Spin-valley locking in the normal state of a transition-metal dichalcogenide superconductor

Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing 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 andorigins have proved controversial. Here, we study a prototypical example, 2H-NbSe₂, 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, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi surface spintexture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe₂ and related TMDCs.