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.Comment: 5 pages, 1 figure, revtex.sty, to be published in J.Phys.Soc.Jpn. Vol.71, No.3 (2002

Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors

Antiferromagnetic superconductors in a magnetic field are studied. We examine a mechanism which significantly reduces the Pauli paramagnetic pair-breaking effect. The mechanism is realized even in the presence of the orbital pair-breaking effect. We illustrate it using a three-dimensional model with an intercalated magnetic subsystem. The upper critical field is calculated for various parameters. It is shown that the upper critical field can reach several times the pure Pauli paramagnetic limit. The possible relevance to the large upper critical field observed in the heavy fermion antiferromagnetic superconductor CePt_3Si discovered recently is briefly discussed. We try to understand the large upper critical field in the compound CePt_3Si and field-induced superconductivity in the compound CePb_3 within a unified framework.Comment: 5 pages, 2 figures, revtex4, minor correction

Enhancement of the Josephson current by magnetic field in superconducting tunnel structures with paramagnetic spacer

The dc Josephson critical current of a (S/M)IS tunnel structure in a parallel magnetic field has been investigated (here S is a superconductor, S/M is the proximity coupled S and paramagnet M bilayer and I is an insulating barrier). We consider the case when, due to the Hund's rule, in the M metal the effective molecular interaction aligns spins of the conducting electrons antiparallel to localized spins of magnetic ions. It is predicted that for tunnel structures under consideration there are the conditions when the destructive action of the internal and the applied magnetic fields on Cooper pairs is weakened and the increase of the applied magnetic field causes the field-induced enhancement of the tunnel critical current. The experimental realization of this interesting effect of the interplay between superconductivity and magnetism is also discussed.Comment: 5 pages 3 figure

New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4

We derive the parallel upper critical field, Hc2, as a function of the temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for the formation of the nonuniform LOFF state. To further check the 2D LOFF model we propose to study the Hc2(T) curve at low T in tilted fields, where the vortex state is described by the high Landau level functions characterized by the index n. We predict a cascade of first order transitions between vortex phases with different n, between phases with different types of the symmetry at given n and the change of the superconducting transition from the second order to the first order as FeCl4 ions are replaced partly by GaCl4 ions.Comment: 4 pages, 3 figures, to be published in PR

Ferromagnetism and Superconductivity in Uranium Compounds

Recent advances on ferromagnetic superconductors, UGe2, URhGe and UCoGe are presented. The superconductivity (SC) peacefully coexists with the ferromagnetism (FM), forming the spin-triplet state of Cooper pairs. The striking new phenomena, such as SC reinforced by the magnetic field, are associated with Ising-type ferromagnetic fluctuations. A variety of ferromagnetic ordered moments between UGe2, URhGe and UCoGe affords to understand the relation between FM, tricriticality and SC.Comment: 11 pages, 16 figures, accepted for publication in J. Phys. Soc. Jpn. as a review article of Special Topics of "Recent developments in superconductivity