First-order topological transition and Andreev edge states in the spin-orbit-coupled polarized 2D Fermi superfluid

Speaker: Professor Uli Zülicke
Affiliation: Victoria University of Wellington

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 [1]. 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 [2]. For larger Cooper-pair bound-state energies, the system undergoes the BCS–BEC crossover, in the course of which the topological transition becomes first-order [3]. As a result, topological and nontopological superfluids coexist in an extended region of phase space spanned by the two-particle s-wave bound-state energy and the Zeeman energy.

We have investigated the topological-superfluid – nontopological-superfluid coexistence region theoretically using a zero-temperature mean-field approach, revealing intriguing physical properties [4]. The internal interface between topological and nontopological parts of the system is found to host unconventional Andreev bound states [5] — including a Majorana mode — whose properties can be tailored by tuning parameters that are routinely accessible in ultracold-atom experiments. Our results thus extend current understanding of spin-imbalanced superfluid Fermi systems and may inform experimental efforts aimed at realizing Majorana quasiparticles.

[1] M. Sato, Y. Ando, Rep. Prog. Phys. 80, 076501 (2017).

[2] J. D. Sau, R. M. Lutchyn, S. Tewari, S. Das Sarma, Phys. Rev. Lett. 104, 040502 (2010); J. Alicea, Phys. Rev. B 81, 125318 (2010).

[3] K. Thompson, J. Brand, U. Zülicke, Phys. Rev. A 101, 013613 (2020).

[4] K. Thompson, U. Zülicke, J. Brand, Phys. Rev. Research 4, 043201 (2021).

[5] K. Thompson, J. Brand, U. Zülicke, SciPost Phys. 14, 115 (2023).