Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields

TL;DR

This paper explores topological phase transitions in ultra-cold Fermi superfluids influenced by interactions and artificial spin-orbit fields, revealing complex phase diagrams and the emergence of Majorana edge states.

Contribution

It constructs phase diagrams for imbalanced systems and analyzes the conditions for topological transitions and Majorana fermions in ultra-cold Fermi superfluids.

Findings

Spin-orbit fields induce triplet components in the order parameter.

Topological phase transitions occur with the emergence of nodes in the quasiparticle spectrum.

Majorana fermion edge states are associated with certain gapped phases.

Abstract

We discuss topological phase transitions in ultra-cold Fermi superfluids induced by interactions and artificial spin orbit fields. We construct the phase diagram for population imbalanced systems at zero and finite temperatures, and analyze spectroscopic and thermodynamic properties to characterize various phase transitions. For balanced systems, the evolution from BCS to BEC superfluids in the presence of spin-orbit effects is only a crossover as the system remains fully gapped, even though a triplet component of the order parameter emerges. However, for imbalanced populations, spin-orbit fields induce a triplet component in the order parameter that produces nodes in the quasiparticle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type. Additionally a fully gapped phase exists, where a crossover from indirect to direct gap occurs, but a topological transition to a gapped phase possessing Majorana fermions edge states does not occur.