(Invited) Ab Initio Exciton and Phonon Dynamics in Transition Metal Dichalcogenides

Abstract

Interest in the properties of transition metal dichalcogenides (TMDs) has increased due to the discovery of the coupling between spin and valley degrees of freedom, which can be seen experimentally using a circularly polarised laser. After excitation the newly formed carrier populations must move towards the other valley until balance is reached. However, this relaxation process is not entirely understood in the literature, where the relative importance of the electron-electron (e-e) or electron-phonon (e-p) interactions is still a subject of debate. Previous works on WSe2 [A. Molina- Sánchez, et al - Nano letters, 2017] have shown that the e-p interaction is a good candidate to describe the relaxation process. Using a fully ab-initio framework based on the Baym-Kadanoff equations [P. M. M. C. de Melo and A. Marini, Phys. Rev. B 93, 155102 (2016)] we study the influence of the e-p interaction on MoSe2 after its excitation by a laser field. We show how phonons allow carrier relaxation and how the Kerr signal and total magnetisation are affected at different temperatures, with the latter exhibiting a non-monotonic behaviour as the temperature increases [M Ersfeld et al - Nano Letters 2019 19 (6), 4083-4090]. An important conclusion is that long lived spin states probably reside within defects. We calculate the spectral signatures of point defects in TMDs, finding two main classes based on the presence of in-gap states, and estimating the experimental resolution needed to provide quantification of the defect concentration [P de Melo et al. https://arxiv.org/abs/2010.10222]. The localization of excitonic states around the defect provides a benchmark for scanning probe characterization (figure shows a S vacancy exciton wave function, the green ball is the hole position). Figure 1