Dynamics and Spin-Valley Locking Effects in Monolayer Transition Metal Dichalcogenides

TL;DR

This study uses ab initio calculations to analyze spin-valley locking and carrier dynamics in monolayer transition metal dichalcogenides, revealing enhanced carrier lifetimes and mobility due to spin-orbit coupling, with implications for valleytronic applications.

Contribution

It provides a detailed ab initio analysis of spin-orbit effects on electron and hole dynamics in monolayer TMDCs, highlighting the role of spin-valley locking in transport properties.

Findings

Spin-orbit coupling increases valence band carrier lifetimes by an order of magnitude.

Intervalley scattering times are around 100 ps at 50 K, up to 140 ps in WSe₂.

Direct optical transitions dominate across energies, even in indirect band gap TMDCs.

Abstract

Transition metal dichalcogenides have been the primary materials of interest in the field of valleytronics for their potential in information storage, yet the limiting factor has been achieving long valley decoherence times. We explore the dynamics of four monolayer TMDCs (MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$) using ab initio calculations to describe electron-electron and electron-phonon interactions. By comparing calculations which both omit and include relativistic effects, we isolate the impact of spin-resolved spin-orbit coupling on transport properties. In our work, we find that spin-orbit coupling increases carrier lifetimes at the valence band edge by an order of magnitude due to spin-valley locking, with a proportional increase in the hole mobility at room temperature. At temperatures of 50~K, we find intervalley scattering times on the order of 100 ps, with a maximum value ~140 ps in WSe$_2$. Finally, we calculate excited-carrier generation profiles which indicate that direct transitions dominate across optical energies, even for WSe$_2$ which has an indirect band gap. Our results highlight the intriguing interplay between spin and valley degrees of freedom critical for valleytronic applications. Further, our work points towards interesting quantum properties on-demand in transition metal dichalcogenides that could be leveraged via driving spin, valley and phonon degrees of freedom.