Hirofumi Sakakibara

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)CuO4 (T(c)≃40  K) and HgBa2CuO(4+δ) (T(c)≃90  K) have such a significant difference in T(c), we study a two-orbital model that incorporates the d(z2) orbital on top of the d(x2-y2) orbital. It is found, with the fluctuation exchange approximation, that the d(z2) 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.

Origin of the material dependence of Tc in the single-layered cuprates

In order to understand the material dependence of Tc within the single-layered cuprates, we study a two-orbital model that considers both dx2−y2 and dz2 orbitals. We reveal that a hybridization of dz2 on the Fermi surface substantially affects Tc in the cuprates, where the energy difference �E between the dx2−y2 and dz2 orbitals is identified to be the key parameter that governs both the hybridization and the shape of the Fermi surface. A smallerE tends to suppress Tc through a larger hybridization, whose effect supersedes the effect of diamond-shaped (better-nested) Fermi surface. The mechanism of the suppression of d-wave superconductivity due to dz2 orbital mixture is clarified from the viewpoint of the ingredients involved in the Eliashberg equation, i.e., the Greens functions and the form of the pairing interaction described in the orbital representation. The conclusion remains qualitatively the same if we take a three-orbital model that incorporates Cu 4s orbital explicitly, where the 4s orbital is shown to have an important effect of making the Fermi surface rounded. We have then identified the origin of the material and lattice-structure dependence ofE, which is shown to be determined by the energy differenceEd between the two Cu3d orbitals (primarily governed by the apical oxygen height), and the energy differenceEp between the in- plane and apical oxygens (primarily governed by the interlayer separation d).

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

Many important questions for high-Tc 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 La2CuO4. Our results suggest that the strong correlation enough to induce Mott gap may not be a prerequisite for the high-Tc superconductivity.

Coexistence of light and heavy surface states in a topological multiband 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

Orbital mixture effect on the Fermi-surface–Tccorrelation in the cuprate superconductors: Bilayer vs. single layer

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Corrugated flat band as an origin of large thermopower in hole doped PtSb2

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

Three-orbital study on the orbital distillation effect in the high Tc cuprates

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Ideal band shape in the potential thermoelectric material CuAlO2: Comparison to NaxCoO2

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

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Theoretical Expectation of Large Seebeck Effect in PtAs2 and PtP2

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