Unveiling valley lifetimes of free charge carriers in monolayer WSe$_2$

TL;DR

This study reveals gate-tunable valley lifetimes in monolayer WSe$_2$, showing how electronic states and scattering mechanisms influence the longevity of valley polarization, with implications for spintronic and valleytronic applications.

Contribution

It provides the first direct measurement of nanosecond valley lifetimes in free charge carriers in monolayer WSe$_2$ and explains the underlying scattering processes.

Findings

Valley lifetimes reach over 10 ns with gating.

Valley polarization increases with Fermi level tuning.

Different scattering channels affect valley relaxation.

Abstract

We report on nanosecond long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe$_2$, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi level-dependent scattering channels of optically excited, valley polarized bright trions either via dark or bound states. By electrostatic gating we demonstrate that the transition metal dichalcogenide WSe$_2$ can be tuned to be either an ideal host for long-lived localized spin states or allow for nanosecond valley lifetimes of free charge carriers (> 10 ns).