Symmetry-breaking has been known to play a key role in noncentrosymmetric superconductors with strong spin-orbit-interaction (SOI). The studies, however, have been so far mainly focused on a particular type of SOI, known as Rashba SOI, whereby the electron spin is locked to its momentum at a right-angle, thereby leading to an in-planar helical spin texture. Here we discuss electric-field-induced superconductivity in molybdenum disulphide (MoS2), which exhibits a fundamentally different type of intrinsic SOI manifested by an out-of-plane Zeeman-type spin polarization of energy valleys. We find an upper critical field of approximately 52 T at 1.5 K, which indicates an enhancement of the Pauli limit by a factor of four as compared to that in centrosymmetric conventional superconductors. Using realistic tight-binding calculations, we reveal that this unusual behaviour is due to an inter-valley pairing that is symmetrically protected by Zeeman-type spin-valley locking against external magnetic fields. Our study sheds a new light on the interplay of inversion asymmetry with SOI in confined geometries, and its unprecedented role in superconductivity.
@article{arxiv.1506.04146,
title = {Superconductivity protected by spin-valley locking in ion-gated MoS2},
author = {Yu Saito and Yasuharu Nakamura and Mohammad Saeed Bahramy and Yoshimitsu Kohama and Jianting Ye and Yuichi Kasahara and Yuji Nakagawa and Masaru Onga and Masashi Tokunaga and Tsutomu Nojima and Youichi Yanase and Yoshihiro Iwasa},
journal= {arXiv preprint arXiv:1506.04146},
year = {2016}
}