Interaction-range effects and universality in the BCS-BEC crossover of spin-orbit–coupled Fermi gases

¹ Dipartimento di Fisica “Enrico Fermi”, Università di Pisa and INFN - Largo B. Pontecorvo 3, I-56127 Pisa, Italy
² Dipartimento di Ingegneria Civile e Industriale, Università di Pisa and INFN - Largo L. Lazzarino, I-56122 Pisa, Italy

Received: 13 June 2018
Accepted: 19 September 2018

Abstract

We explore the evolution of an ultracold quantum gas of interacting fermions crossing from a Bardeen-Cooper-Schrieffer (BCS) superfluidity to a Bose-Einstein condensation (BEC) of molecular bosons in the presence of a tunable-range interaction among the fermions and of an artificial magnetic field, which can be used to simulate a pseudo-spin-orbit coupling (SOC) and to produce topological states. We find that the crossover is affected by a competition between the finite range of the interaction and the SOC and that the threshold for the topological transition is affected by the interactions only in the small pair size, BEC-like, regime. Below , we find persistence of universal behavior in the critical temperature, chemical potential, and condensate fraction, provided that the pair correlation length is used as a driving parameter. Above threshold, universality is lost in the regime of large pair sizes. Here, the limiting ground state departs from a weakly interacting BCS-like one so that a different description is required. Our results can be relevant in view of current experiments with cold atoms in optical cavities, where tunable-range effective atomic interactions can be engineered.