Peculiar vortex structures in Fulde-Ferrell-Larkin-Ovchinnikov phase

Abstract
A long time ago, it was predicted by Larkin and Ovchinnikov and Fulde and Ferrell that the non-uniform superconducting state (FFLO state) must appear in the magnetic field acting on the electron spins. Up to now, there have been no unambiguous experimental proofs in the favour of this state observation. We discuss the unusual properties of such a state, which can permit its identification. It is demonstrated that in 2D (or quasi 2D) superconductors the FFLO state leads to an appearance of a very special oscillatory - like dependence of the upper critical field versus the angle with the respect to the layers. The new solutions, corresponding to the higher Landau level functions are realized, and the vortex lattice structures are quite exotic. Corresponding vortex states reveal the zeros of superconducting order parameter with high winding numbers. The predicted quasi-oscillatory angular and temperature dependence of B , as well as a cascade of first order transitions must permit the unambiguous identification of mysterious FFLO state. Very recently the magnetic-field-induced superconductivity has been observed in the quasi two-dimensional (2d) organic conductor (BETS) ₂FeCI ₄ which is an excellent candidate for the observation of the discussed effects.