Long valley lifetime of free carriers in monolayer WSe2

TL;DR

This study measures the valley lifetime of free carriers in monolayer WSe2, finding it to be around 2 nanoseconds at 70 K, significantly longer than exciton lifetimes, with implications for valleytronic applications.

Contribution

It provides the first direct measurement of free carrier valley lifetime in monolayer WSe2 using time-resolved Kerr rotation spectroscopy.

Findings

Free carrier valley lifetime is about 2 ns at 70 K.

Exciton lifetime is approximately 2 ps.

Exciton binding energy is 0.60 eV.

Abstract

Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spin-orbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominates the optical response due to the enhanced Coulomb interaction in two dimensions. The valley lifetime of free carriers remains in ambiguity. In this work, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselinide. The valley lifetime of free carriers is found around 2 ns at 70 K, about 3 orders of magnitude longer than the excitons of about 2 ps. The extended valley lifetime of free carriers evidences that exchange interaction dominates the valley relaxation in optical excitation. The pump-probe spectroscopy also reveals the exciton binding energy of 0.60 eV in monolayer WSe2.