Microscopic origin of Rashba coupling from first principles: Layer-resolved orbital asymmetry in transition metal dichalcogenides

TL;DR

This paper investigates the microscopic origins of Rashba spin splitting in transition metal dichalcogenides, revealing how orbital asymmetry and interlayer interactions influence spin splitting in bilayer and monolayer structures.

Contribution

It introduces a microscopic model explaining Rashba coupling in TMDs, emphasizing orbital polarization imbalance and layer-resolved asymmetry as key factors.

Findings

Rashba splitting is smaller in bilayers than monolayers at typical fields.

Orbital polarization imbalance determines spin ordering of Rashba states.

Quantitative analysis clarifies the origin of Rashba coupling in TMDs.

Abstract

Spin-orbit coupling in two-dimensional materials gives rise to a Rashba spin splitting when inversion and mirror symmetries are broken, yet its microscopic origin and quantitative characterization in transition metal dichalcogenides remains incomplete. Both symmetries are broken in certain bilayer structures, enabling Rashba splittings in the absence of external electric fields. We determine this zero-field offset and the Rashba parameters that dictate the spin splitting in the linear regime. Surprisingly, the splitting is substantially smaller in bilayers than in monolayers at typical fields. This is clarified within a perturbative microscopic model, revealing that the spin splitting results from a competition between internal polarization and interlayer hybridization. We further introduce the orbital polarization imbalance as an order parameter that captures the asymmetry of the valence bands and determines the spin ordering of the Rashba-split states. Our results are both quantitative and qualitative, as they clarify the nature and origin of Rashba coupling in transition metal dichalcogenides.