arXiv subjects

Hiroshi Shimahara

Publications and source records attributed to Hiroshi Shimahara.

29 records · Page 2Linked to original sources

arXiv2003

Fulde-Ferrell-Larkin-Ovchinnikov State and Field-Induced Superconductivity in an Organic Superconductor

An experimental phase diagram of the field-induced superconductivity in the lambda-(BETS)_2FeCl_4 compound is theoretically reproduced by a combination of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state and the Jaccarino-Peter mechanism. Semi-quantitative agreement is obtained in the presence of a very weak subdominant triplet pairing interaction between electrons with antiparallel spins, in addition to a dominant singlet pairing interaction. An order-parameter mixing effect enhances the FFLO state, and plays an essential role in the semi-quantitative agreement. As a possible origin of the small triplet component, pairing interactions mediated by spin fluctuations are proposed. It is discussed that in this mechanism of the pairing interactions equal spin pairing interactions are much weaker than the antiparallel spin pairing interactions in the strong magnetic field, which is consistent with the absence of observation of the equal spin state.

cond-mat.supr-con↗

arXiv2002

Temperature dependence of the upper critical field of an anisotropic singlet superconductivity in a square lattice tight-binding model in parallel magnetic fields

Upper critical field parallel to the conducting layer is studied in anisotropic type-II superconductors on square lattices. We assume enough separation of the adjacent layers, for which the orbital pair-breaking effect is suppressed for exactly aligned parallel magnetic field. In particular, we examine the temperature dependence of the critical field H_c(T) of the superconductivity including the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state, in which the Cooper pairs have non-zero center-of-mass momentum q. In the system with the cylindrically symmetric Fermi-surface, it is known that H_c(T) of the d-wave FFLO state exhibits a kink at a low temperature due to a change of the direction of q in contrast to observations in organic superconductors. It is shown that the kink disappears when the Fermi-surface is anisotropic to some extent, since the direction of q is locked in an optimum direction independent of the temperature.

cond-mat.supr-con↗

arXiv2002

Enhancement of the upper critical field and a field-induced superconductivity in antiferromagnetic conductors

We propose a mechanism by which the paramagnetic pair-breaking effect is largely reduced in superconductors with coexisting antiferromagnetic long- range and short-range orders. The mechanism is an extension of the Jaccarino and Peter mechanism to antiferromagnetic conductors, but the resultant phase diagram is quite different. In order to illustrate the mechanism, we examine a model which consists of mobile electrons and antiferromagnetically correlated localized spins with Kondo coupling between them. It is found that for weak Kondo coupling, the superconductivity occurs over an extraordinarily wide region of the magnetic field including zero field. The critical field exceeds the Chandrasekhar and Clogston limit, but there is no lower limit in contrast to the Jaccarino and Peter mechanism. On the other hand, for strong Kondo coupling, both the low-field superconductivity and a field-induced superconductivity occur. Possibilities in hybrid ruthenate cuprate superconductors and some organic superconductors are discussed.

cond-mat.supr-con↗

arXiv2001

Triplet superconductivity induced by screened phonon interactions in ferromagnetic compounds

We propose that screened pairing interactions mediated by phonons could give rise to a spin triplet superconductivity in ferromagnetic compounds such as UGe_2. It is pointed out that the pairing interactions include anisotropic components such as those of p, d, f-waves in addition to dominant s-wave component due to the momentum dependence. Since the ferromagnetic long-range order coexists, there is a large splitting of the Fermi surfaces of up and down spin electrons, which suppresses singlet pairing. Therefore, triplet pairing occurs at last due to the sub-dominant anisotropic interactions, even in the absence of magnetic contribution to the pairing interactions.

cond-mat.supr-con↗

arXiv2001

Anisotropic superconductivity mediated by phonons in layered compounds with weak screening effect

