Youichi Yanase

Theory of pseudogap phenomena in High-Tc cuprates based on the strong coupling superconductivity

In this paper we study the effects of the strong coupling superconductivity on the normal state electronic structure. We point out that the pseudogap phenomena in High- T c cuprates are naturally explained as a precursor of the strong coupling superconductivity. We expand the reciprocal of the T-matrix with respect to the momentum and the frequency, and we discuss the properties of the expansion parameters. We estimate these parameters and carry out the calculations for the single particle self-energy using the T-matrix and the self-consistent T-matrix approximations, respectively. We show that the superconducting fluctuations, which are strongly diffusive within the conventional weak coupling theory, become propagative and have the character of the pre-formed pairs in the strong coupling case. The spectral weight and the density of states show the gap-like features by the effects of the resonances with the thermally excited weakly-damped pre-formed pairs. The spectral weight at the Fermi energy is strong...

Superconductivity in NaxCoO2·yH2O due to Charge Fluctuation

A new mechanism of superconductivity due to charge fluctuation in the newly discovered Co-based oxide is proposed. A single-band extended Hubbard model on a triangular lattice is studied within the random phase approximation. f -wave triplet superconductivity is stabilized in the vicinity of charge–density–wave instability, which is in sharp contrast to the square-lattice case. The physical origin of the realization of the f -wave triplet state as well as its relevance to experiments are discussed.

Pseudogap Phenomena and Superconducting Fluctuations in Hubbard Model for High-Tc Cuprates

The pseudogap phenomena in High- T c cuprates are investigated on the basis of the Hubbard model which includes only the on-site repulsive interaction U . We consider the pairing scenario for the p...

Multi-orbital Analysis on the Superconductivity in NaxCoO2·yH2O

We preform a multi-orbital analysis on the novel superconductivity in Na x CoO 2 · y H 2 O. We construct a three-orbital model which reproduces the band structure expected from the LDA calculation. The effective interaction leading to the pairing is estimated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region. This is basically owing to the ferromagnetic character of spin fluctuation. The p -wave and f -wave superconductivity are nearly degenerate. The former is realized when the Hunds rule coupling is large, and vice versa . In a small part of the parameter space, the d -wave superconductivity is also stabilized. We point out that the orbital degeneracy plays an essential role for these results. In particular, the momentum dependence of wave function of quasi-particles is quite important. The nearly degeneracy of p -wave and f -wave superconductivity is explained by analysing the microscopic character of each Fermi surface....

Microscopic Identification of the D-vector in Triplet Superconductor Sr2RuO4

Triplet superconductivity in Sr 2 RuO 4 is investigated with main interest on its internal degree of freedom. We perform a microscopic calculation to investigate how the chiral state \(\hat{d}(k)=(k_{x}\pm \text{i}k_{y})\hat{z}\) is realized among the underlying six degenerate states. Starting from the three band Hubbard model with spin–orbit interaction, we use a perturbation theory in order to calculate the pairing interaction. The p -wave superconductivity with T c ∼1.5 K is obtained in the moderately weak coupling region. It is shown that the orbital dependent superconductivity (ODS) robustly appears in Sr 2 RuO 4 . We determine the stabilized state by solving the Eliashberg equations. We find that the Hund coupling term as well as the spin–orbit interaction is necessary for the “symmetry breaking interaction”. The main result is that the chiral state is stabilized in case of the p -wave symmetry with the main γ-band, which is obtained in the perturbation theory. When we assume the other pairing symme...

Ferromagnetic Fluctuation and Possible Triplet Superconductivity in Na~xCoO~2 · yH~2O: Fluctuation-Exchange Study of the Multiorbital Hubbard Model

Spin and charge fluctuations and superconductivity in NaxCoO2.yH(2)O are studied based on a multiorbital Hubbard model. Tight-binding parameters are determined to reproduce the results of band calculations. By applying the fluctuation-exchange approximation, we show that the Hunds-rule coupling between the Co t(2g) orbitals causes ferromagnetic (FM) spin fluctuation. Triplet fy((y(2)-3x(2)))-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in NaxCoO2.yH(2)O, the Co t(2g) orbitals and interorbital Hunds-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor in which the orbital degrees of freedom play substantial roles.

Theory of Transport Phenomena in High-Tc Cuprates.

In this paper we point out that the anomalous transport phenomena in high- T c cuprates are comprehensively understood on the basis of Fermi liquid theory by taking account of a strong anti-ferromagnetic spin fluctuation. We assume phenomenologically the spectrum of magnetic excitation from experiment and calculate the self-energy on the basis of the one loop approximation. We show that the T -linear term of damping rate, the momentum dependence of lifetime and the transformation of Fermi surface are important to explain the anomalous transport phenomena such as resistivity, Hall coefficient and c -axis resistivity. In particular, the strong spin fluctuation leads to the transformation of Fermi surface which leads to a form more appropriate to nesting. This effect plays an important role. At last we show that our theory is applicable to not only the normal state but also the pseudogap state.

Non-centrosymmetric Superconductivity and Antiferromagnetic Order: Microscopic Discussion of CePt3Si

The influence of antiferromagnetic order on the superconductivity in the non-centrosymmetric heavy fermion compound CePt 3 Si and related materials is discussed. Based on our RPA analysis for the extended Hubbard model two phases could be stabilized by a spin fluctuation induced pairing, with either dominantly p -wave or d -wave symmetry. The antiferromagnetic order plays an essential role for the low-energy physics, in particular, for the appearance of line nodes in the gap and the enhancement of spin susceptibility below T c . Various properties and possible phase diagrams under pressure are analyzed. The present experimental situation suggests that the p -wave phase is most likely realized in CePt 3 Si.

Superconductivity and Magnetism in Non-centrosymmetric System: Application to CePt3Si

Superconductivity and magnetism in the non-centrosymmetric heavy fermion compound CePt 3 Si and related materials are theoretically investigated. On the basis of the random phase approximation (RPA) analysis of the extended Hubbard model, we describe the helical spin fluctuation induced by the Rashba-type anti-symmetric spin–orbit coupling and identify two stable superconducting phases with either the dominant p -wave ( s + P -wave) symmetry or the d -wave ( p + D + f -wave) symmetry. The effect of the coexistent antiferromagnetic order is investigated in both states. The superconducting order parameter, quasiparticle density of state, NMR 1/ T 1 T , specific heat, anisotropy of H c2 , and possible multiple phase transitions are discussed in detail. A comparison with experimental results indicates that the s + P -wave superconducting state is likely realized in CePt 3 Si.

Superconductivity due to charge fluctuation in θ-type organic conductors

Superconductivity close to charge ordering in quasi-two-dimensional 3/4-filled organic compounds is investigated for the θ-type structure. The effect of spin and charge fluctuations is calculated within random phase approximation for a relevant extended Hubbard model with both on-site ( U ) and intersite ( V ) Coulomb interactions on an anisotropic triangular lattice. It is found that both spin and charge fluctuations induce superconductivity with extended s -wave singlet pairing, which is similar to the d x y pairing state in the square lattice. In the vicinity of charge-density-wave instability, f -wave triplet pairing is also stable due to the effect of nearest-neighbor Coulomb interaction. The temperature dependences of superconductivity and charge-density-wave phase boundaries are also examined.