Unconventional pairing in Ising superconductors: Application to monolayer NbSe$_2$

TL;DR

This paper develops a microscopic model for Ising superconductors, specifically applied to monolayer NbSe$_2$, revealing how spin-orbit coupling and fluctuations influence pairing symmetry and critical magnetic fields.

Contribution

It introduces a new formalism for Ising superconductors that accounts for momentum-dependent fluctuations and applies it to monolayer NbSe$_2$ to analyze pairing states.

Findings

Strong paramagnetic fluctuations in NbSe$_2$ influence pairing.

Mixing of even- and odd-parity states varies with Coulomb interaction.

Ising spin-orbit coupling enhances critical magnetic field at low temperatures.

Abstract

The presence of a non-centrosymmetric crystal structure and in-plane mirror symmetry allows an Ising spin-orbit coupling to form in some two-dimensional materials. Examples include transition metal dichalcogenide superconductors like monolayer NbSe$_2$, MoS$_2$, TaS$_2$, and PbTe$_2$, where a nontrivial nature of the superconducting state is currently being explored. In this study, we develop a microscopic formalism for Ising superconductors that captures the superconducting instability arising from a momentum-dependent spin- and charge-fluctuation-mediated pairing interaction. We apply our pairing model to the electronic structure of monolayer NbSe$_2$, where first-principles calculations reveal the presence of strong paramagnetic fluctuations. Our calculations provide a quantitative measure of the mixing between the even- and odd-parity superconducting states and its variation with Coulomb interaction. Further, numerical analysis in the presence of an external Zeeman field reveals the role of Ising spin-orbit coupling and mixing of odd-parity superconducting state in influencing the low-temperature enhancement of the critical magnetic field.