Anisotropic pairing interactions mediated by phonons are examined in layer systems. It is shown that the screening effects become weaker when the layer spacing increases. Then the anisotropic components of the pairing interactions increase with the screening length since the momentum dependence changes. As a result, various types of anisotropic superconductivity occur depending on the parameter region. For example, p-wave superconductivity occurs when the short-range part of Coulomb repulsion is strong and the layer spacing is large. Two kinds of inter-layer pairing may occur when the layer spacing is not too large. Although the phonon contribution to the d-wave pairing interaction is weaker than the p-wave interaction, it increases with the layer spacing. Relevance of the present results to organic superconductors, high-T_c cuprates, and Sr_2RuO_4 is discussed.

cond-mat.supr-con↗

arXiv2000

Pseudogap due to antiferromagnetic fluctuations and the phase diagram of the high temperature oxide superconductors

A reduction of the density of states near the Fermi energy in the normal state (pseudogap) of high-temperature oxide superconductors is examined on the basis of the two-dimensional tight-binding model with effective interactions due to antiferromagnetic fluctuations. By using antiferromagnetic correlation lengths which are phenomenologically assumed, the doping dependence of the pseudogap is obtained. The superconducting transition temperature decreases and eventually vanishes due to the pseudogap as the hole concentration is reduced.

cond-mat.supr-con↗

arXiv2000

Coexistence of Singlet and Triplet Attractive Channels in the Pairing Interactions Mediated by Antiferromagnetic Fluctuations

We propose a phase diagram of quasi-low-dimensional type II superconductors in parallel magnetic fields, when antiferromagnetic fluctuations contribute to the pairing interactions. We point out that pairing interactions mediated by antiferromagnetic fluctuations necessarily include both singlet channels and triplet channels as attractive interactions. Usually, a singlet pairing is favored at zero field, but a triplet pairing occurs at high fields where the singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect. As a result, the critical field increases divergently at low temperatures. A possible relation to experimental phase diagrams of a quasi-one-dimensional organic superconductor is briefly discussed. We also discuss a possibility that a triplet superconductivity is observed even at zero field.

cond-mat.supr-con↗

arXiv2000

Upper critical fields of quasi-low-dimensional superconductors with coexisting singlet and triplet pairing interactions in parallel magnetic fields

Quasi-low-dimensional type II superconductors in parallel magnetic fields are studied when singlet pairing interactions and relatively weak triplet pairing interactions coexist. Singlet and triplet components of order parameter are mixed at high fields, and at the same time an inhomogeneous superconducting state called a Fulde-Ferrell-Larkin-Ovchinnikov state occurs. As a result, the triplet pairing interactions enhance the upper critical field of superconductivity remarkably even at temperatures far above the transition temperature of parallel spin pairing. It is found that the enhancement is very large even when the triplet pairing interactions are so weak that a high field phase of parallel spin pairing may not be observed in practice. A possible relvance of the result in organic superconductors and a hybrid-ruthenate-cuprate superconductor is discussed.

cond-mat.supr-con↗

arXiv2000

Superconductivity in a Ferromagnetic Layered Compound

We examine superconductivity in layered systems with large Fermi-surface splitting due to coexisting ferromagnetic layers. In particular, the hybrid ruthenate-cuprate compound RuSr_2GdCu_2O_8 is examined on the coexistence of the superconductivity and the ferromagnetism, which has been observed recently. We calculate critical fields of the superconductivity taking into account the Fulde-Ferrell-Larkin-Ovchinnikov state in a model with Fermi-surfaces which shapes are similar to those obtained by a band calculation. It is shown that the critical field is enhanced remarkably due to a Fermi-surface effect, and can be high enough to make the coexistence possible in a microscopic scale. We also clarify the direction of the spatial oscillation of the order parameter, which may be observed by scanning tunneling microscope experiments.

cond-mat.supr-con↗

arXiv1999

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.

cond-mat.supr-con↗

arXiv1998

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 type-II superconductors near the second order phase transition line between the normal and the superconducting states. The optimum spatial structure of the orderparameter is examined in systems with cylindrical symmetric Fermi surfaces. It is found that states with two-dimensional structures have lower free energies than the traditional one-dimensional solutions, at low temperatures and high magnetic fields. For s-wave pairing, triangular, square, hexagonal states are favored depending on the temperature, while square states are favored at low temperatures for d-wave pairing. In these states, orderparameters have two-dimensional structures such as square and triangular lattices.

cond-mat.supr-con↗