Long valley lifetime of dark excitons in single-layer WSe2

Yanhao Tang,Jie Shan,Kin Fai Mak

doi:10.1038/s41467-019-12129-1

Abstract

Single-layer transition metal dichalcogenides provide a promising material system to explore the electron’s valley degree of freedom as a quantum information carrier. The valley degree of freedom can be directly accessed by means of optical excitation. However, rapid valley relaxation of optically excited electron-hole pairs (excitons) through the exchange interaction has been a major roadblock. Theoretically such valley relaxation is suppressed in dark excitons, suggesting a potential route for long valley lifetimes. Here we develop a waveguide-based method to detect time-resolved and energy-resolved dark exciton emission in single-layer WSe2, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for the dark charged excitons.

Highlights

Single-layer transition metal dichalcogenides provide a promising material system to explore the electron’s valley degree of freedom as a quantum information carrier
We show that the valley-polarized dark excitons can be initiated through scattering of the valley-polarized bright excitons
Single-layer WSe2 is contacted with a few-layer graphite electrode, and can be gated by a top and a bottom graphite gate with hexagonal boron nitride gate dielectrics

Summary

Introduction

Single-layer transition metal dichalcogenides provide a promising material system to explore the electron’s valley degree of freedom as a quantum information carrier. Rapid valley relaxation of optically excited electron-hole pairs (excitons) through the exchange interaction has been a major roadblock. The bright excitons exhibit strong valley circular dichroism (i.e. each handedness of circularly polarized light couples only to one of the two valleys), which provides an effective means to access the valley degree of freedom[3,4,5]. Such valley circular dichroism has triggered intense interest in single-layer TMDs as potential candidates for valleytronic applications[6,7,8], which desire a long valley lifetime. The long valley lifetimes revealed by our experiment for dark charged excitons have a distinct physical origin from that found in localized emitters[24], resident carriers[25,26,27,28,29] and interlayer excitons[30]

Methods

Results

Conclusion