Non-Hermitian BCS-BEC evolution with a complex scattering length

Abstract

Having both elastic and inelastic two-body processes that are characterized by a complex s-wave scattering length between ↑ and ↓ fermions in mind, here we apply the non-Hermitian extension of the mean-field theory to the BCS-BEC evolution at zero temperature. We construct the phase diagram of the system, where we find a reentrant superfluid (SF) transition that is intervened by a normal and/or a metastable phase as a function of increasing inelasticity. This transition occurs in a large parameter regime away from the unitarity, i.e., on both the BCS and BEC sides of the resonance, and it is mostly governed by the exceptional points. In addition, except for the strongly inelastic regime, we also show that the SF phase can be well described by the condensation of weakly interacting bosonic pairs in the two-body bound state with a complex binding energy.