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 two-leg and three-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, but not within, 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 wide applicability of the high Tc pairing mechanism due to coexisting wide and "incipient" narrow bands in quasi-one-dimensional systems.

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 Tc.

In the hole-doped cuprate superconductors, the superconducting transition temperature Tc exhibits a dome-like feature against the doping rate. By contrast, recent experiments reveal that Tc 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 CuO2 plane. This is explained as a combined effect of the intrinsic electron-hole asymmetry in systems comprising Cu3d and O2p orbitals and the band-filling-dependent vertex correction.PACS numbers: 74.25. Dw, 74.20.Pq Despite the long history, there still remain various unsolved problems in the study of the high-T c cuprate superconductors. The striking difference in the doping dependence of the superconducting transition temperature T c between the hole-and the electron-doped materials is among those unresolved issues. It is well known that in the hole-doped case, T c exhibits a domelike feature against the doping rate, namely, T c first increases upon doping (underdoped), then yields a maximum value (optimal), and finally decreases with further doping (overdoped). On the other hand, it was known for the electron-doped cases that T c abruptly appears as soon as the antiferromagnetism is lost with doping, and monotonically decreases as the doping rate increases. Recent experiments show that the antiferromagnetism can be suppressed down to very small doping rate, or even in the mother compound, when the apical oxygens are ideally removed in the T -type crystal structure of the electron doped cuprates. Then, it has been revealed that T c monotonically increases with decreasing the electron doping at least up to a very small doping rate (less than 5 percent), and is suggested to be superconducting even in the non-doped mother compound 1-4 .

We study the spin-fluctuation-mediated s±-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.

We introduce the concept of "hidden ladders" for the bilayer Ruddlesden-Popper type compounds: While the crystal structure is bilayer, dxz (dyz) orbitals in the relevant t2g sector of the transition metal form a two-leg ladder along x (y), since the dxz (dyz) electrons primarily hop in the leg (x, y) direction along with the rung (z) direction. This leads us to propose that Sr3Mo2O7 and Sr3Cr2O7 are candidates for the hidden-ladder material, with the right position of EF from a first-principles band calculation. Based on the analysis of Eliashberg equation, we predict the possible occurrence of high temperature superconductivity in these non-copper materials arising from the interband pair-scattering processes between a wide band and an "incipient" narrow band on the ladder.

For the recently discovered cuprate superconductor 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 intraorbital and interorbital 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±-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 sand 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.

We study the electronic structure of delafossite PtCoO2 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 + px/py orbitals, and the other from Pt d 3z 2 −r 2 + dxy/d x 2 −y 2 orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt s + px/py 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 + px/py band. In addition, 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 PtCoO2. PACS numbers:

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