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.