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

TL;DR

This study reveals that in the normal state of the superconductor NbSe2, quasiparticles exhibit spin-valley locking due to strong spin-orbit coupling and inversion symmetry breaking, impacting understanding of its electronic phases.

Contribution

The paper provides the first detailed spin-resolved analysis showing spin-valley locking in NbSe2's normal state, highlighting the role of spin-orbit interactions and interlayer coupling.

Findings

Quasiparticles exhibit spin-valley locking in the normal state.

Spin texture varies with three-dimensional momentum due to interlayer coupling.

Results challenge existing theories of charge order and superconductivity in TMDCs.

Abstract

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.