Condensed Matter Seminar Series

Topological Phase Transitions in Ultra-Cold Fermi Superfluids:
The Evolution from BCS to BEC under Zeeman and Spin-Orbit Fields

Carlos A. R. Sa de Melo

Georgia Tech

Friday October 14,  1:30 pm,  Room 298,  Physics Building
NOTE IRREGULAR DAY AND TIME!

Abstract:  Topological quantum phase transitions in superfluids with non-s-wave order parameters have been conjectured theoretically in the context of p-wave [1] and d-wave [2] systems for many years, but never observed experimentally due to the absence of tunable parameters. In this talk, I discuss topological phase transitions in ultra-cold Fermi superfluids induced by interactions, artificial spin orbit, and Zeeman fields, which are tunable parameters in the context of ultra-cold atom physics. I construct the phase diagram of ultracold Fermi superfluids at zero and finite temperatures, and analyze spectroscopic and thermodynamic properties to characterize various phase transitions. For systems with zero Zeeman field, the evolution from BCS to BEC superfluids in the presence of spin-orbit effects is only a crossover as the system remains fully gapped [3], even though a triplet component of the order parameter emerges. However, for finite Zeeman fields, 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 [4]. 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.

[1] G. E. Volovik, Exotic Properties of Superfluid 3He, World Scientific, Singapore (1992).
[2] R. D. Duncan and C. A. R. Sa de Melo, Phys. Rev. B 62, 9675 (2000).
[3]  Li Han, C. A. R. Sa de Melo, arXiv:1106.3613v1.
[4]  Kangjun Seo, Li Han and C. A. R. Sa de Melo, arXiv:1108.4068v2.