Seminar über Theoretische Festkörperphysik
Room 10.01, 10th Floor, Bldg. 30.23, KIT Campus South
CNRS-LNCMI Grenoble, France
PD Dr. Markus Garst
Semiconducting transition metal dichalcogenides (S-TMD) are layered materials with the chemical composition MX$_2$, where M is a transition metal (such as molybdenum or tungsten), and X is a chalcogen (sulfur, selenium, or tellurium). The interest to S-TMDs has been sparked by the discovery of the monolayer MX$_2$ being a direct-gap semiconductor, in contrast to its bulk indirect-gap counterpart. The energy gap in the visible light range together with tightly bound excitons in S-TMD monolayers make these materials promising for optical applications. There are two types of excitons in such systems: “bright” (which can be created by absorption of light) and “dark” ones (which are formally decoupled from photons). The latter poses a question of whether the dark exciton states are optically inactive, or there are still some mechanisms for their radiative decay.
We perform a theoretical study of radiative decay of dark intravalley excitons in S-TMD monolayers. This decay necessarily involves an electronic spin flip.
We study three possible spin-flip mechanisms. The first intrinsic decay mechanism owing to interband spin-flip dipole moment perpendicular to the plane of the monolayer gives a rate of about 100–1000 times smaller than that of bright excitons. We found that this mechanism also introduces i) the energy splitting between doubly degenerated by valley dark exciton states due to a local field effect; ii) whole oscillator strength is contained in the higher energy component, whereas the lowest energy state remains dark and needs an extrinsic spin-flip mechanism for the decay. The second mechanism appears from the Rashba effect due to a perpendicular electric field or a dielectric substrate. It gives a negligible radiative decay rate (about $10^7$ times less than that of bright excitons). The third spin-flip process, due to the Zeeman effect in a sufficiently strong in-plane magnetic field, can give a decay rate comparable to that owing to the intrinsic interband spin-flip dipole.