Pairing with dominant triplet component and possible weak topological superconductivity in BiS$_2$-based superconductors

TL;DR

This paper proposes that BiS$_2$-based superconductors may host a dominant triplet pairing component with potential weak topological superconductivity, explaining experimental observations and the nature of their gap structure.

Contribution

It introduces the possibility of dominant triplet pairing and weak topological superconductivity in BiS$_2$-based materials, highlighting the role of spin-orbit coupling and Fermi surface topology.

Findings

Dominant triplet pairing component with sign changes between Fermi pockets

Superconducting gap is nodeless and weak topological below Lifshitz level

Gap becomes nodal above Lifshitz points, consistent with experimental critical fields

Abstract

We show that the newly discovered BiS$_2$-based superconductors may have a dominant triplet pairing component, in addition to a subdominant singlet component arising from the spin-orbital coupling. The pairing respects time-reversal symmetry. The dominant triplet gap causes gap sign changes between the spin-split Fermi pockets. Within a pocket, the gap function respects $d^*_{x^2-y^2}$-wave symmetry, where the star indicates joint spin-lattice rotations. Below the Lifshitz filling level the gap is nodelss and the superconducting state is weak topological. Above the Lifshitz points the gap becomes nodal. The dominant triplet pairing is consistent with the experimental upper critical field exceeding the Pauli limit.