Unconventional singlet-triplet superconductivity

TL;DR

This paper explores how additional degrees of freedom in BCS theory allow for coexistence and conversion between spin singlet and triplet superconductivity, with implications for specific materials.

Contribution

It introduces a symmetry-based framework for understanding coexistence of singlet and triplet pairing, and discusses real material examples exhibiting this phenomenon.

Findings

Magnetic field can convert between singlet and triplet states.

Symmetry arguments determine the coexistence conditions.

CeRh₂As₂ and bilayer-NbSe₂ are potential singlet-triplet superconductors.

Abstract

Have you been lying awake wondering what symmetries determine whether a superconductor is spin singlet, triplet, or both? We show that if BCS theory is supplied with additional degrees of freedom, spin singlet can coexist with spin triplet superconductivity. In the first part, we didactically guide the reader to the most general superconducting state using symmetry arguments. If both singlet and triplet pairing channels are present, a magnetic field can convert between spin singlet and triplet states. In the second part, we discuss two possible singlet-triplet superconductors: CeRh$_2$As$_2$ and bilayer-NbSe$_2$.