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

L Bawden,S P Cooil,J M Riley,C M Polley,P D C King,V Sunko,T Balasubramanian,L J Collins-Mcintyre,K W B Hunvik,M Leandersson,M Hoesch,T K Kim,M S Bahramy,F Mazzola,G Balakrishnan,J W Wells

doi:10.1038/ncomms11711

Abstract

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 and origins 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 characterized 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 spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.

Highlights

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
Each layer forms a graphene-like honeycomb structure with Nb occupying the A sublattice and two Se atoms situated on the B sublattice
The corresponding normal-state Fermi surface is shown in Fig. 1c, as measured by angle-resolved photoemission (ARPES)

Summary

Introduction

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. We find that the normal state, from which its hallmark collective phases emerge, is characterized 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 spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs. In combination with broken structural inversion symmetry, spin–orbit coupling (SOC) provides a powerful route to stabilize spin-polarized electronic states without magnetism. This yields a rich underlying spin-polarized electronic landscape from which charge order and superconductivity emerge upon cooling

Methods

Results

Conclusion