Exotic Pairing States in Two-dimensional Fermi Gases with Rashba Spin-Orbit Coupling

A/Prof. Wei Zhang

Renmin University of China, Beijing

3:30 pm Friday, 18 October 2013, EN313 Lecture Theatre (EN Building), Hawthorn.

The realization of synthetic spin-orbit coupling (SOC) adds another important piece to the already versatile toolbox of controllability in cold atoms. Theoretical investigation has demonstrated that the interplay of SOC, pairing superfluidity and effective Zeeman fields can lead to exotic superfluid phases in various dimensions. In these systems, as SOC mixes different spin states, both intra- and inter-branch pairings can take place and the competition between them results in rich phase structures.

In this talk, I will investigate the pairing physics in a two-dimensional Fermi gas with Rashba SOC. In the presence of an out-of-plane Zeeman field, a topological superfluid (TSF) state will be stabilized in a large parameter region on the phase diagram by tuning through the Feshbach resonance. The TSF state acquires a chiral Majorana edge mode protected by the gap in the bulk, and can be understood in the weak-coupling limit as a consequence of intra-branch pairing when the chemical potential lies within or below the gap opened by the out-of-plane Zeeman field. As both the inversion and the time-reversal symmetries are broken in the system, the TSF state belongs to class D. The TSF state will evolve into a topological Fulde-Ferrell (FF) state if an additional in-plane Zeeman field is applied. Similar to the case of a TSF state, in the weak-coupling limit, the emergence of the topological FF state can be understood as a result of single-branch pairing within the lower helicity branch. The resulting pairing state would preserve all topological properties provided that the deformation of the Fermi surface should not be drastic enough to violate the single-branch pairing scenario.

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