Spin-orbit-coupled superconductivity with spin-singlet non-unitary pairing

TL;DR

This paper explores the theoretical possibility of spin-singlet superconductors with non-unitary pairing, revealing new phenomena like spin-locking, topological edge states, and spin-polarized states driven by spin-orbit coupling.

Contribution

It introduces a framework for spin-singlet superconductors with non-unitary pairing and demonstrates how spin-orbit coupling leads to novel exotic phenomena.

Findings

Discovery of spin-orbit-coupled non-unitary superconductivity

Prediction of Type II Ising superconductivity with high in-plane critical field

Existence of topological Majorana edge states without external magnetic fields

Abstract

An unconventional superconductor is distinguished with two types of gap functions: unitary and non-unitary. This core subject has been concentrated on purely spin-triplet or singlet-triplet mixed superconductors. However, the generalization to a purely spin-singlet superconductor has remained primarily of theoretical interest, which requires at least a multi-orbital correlated electronic systems. In this work, we present a possible establishment of both unitary and non-unitary pairings for spin-singlet superconductors with two atomic orbitals. Then we investigate the effects of atomic spin-orbit coupling and find a new spin-orbit-coupled non-unitary superconductor that supports exotic phenomena. Remarkably, there are mainly three features. Firstly, the atomic spin-orbit coupling locks the electron spins to be out-of-plane, which could give birth to the Type II Ising superconductivity with a large in-plane upper critical field compared to the Pauli limit. Secondly, topological chiral or helical Majorana edge state could be realized even in the absence of external magnetic fields or Zeeman fields. In addition, a spin-polarized superconducting state could even be generated by spin-singlet non-unitary pairings with spontaneous time-reversal breaking, which severs as a smoking gun to detect this exotic state by measuring the spin-resolved density of states. Therefore, our work might pave a new avenue to spin-orbit-coupled superconductors with spin-singlet non-unitary pairing symmetries.