Fulde–Ferrell–Larkin–Ovchinnikov states in quasi-two-dimensional d + s-wave superconductors: Enhancement of the upper critical field

We investigate the spatially nonuniform Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting state in a quasi-two-dimensional (Q2D) d + s-wave superconductor. In the d + s-wave superconductor, the shape of the superconducting gap function and the nature of FFLO state depend on the degree of s-wave (or d-wave) admixture. The magnetic field versus temperature phase diagrams for the d + s-wave states with different shape of gap function are studied systematically. We find in particular that, a slight deviation of gap function from that of d-wave state may lead to significant change of FFLO state and remarkable enhancement of the upper critical magnetic field. The superconducting density of states (DOS) for d + s-wave superconductor is also calculated and found to be strikingly different from that for pure s-wave and pure d-wave superconductors.