# Fulde-Ferrell superfluids in spinless ultracold Fermi gases

## TL;DR

This paper demonstrates that Fulde-Ferrell superfluid phases can emerge in spinless ultracold Fermi gases within optical lattices due to split Fermi surfaces, with potential for stable finite-temperature realization.

## Contribution

It introduces a novel mechanism for FF state emergence in spinless Fermi gases via Fermi surface splitting, differing from previous spinful implementations.

## Key findings

- FF state arises from split Fermi surfaces in spinless gases
- Finite-momentum pairing remains robust beyond mean-field approximations
- Phase transition involves Chern number changes without band gap closing

## Abstract

The Fulde-Ferrell (FF) superfluid phase, in which fermions form finite-momentum Cooper pairings, is well studied in spin-singlet superfluids in past decades. Different from previous works that engineer the FF state in spinful cold atoms, we show that the FF state can emerge in spinless Fermi gases confined in optical lattice associated with nearest-neighbor interactions. The mechanism of the spinless FF state relies on the split Fermi surfaces by tuning the chemistry potential, which naturally gives rise to finite-momentum Cooper pairings. The phase transition is accompanied by changed Chern numbers, in which, different from the conventional picture, the band gap does not close. By beyond-mean-field calculations, we find the finite-momentum pairing is more robust, yielding the system promising for maintaining the FF state at finite temperature. Finally we present the possible realization and detection scheme of the spinless FF state.