Stabilization of the Fulde-Ferrell-Larkin-Ovchinnikov State by Tuning In-plane Magnetic-Field Direction: Application to a Quasi-One-Dimensional Organic Superconductor

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in quasi-one-dimensional systems with warped Fermi surfaces is examined in strong parallel magnetic fields. It is shown that the state is extremely stable for field directions around nontrivial optimum directions, at which the upper critical field exhibits cusps, and that the stabilization is due to a Fermi-surface effect analogous to the nesting effect for the spin density wave and charge density wave. Interestingly, the behavior with cusps is analogous to that in a square lattice system in which the hole density is controlled. For the organic superconductor (TMTSF)_2ClO_4, when the hopping parameters obtained by previous authors based on X-ray crystallography results are assumed, the optimum directions are in quadrants consistent with the previous experimental observations. Furthermore, near this set of parameters, we also find sets of hopping parameters that more precisely reproduce the observed optimum in-plane field directions. These results are consistent with the hypothesis that the FFLO state is realized in the organic superconductor.