Chiral And Helical P-Wave Superconductivity In Doped Bilayer Bih

We investigate the superconductivity (SC) driven by correlation effects in electron-doped bilayer BiH near a type-II Van Hove singularity (VHS). Using the functional renormalization group, we find triplet p-wave pairing prevails in the interaction parameter space, except for a small-momentum spin density wave 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 twofold 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 and can drive the chiral p wave into helical p wave in the SC state, although we find its effect is of very high order. Interestingly, there is a sixfold degeneracy in the helical states, reflected by the relative phase angle theta = n pi/3 (for integer n) between the two spin components of the helical order parameter. This relative phase is shown to be locked in the mean-field theory even if the phase of the helical state is forced to wind, e.g., in the vortex state under a magnetic field.