Chiral and helical $p$-wave superconductivity in doped bilayer BiH

TL;DR

This paper explores chiral and helical p-wave superconductivity in doped bilayer BiH, revealing dominant triplet pairing driven by correlations, with topologically trivial bands supporting edge states, and a six-fold degeneracy in helical states.

Contribution

It demonstrates the emergence of chiral and helical p-wave pairing in doped bilayer BiH, highlighting the effects of spin-orbit coupling and the stability of phase degeneracies.

Findings

Triplet p-wave pairing dominates near the van Hove singularity.

Chiral p-wave state supports in-gap edge states despite trivial topology.

Helical p-wave states exhibit a six-fold degeneracy in phase angle.

Abstract

We investigate the superconductivity (SC) driven by correlation effects in electron-doped bilayer BiH near a type-II van Hove singularity (vHS). By functional renormalization group, we find triplet $p$-wave pairing prevails in the interaction parameter space, except for spin density wave (SDW) closer to the vHS or when the interaction is too strong. Because of the large atomic spin-orbital coupling (SOC), the $p$-wave pairing occurs between equal-spin electrons, and is chiral and two-fold degenerate. The chiral state supports in-gap edge states, even though the low energy bands in the SC state are topologically trivial. The absence of mirror symmetry allows Rashba SOC that couples unequal spins, but we find its effect is of very high order, and can only drive the chiral $p$-wave into helical $p$-wave deep in the SC state. Interestingly, there is a six-fold degeneracy in the helical states, reflected by the relative phase angle $\theta=n\pi/3$ (for integer $n$) between the spin components of the helical pairing function. The phase angle is shown to be stable in the vortex state.