Distinguishing nodal and nonunitary superconductivity in quasiparticle interference of an Ising superconductor with Rashba spin-orbit coupling: an example of NbSe$_2$

TL;DR

This paper investigates the superconducting pairing symmetries in NbSe₂ monolayers with Rashba spin-orbit coupling, revealing new gap functions and using quasiparticle interference to distinguish pairing types and symmetry violations.

Contribution

It introduces new superconducting pairing functions beyond nearest-neighbor models, including nodal singlet and non-unitary triplet pairings, and analyzes their quasiparticle interference signatures.

Findings

Identification of nodal singlet gap functions.

Discovery of triplet non-unitary pairing breaking time-reversal symmetry.

Characteristic quasiparticle interference patterns distinguishing pairing types.

Abstract

The NbSe$_2$ monolayer with Rashba spin-orbit coupling represents a paradigmatic example of an Ising superconductor on a substrate. Using a single-band model and symmetry analysis, we present general superconducting pairing functions beyond the nearest-neighbor approximation, uncovering new types of gap functions, including the nodal singlet gap function and the triplet non-unitary pairing function that breaks time-reversal symmetry. The non-unitarity builts in the asymmetrical band dispersion in the superconducting quasiparticle energy spectra. Performing exact T-matrix calculations of quasiparticle interference due to a single scalar impurity scattering, we found that the interference patterns possess characteristic features distinguishing the type of pairing and possible nematic and chiral symmetry violations.