Valley lifetimes of conduction band electrons in monolayer WSe$_2$

TL;DR

This study investigates how valley lifetimes of conduction electrons in monolayer WSe$_2$ depend on gate voltage, revealing the impact of scattering mechanisms and emphasizing the need for modified measurement techniques.

Contribution

It introduces a combined experimental approach to measure gate-dependent valley lifetimes and highlights the importance of accounting for photo-induced gate screening effects.

Findings

Valley lifetimes decrease with Fermi energy into the conduction band.

Scattering of bright trions into dark states explains valley polarization formation.

Modified Kerr rotation measurement scheme is necessary to avoid erroneous conclusions.

Abstract

One of the main tasks in the investigation of 2-dimensional transition metal dichalcogenides is the determination of valley lifetimes. In this work, we combine time-resolved Kerr rotation with electrical transport measurements to explore the gate-dependent valley lifetimes of free conduction band electrons of monolayer WSe$_2$. When tuning the Fermi energy into the conduction band we observe a strong decrease of the respective valley lifetimes which is consistent with both spin-orbit and electron-phonon scattering. We explain the formation of a valley polarization by the scattering of optically excited valley polarized bright trions into dark states by intervalley scattering. Furthermore, we show that the conventional time-resolved Kerr rotation measurement scheme has to be modified to account for photo-induced gate screening effects. Disregarding this adaptation can lead to erroneous conclusions drawn from gate-dependent optical measurements and can completely mask the true gate-dependent valley dynamics.