# Chiral twodimensional p-wave superfluid from s-wave pairing in the BEC regime

## TL;DR

This paper explores the BCS-to-BEC crossover in two-dimensional spin-orbit-coupled Fermi gases with s-wave pairing, revealing conditions under which robust topological chiral p-wave superfluidity persists even in the BEC regime, aiding experimental realization.

## Contribution

It provides phase diagrams and analysis showing that topological chiral p-wave superfluidity remains stable in the BEC regime, unlike in the BCS limit, and identifies optimal parameters for experimental detection.

## Key findings

- Topological phase retains Fermi surface features in the BEC regime.
- Chiral p-wave order parameter is larger in the BEC regime, facilitating detection.
- Moderate spin-orbit coupling suffices for topological superfluidity in the BEC regime.

## Abstract

Twodimensional spin-orbit-coupled Fermi gases subject to s-wave pairing can be driven into a topological phase by increasing the Zeeman spin splitting beyond a critical value. In the topological phase, the system exhibits the hallmarks of chiral p-wave superfluidity, including exotic Majorana excitations. Previous theoretical studies of this realization of a twodimensional topological Fermi superfluid have focused on the BCS regime where the s-wave Cooper pairs are only weakly bound and, hence, the induced chiral p-wave order parameter has a small magnitude. Motivated by the goal to identify potential new ways for the experimental realization of robust topological superfluids in ultra-cold atom gases, we study the BCS-to-BEC crossover driven by increasing the Cooper-pair binding energy for this system. In particular, we obtain phase diagrams in the parameter space of two-particle bound-state energy and Zeeman spin-splitting energy. Ordinary characteristics of the BCS-to-BEC crossover, in particular the shrinking and eventual disappearance of the Fermi surface, are observed in the nontopological phase. In contrast, the topological phase retains all features of chiral p-wave superfluidity, including a well-defined underlying Fermi surface, even for large s-wave pair-binding energies. Compared to the BCS limit, the topological superfluid in the BEC regime turns out to be better realizable even for only moderate magnitude of spin-orbit coupling because the chiral p-wave order parameter is generally larger and remnants of s-wave pairing are suppressed. We identify optimal parameter ranges that can aid further experimental investigations and elucidate the underlying physical reason for the persistence of the chiral p-wave superfluid.