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.
Highlights
The superfluidity of polarized, i.e., species-imbalanced, Fermi gases underpins a wide range of topics being focused on in current research [1,2]
We present a detailed study of the first-order phasecoexistence region in 2D Fermi superfluids with Zeeman spin splitting and spin-orbit coupling in Sec
Across the first-order phase transition, we find the density nw of the weak superfluid phase to be always smaller than the density ns of the strong-superfluid phase, regardless of whether the weak superfluid phase is a topological superfluid (TSF) or an nontopological superfluid (NSF)
Summary
The superfluidity of polarized, i.e., species-imbalanced, Fermi gases underpins a wide range of topics being focused on in current research [1,2]. The interaction strength can be parameterized in terms of the energy Eb of the two-particle bound state in vacuum without spin-orbit coupling and Zeeman splitting, which exists in a 2D system at any nonzero strength of s-wave interactions [51,52]. In the absence of spin-orbit coupling, raising the Zeeman energy splitting 2h between opposite-spin states drives a first-order transition from the s-wave superfluid phase to the normal phase, regardless of the magnitude of Eb [16].
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