Hiroshi Shimahara

Fulde–Ferrell–Larkin–Ovchinnikov State in Heavy Fermion Superconductors

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state is a novel superconducting state in a strong magnetic field characterized by the formation of Cooper pairs with nonzero total momentum ( k ↑,- k + ...

Green's Function Theory of the Two-Dimensional Heisenberg Model : Spin Wave in Short Range Order

The two dimensional Heisenberg model is studied with a Greens function decoupling scheme in the ferromagnetic and the antiferromagnetic cases. We employ Kondo and Yamajis formulation and improve it quantitatively. The present theory does not violate the rotational symmetry and the sum rule of the correlation function. We calculate excitation spectrum, long range order, correlation functions, correlation length, energy, and spin susceptibility. The results are compared with those of the Monte Carlo simulations and the experiments of La 2 CuO 4 . Qualitative and semi-quantitative agreements are obtained. In particular, the temperature dependence of the susceptibility indicated by Monte Carlo simulations is reproduced over all temperature region.

Superconductivity and Spin Density Wave in Two Dimensional Hubbard Model

An interplay between the superconductivity and spin density wave (SDW) is studied from the perturbative point of view based on the two dimensional Hubbard model. It is shown that the d-pairing superconductivity is induced by spin fluctuations near the SDW boundary. The superconducting transition temperature ( T c ) is found to be remarkably suppressed by the renormalization through the electron self-energy, and to be sensitive to the band parameters due to nesting of the Fermi surface and the presence of the van Hove singularity as well as to the Fermi energy and the on-site interaction. As a consequence T c is enhanced by the next-nearest-neighbour hopping integral. The relation between the present result and experiments of high T c superconductors is discussed.

Crossover from Vortex States to the Fulde-Ferrell- Larkin-Ovchinnikov State in Two-Dimensional s- and d-Wave Superconductors

The effects of a tilted magnetic field on two-dimensional type-II superconductors of s - and d -wave pairing are studied. This includes a study of the crossover from vortex states to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state when the direction of the magnetic field is changed from perpendicular to the conducting plane to a parallel orientation. The FFLO state is obtained in the limit of a parallel magnetic field, and vortex phases with a large quantum number n >0 (a higher Landau level) appear for tilted magnetic fields. At fixed tilt angle, the critical field exhibits a characteristic gtemperature dependence due to the change of the discrete quantum number. The relevance of the results for the observation and the identification of the FFLO state in the organic and high- T c superconductors is discussed.

Structure of the Fulde-Ferrell-Larkin-Ovchinnikov State in Two-Dimensional Superconductors

Nonuniform superconducting state due to strong spin magnetism is studied in two-dimensional (2D) type-II superconductors near the second order phase transition line between the normal and superconducting states. The optimum spatial structure of the order parameter is examined in systems with cylindrically symmetric Fermi surfaces. It is found that states with 2D structures have lower free energies than the traditional one-dimensional solutions, at low temperatures and high magnetic fields. For s -wave pairing, triangular, square, and hexagonal states are favored depending on the temperature, while square states are favored at low temperatures for d -wave pairing. In these states, the order parameters have 2D structures such as square and triangular lattices.

Long-Range Spin-Fluctuations and Superconductivity in Quasi-One-Dimensional Organic Compounds

The superconductivity mediated by antiferromagnetic spin-fluctuations (AFSF) is studied in the quasi-one-dimensional Hubbard model. The superconducting transition temperature and the momentum dependence of the order parameter are calculated numerically. An effect of the long-range (≫the lattice constant) AFSF is examined. The obtained phase diagram is compared with that of the organic superconductors (TMTSF) 2 X and (DMET) 2 X, and some qualitative agreements between the theory and experiments are obtained.

Fulde-Ferrell-Larkin-Ovchinnikov State in a Quasi-Two-Dimensional Organic Superconductor

The possibility of the existence Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in quasi-low-dimensional organic superconductors is discussed. The critical field and the wave number q of the spatial oscillation are calculated on the basis of a tight binding model of κ- [BEDT- TTF] 2 I 3 , as an example. The critical fields H c at T =0 are estimated to be roughly 1.5, 2, and 2.5 times the Pauli paramagnetic limit H P for s -wave, d x y -wave, and d x 2 - y 2 -wave FFLO states, respectively. We discuss which properties of the structure of the Fermi surface enhance the critical field and are relevant to the direction of q . We show that the number of the lines on the displaced Fermi surfaces due to the Zeeman energy which simaltaneously touch by translation p → p + q are important in addition to the curvature of the Fermi surface and the density of states.

Fragility of the antiferromagnetic long-range-order and spin correlation in the two-dimensional t-J model

The two dimensional (2D) t - J model is studied in a Greens function decoupling scheme, which is an extension of the Kondo and Yamajis theory (KY theory). The KY theory was originally proposed for spin systems, and is applicable to cases in which there is no symmetry breaking. We derive the lowest order corrections by the electron hopping energy t to self-consistent equations for the correlation function. Hole concentration dependences of the antiferromagnetic (AF) long-range-order (LRO) and spin correlation length at zero temperature are obtained. It is found that they decrease sensitively with doping holes even in a situation very favorable for the antiferromagnetism. The critical hole concentration at which AF LRO vanishes is estimated to be much smaller than in the three-dimensional case. A characteristic feature of 2D electron systems with strong correlations is discussed.

Order Parameter Mixing Effectin the Fulde-Ferrell State

It is shown that when the triplet pairing interaction is finite in the singlet pairing superconductor, the Fulde-Ferrell (FF) state is significantly enhanced by the mixing effect which is inherent to nonuniform superconducting states in a strong magnetic field. The mixed FF state can exist at higher temperatures than the pure singlet FF state. This may explain the behavior of the recently observed high-field phase of the heavy fermion superconductor UPd 2 Al 3 . The phase diagram of the second-order phase transition is given for a simple model of s -wave- p -wave mixing state.

Nodeless d-wave superconductivity in a quasi-one-dimensional organic superconductor under anion order

We propose a mechanism of nodeless d-wave superconductivity in a quasi-one-dimensional organic superconductor under anion order. It is shown that split of the Fermi surfaces due to the anion order eliminates the line nodes of a d-wave gap function on the Fermi surfaces, and a small finite energy gap appears in the quasi-particle excitations of superconductivity. We discuss that temperature dependences of thermal conductivity and NMR relaxation rate observed in a quasi-one-dimensional organic superconductor (TMTSF)2ClO4 can be explained consistently by the small energy gap.