/\partial P_c<0$, with a hydrostatic response $\partial T_c/\partial P>0$ for both La214 and Hg1201 cuprates, in qualitative agreement with experiments. Physically, this is shown to come from a unified picture in which higher $T_c$ is achieved with an "orbital distillation", namely, the less the $d_{x^2-y^2}$ main band is hybridized with the $d_{z^2}$ and $4s$ orbitals higher the $T_c$. Some implications for obtaining higher $T_c$ materials are discussed.

cond-mat.supr-con↗

arXiv2012

Corrugated flat band as an origin of large thermopower in hole doped PtSb$_2$

The origin of the recently discovered large thermopower in hole-doped PtSb$_2$ is theoretically analyzed based on a model constructed from first principles band calculation. It is found that the valence band dispersion has an overall flatness combined with some local ups and downs, which gives small Fermi surfaces scattered over the entire Brillouin zone. The Seebeck coefficient is calculated using this model, which gives good agreement with the experiment. We conclude that the good thermoelectric property originates from this "corrugated flat band", where the coexistence of large Seebeck coefficient and large electric conductivity is generally expected.

cond-mat.mtrl-sci↗

arXiv2010

Two-Orbital Model Explains the Higher Transition Temperature of the Single-Layer Hg-Cuprate Superconductor Compared to That of the La-Cuprate Superconductor

In order to explore the reason why the single-layered cuprates, La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ ($T_c\simeq$ 40K) and HgBa$_2$CuO$_{4+δ}$ ($T_c\simeq$ 90K), have such a significant difference in $T_c$, we study a two-orbital model that incorporates the $d_{z^2}$ orbital on top of the $d_{x^2-y^2}$ orbital. It is found, with the fluctuation exchange approximation, that the $d_{z^2}$ orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of $T_c$ in the cuprates.

cond-mat.supr-con↗