Spin-valley locking in the normal state of a transition-metal   dichalocogenide superconductor
  The metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing upon cooling from a charge density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based, and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved highly controversial. Here, we study a prototypical example, $2H$-NbSe$_2$, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterised by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin-orbit interactions and local inversion symmetry breaking. Non-negligible interlayer coupling further drives a rich three-dimensional momentum-dependence of the underlying Fermi surface spin texture. Together, these findings necessitate a fundamental re-investigation of the nature of charge order and superconducting pairing in NbSe$_2$ and related TMDCs. 
L. Bawden, S.P. Cooil, F. Mazzola, J.M. Riley, L.J. Collins-McIntyre,   V. Sunko, K. Hunvik, M. Leandersson, C.M. Polley, T....
cond-mat.supr-con cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el