Magnetic brightening of light-like excitons in a monolayer semiconductor

TL;DR

This study uses magneto-photoluminescence spectroscopy to observe magnetic brightening of light-like excitons in monolayer WSe$_2$, revealing a new spin-singlet exciton with a linear dispersion relation influenced by magnetic fields.

Contribution

It demonstrates magnetic-field-induced coupling between bright and light-like excitons, leading to brightening of dark excitons in monolayer WSe$_2$, a novel observation in 2D semiconductors.

Findings

Emergence of a new blue-detuned emission peak under magnetic field.

Identification of a spin-singlet light-like exciton with smaller mass.

Intensity of the new peak scales as B^2, indicating linear dispersion.

Abstract

Monolayer transition-metal dichalcogenides, such as WSe$_2$, are direct gap, multi-valley semiconductors. Long-range electron-hole exchange interactions mix the valleys, yielding dispersion relations for massive ($\propto Q^2$) as well as light-like ($\propto Q$) excitons. We report magneto-photoluminescence spectroscopy of excitons in the monolayer semiconductor WSe$_2$ to $B = \pm25$T. The magnetic field-dependent line shape of the neutral exciton reveals the emergence of a new blue-detuned emission peak in both field orientations. Analyzing the distinct magnetic field-dependent shifts of both peaks facilitates the identification of the emergent feature as a spin-singlet with a significantly smaller reduced exciton mass as compared to the neutral exciton. The intensity of the emergent feature increases with magnetic field according to $\propto B^2$, as expected for a linear dispersion relation. The density-dependent diamagnetic shift ratios of both features follow the expected density dependence of the electron-hole exchange interactions. We interpret our observations within a picture of magnetic-field-induced coupling between the bright massive and quasi dark light-like exciton, leading to its brightening.