Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy

TL;DR

This study investigates the temperature-dependent valley relaxation dynamics in single-layer WS2, revealing that hole valley lifetimes are significantly longer than electron lifetimes and are influenced by thermally activated processes.

Contribution

It provides the first detailed measurement of valley relaxation times in WS2 and proposes a theoretical model involving exciton scattering and thermal activation mechanisms.

Findings

Electron valley relaxation is less than 1 ps.

Hole valley lifetime is at least 100 times longer and temperature-dependent.

Two potential relaxation processes are identified: exciton scattering and Gamma valley scattering.

Abstract

We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.