Evidence for spin-triplet odd-parity superconductivity close to type-II van Hove singularities

TL;DR

This study provides evidence that near type-II van Hove singularities, a spin-triplet odd-parity p-wave superconducting state can emerge due to repulsive interactions, beyond weak coupling approximations.

Contribution

It demonstrates the emergence of spin-triplet p-wave pairing near type-II van Hove singularities using multiple advanced computational methods beyond weak coupling approximations.

Findings

Spin-triplet p-wave pairing appears near type-II van Hove filling.

The pairing state is degenerate and occurs with repulsive interactions.

Results are relevant to certain recently discovered superconductors.

Abstract

Searching for unconventional Cooper pairing states has been at the heart of superconductivity research since the discovery of BCS superconductors. In particular, spin-triplet odd-parity pairing states were recently revisited due to the possibility of tuning towards topological superconductors. In this context, it is interesting to note a recent proposal that such a spin-triplet pairing instability occurs when the band filling is near van Hove singularities (vHS) associated with momenta away from time-reversal invariant momenta named type-II vHS. However, this result was obtained within a weak coupling renormalization group with Fermi surface patch approximation. To explore superconducting instabilities beyond this weak coupling Fermi surface patch approximation, we perform systematic study on Hubbard model in a two-dimensional square lattice using three different methods: random phase approximation, large-scale dynamical mean field theory simulations with continuous time quantum Monte Carlo (CTQMC) impurity solver, and large-scale dynamical cluster simulations with CTQMC cluster solver. We find, in a wide doping range centered around the type-II van Hove filling, a two-fold degenerate, spin-triplet, odd-parity $p$-wave pairing state emerges due to repulsive interaction, when the Fermi surface is not sufficiently nested. Possible relevance of our findings to the recently discovered superconductors LaO$_{1-x}$F$_{x}$BiS$_{2}$, Ir$_{1-x}$Pt$_{x}$Te$_{2}$ and proposed doped BC$_{3}$ are also discussed.