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 NbSe2 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 NbSe2 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.
@article{arxiv.1507.08731,
title = {Evidence of Ising pairing in superconducting NbSe$_2$ atomic layers},
author = {Xiaoxiang Xi and Zefang Wang and Weiwei Zhao and Ju-Hyun Park and Kam Tuen Law and Helmuth Berger and László Forró and Jie Shan and Kin Fai Mak},
journal= {arXiv preprint arXiv:1507.08731},
year = {2016}
}