Probing the exciton spin-valley depolarization with spin-momentum-valley locked unbound states using tr-ARPES

Abstract

Time-resolved, angle-resolved photoemission spectroscopy (tr-ARPES) is a powerful technique to access the ultrafast dynamics of intervalley couplings and population exchanges. In monolayer transition metal dichalcogenides, this is advantageous due to the fact that excitons have a large binding energy, allowing them to be resonantly excited by a short-pulse pump without also putting into play a large set of dissociated electron-hole pairs, while at the same time remaining relatively localized in momentum space so that the outgoing currents related to different local valleys can be safely distinguished in the energy-momentum resolved tr-ARPES signal. In this work, we present a detailed theoretical description of the high-energy states that form the escape continuum for photoejected electrons in a tr-ARPES experiment, paying particular attention to their momentum dispersions, time-reversal symmetries, and spin characteristics. The analysis of the various symmetries fulfilled by such unbound states reveals the existence of a momentum-valley locking for the unbound final electron states, sharing various features with the valley Hall effect. Moreover, the photoextracted current under a circularly polarized pump ideally presents a robust triple locking of momentum direction, valley origin, and spin orientation, which permits envisioning an on-demand source of highly spin-polarized electrons on vacuum. We also present a study of the robustness of such findings against intervalley processes that tend to mitigate the valley-spin polarization of the intermediate exciton states. To further investigate this aspect, we consider in detail the role of Coulomb-exchange driven intervalley coupling. That allows us to quantify the dependence of the tr-ARPES measurements upon various setup parameters, such as the incident angle and light helicity. Finally, we present a critical discussion about the role of a few intervalley couplings and/or scattering processes. Our findings have broad implications and are anticipated to be applicable to other two-dimensional structures and intervalley scattering events.