Theory of the Fulde–Ferrell–Larkin–Ovchinnikov State and Application to Quasi-Low-dimensional Organic Superconductors

Abstract

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO or LOFF) state is a superconducting state of Cooper pairs with finite center-of-mass momenta that is stabilized by the Zeeman energy at high magnetic fields in clean type II superconductors. In this article, we review recent developments of the theory of the FFLO state in quasi-low-dimensional (QLD) systems, and its application to QLD organic superconductors. For the FFLO state to occur, the orbital pair-breaking effect needs to be sufficiently weak that superconductivity survives up to the Pauli paramagnetic limit. This condition is satisfied in layered superconductors with small interlayer electron hopping energy when the magnetic field is precisely aligned parallel to the most conductive layers. The FFLO state is favored in QLD systems because of the Fermi surface effect, analogous to the nesting effects for charge density waves (CDW) and spin density waves (SDW), arising in the FFLO state due to the finite center-of-mass momenta of the Cooper pairs.