Evidence of Ising pairing in superconducting NbSe$_2$ atomic layers

TL;DR

This paper reports the discovery of Ising superconductivity in monolayer NbSe$_2$, where spin-momentum locking due to strong spin-orbit coupling leads to an in-plane critical field exceeding the Pauli limit, revealing unconventional pairing.

Contribution

It provides experimental evidence of Ising pairing in monolayer NbSe$_2$, highlighting the role of spin-momentum locking in non-centrosymmetric 2D superconductors.

Findings

In-plane critical field exceeds six times the Pauli limit.

Spin-momentum locking protects superconductivity against magnetic fields.

Evidence of unconventional Ising pairing in monolayer NbSe$_2$.

Abstract

Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependent Berry curvature effects have attracted tremendous recent interests. Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied, effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe$_2$ with an in-plane upper critical field over six times of the Pauli paramagnetic limit by magneto-transport measurements. The effect can be understood in terms of the competing Zeeman effect and large intrinsic spin-orbit interactions in non-centrosymmetric NbSe$_2$ monolayers, where the electronic spin is locked to the out-of-plane direction. Our results provide a strong evidence of unconventional Ising pairing protected by spin-momentum locking and open up a new avenue for studies of non-centrosymmetric superconductivity with unique spin and valley degrees of freedom in the exact two-dimensional limit.