Valley Zeeman Effect in Elementary Optical Excitations of a Monolayer WSe2

TL;DR

This study demonstrates the valley Zeeman effect in monolayer WSe2, revealing anisotropic valley splitting under magnetic fields and uncovering unexpected large effects in trions linked to Berry curvature and exchange interactions.

Contribution

It provides experimental evidence of the valley Zeeman effect in monolayer WSe2 and explains the large trion splitting through Berry curvature and exchange interactions.

Findings

Valley Zeeman splitting observed in excitons and trions.

Large valley Zeeman effect in trions exceeds independent electron-hole predictions.

Berry curvature and exchange interactions explain the large trion splitting.

Abstract

A monolayer of a transition metal dichalcogenide (TMD) such as WSe$_2$ is a two-dimensional (2D) direct band-gap valley-semiconductor having an effective Honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly-polarized light fields, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom. Here we report such experiments that demonstrate the valley Zeeman effect -- strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. While the valley-splitting measured using the exciton transition is consistent with the difference of the conduction and valence band orbital magnetic moments, the trion transition exhibits an unexpectedly large valley Zeeman effect which cannot be understood using an independent electron-hole picture. Instead, we find an explanation using the recently predicted large Berry curvature and the associated magnetic moment for the electron-hole exchange interaction modified trion dispersion. Our results raise the possibility of observing optical excitation induced valley Hall effect in monolayer TMDs or topological states of photons strongly coupled to trion excitations in a microcavity.