Topological superfluids with time-reversal symmetry from $s$-wave interaction in a bilayer system

TL;DR

This paper proposes a realistic way to realize topological superfluids with time-reversal symmetry in a bilayer system using only s-wave interactions, supported by detailed calculations and symmetry analysis.

Contribution

It demonstrates that topological phases with TR symmetry can be achieved in a bilayer system with s-wave interactions, avoiding the need for unconventional pairing.

Findings

Topological phases with TR symmetry are realizable in a bilayer system with s-wave interactions.

The gapped phase in DIII class is characterized by a Z2 invariant.

Gapless phases exhibit nontrivial winding numbers and Majorana flat bands.

Abstract

Topological superconducting phases with time-reversal (TR) symmetry have been widely explored in recent years. However the involved unconventional pairings are generally implausible in realistic materials. Here we demonstrate via detailed self-consistent calculation that these topological phases with TR symmetry in DIII and BDI classes can be realized in a spin-orbit coupled bilayer system with only $s$-wave interaction. The bilayer freedom enables the definition of TR symmetry between the layers even in the presence of local Zeeman fields, which we propose to be realized using four laser beams. The gapped phase in DIII class is characterized by $\mathbb{Z}_2$, while all the gapless phases in these two classes are characterized by nontrivial winding numbers and are also manifested from the Majorana flat bands. We also unveil the intimate relation between TR symmetry and mirror symmetry due to phase locking effect between the two layers, which harbors the mirror symmetry protected topological phases. We finally demonstrate that these phases will not be spoiled by interlayer pairings.