The low-temperature Fulde-Ferrell-Larkin-Ovchinnikov phases in 3 dimensions

We consider the nature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases in three dimensions at low temperature. We introduce a new method to handle the quasiclassical equations for superconductors with space-dependent order parameter, which makes use of a Fourier expansion. This allows us to show that, at T = 0, an order parameter given by the linear combination of three cosines oscillating in orthogonal directions is preferred over the standard single-cosine solution. The transition from the normal state to this phase is first order, and quite generally the transition below the tricritical point to the FFLO phases is always first order.