Pair tunneling, phase separation, and dimensional crossover in imbalanced fermionic superfluids in a coupled array of tubes

We study imbalanced fermionic superfluids in an array of one-dimensional tubes at the incipient dimensional crossover regime, wherein particles can tunnel between neighboring tubes. In addition to single-particle tunneling (ST), we consider pair tunneling (PT) that incorporates the interaction effect during the tunneling process. We find that with an increase of PT strength, a system of low global polarization evolves from a structure with a central Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state to one with a central BCS-like fully paired state. For the case of high global polarization, the central region exhibits pairing zeros embedded in a fully paired order. In both cases, PT enhances the pairing gap, suppresses the FFLO order, and leads to spatial separation of fully paired and fully polarized regions, the same as in higher dimensions. Thus we show that PT beyond second-order ST processes is of relevance to the development of signatures characteristic of the incipience of the dimensional crossover.