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

Abstract

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