Spin-flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers

TL;DR

This paper provides a theoretical analysis of the radiative decay mechanisms of dark intravalley excitons in transition metal dichalcogenide monolayers, highlighting intrinsic and extrinsic spin-flip processes and their relative efficiencies.

Contribution

It introduces a detailed theoretical framework for understanding dark exciton decay, including the effects of local fields, external electric fields, and magnetic fields, which was not previously comprehensively analyzed.

Findings

Intrinsic decay rate is 100-1000 times smaller than bright excitons.

Energy splitting due to local field effect shifts oscillator strength to higher-energy component.

External electric fields have negligible impact on decay rate; magnetic fields can enhance decay via Zeeman effect.

Abstract

We perform a theoretical study of radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers. This decay necessarily involves an electronic spin flip. The intrinsic decay mechanism due to interband spin-flip dipole moment perpendicular to the monolayer plane, gives a rate about 100--1000 times smaller than that of bright excitons. However, we find that this mechanism also introduces an energy splitting due to a local field effect, and the whole oscillator strength is contained in the higher-energy component, while the lowest-energy state remains dark and needs an extrinsic spin-flip mechanism for the decay. Rashba effect due to a perpendicular electric field or a dielectric substrate, gives a negligible radiative decay rate (about $10^7$ times slower than that of bright excitons). Spin flip due to Zeeman effect in a sufficiently strong in-plane magnetic field can give a decay rate comparable to that due to the intrinsic interband spin-flip dipole.