Coexistence of topological and nontopological Fermi-superfluid phases

TL;DR

This paper explores the coexistence of topological and nontopological superfluid phases in a spin-imbalanced Fermi gas with spin-orbit coupling, revealing phase transitions and spatial domain separation through theoretical analysis.

Contribution

It provides a theoretical investigation of phase coexistence in spin-imbalanced Fermi superfluids, including numerical results and an analytical description for weak spin-orbit coupling.

Findings

Topological and nontopological superfluids coexist in certain parameter regions.

Phase transitions between superfluid phases can be first-order or second-order.

The study offers insights into potential Majorana excitations at phase interfaces.

Abstract

The two-dimensional spin-imbalanced Fermi gas subject to s-wave pairing and spin-orbit coupling is considered a promising platform for realizing a topological chiral-p-wave superfluid. In the BCS limit of s-wave pairing, i.e., when Cooper pairs are only weakly bound, the system enters the topological phase via a second-order transition driven by increasing the Zeeman spin-splitting energy. Stronger attractive two-particle interactions cause the system to undergo the BCS-BEC crossover, in the course of which the topological transition becomes first-order. As a result, topological and nontopological superfluids coexist in spatially separated domains in an extended region of phase space spanned by the strength of s-wave interactions and the Zeeman energy. Here we investigate this phase-coexistence region theoretically using a zero-temperature mean-field approach. Exact numerical results are presented to illustrate basic physical characteristics of the coexisting phases and to validate an approximate analytical description derived for weak spin-orbit coupling. Besides extending our current understanding of spin-imbalanced superfluid Fermi systems, the present approach also provides a platform for future studies of unconventional Majorana excitations that, according to topology, should be present at the internal interface between coexisting topological and nontopological superfluid parts of the system.