Direct Measurement of Exciton Valley Coherence in Monolayer WSe$_2$ (1509.08810v1)

Abstract: In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time, a key quantity for manipulating the valley pseudospin, has never been measured in any material. In this work, we use a sequence of laser pulses to resonantly generate a coherent superposition of excitons (Coulomb-bound electron-hole pairs) in opposite valleys of monolayer WSe$_2$. The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population decay. Our work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

• The paper directly measures exciton valley coherence dynamics in monolayer WSe2 using two-dimensional coherent spectroscopy.
• It finds that valley coherence persists for approximately 100 femtoseconds, with a decay rate (69±0.2 meV) much faster than exciton recombination.
• The study implies significant valleytronic potential by showing that rapid isotropic momentum scattering can induce a motional narrowing effect to modulate coherence.

Direct Measurement of Exciton Valley Coherence in Monolayer WSe

The paper in question investigates the exciton valley coherence in monolayer tungsten diselenide (WSe) through a meticulous experimental setup involving two-dimensional coherent spectroscopy (2DCS). The paper offers substantial insights into valleytronics—a field garnering attention due to the inherent potential of valley degrees of freedom (DoF) as information carriers akin to electronic spin or charge. Unlike traditional techniques primarily reliant on non-resonant photoluminescence (PL) or pump-probe spectroscopy, this research presents a novel direct measurement of exciton valley coherence dynamics, a fundamental aspect yet unmeasured across materials prior to this work.

This research utilizes laser-induced coherent superposition of excitons within the K and K' valleys of monolayer WSe, following the excitation pattern associated with the valley pseudospin DoF. These excitons, fundamental quasiparticles formed by Coulomb interactions, exhibit significant levels of binding energy due to the diminished dielectric screening in monolayer transition metal dichalcogenides (TMDs). The use of 2DCS, akin to three-pulse four-wave mixing with interferometric stability, facilitates a granular exploration of valley decoherence, circumventing the interpretive challenges presented by traditional techniques that are limited to incoherent exciton dynamics.

A notable finding from the paper is that the valley coherence persists for approximately 100 femtoseconds, a temporal span hastened by the electron-hole exchange interaction, which acts as a significant decoherence mechanism beyond exciton population decay. The coherence time is substantially faster than that of exciton recombination, thereby highlighting the internal processes governing valley pseudospin dynamics. The researchers discern that the valley coherence decay rate (γ_v = 69±0.2 meV) notably surpasses the population recombination rate, signifying the effectiveness of underlying decoherence mechanisms.

Methodologically, the research features the use of WSe flakes grown on a sapphire substrate, characterized by chemical vapor deposition, and focuses on samples thin to monolayers—a thickness confirmed via atomic force microscopy. The experiments occur at remarkably low average excitation power to mitigate broadened interactions involving exciton-phonon or exciton-exciton coupling. The paper posits that the observed valley coherence time aligns well with predictions based on momentum scattering rates, modeled effectively by the Maialle-Silva-Sham mechanism, involving electron-hole exchange interplays constituting an effective magnetic field 𝛀.

Given the intrinsic coupling between exciton center-of-mass momentum and the valley DoF via this magnetic field, the work uncovers that rapid isotropic momentum scattering increases the valley coherence time contrary to naive expectations, akin to a motional narrowing effect documented in other solid-state systems.

The implications of these findings are substantial: they encourage the exploration of valleytronic applications reliant on the synchronous manipulation of valley pseudospins. The paper underscores the necessity of conducting coherent operations within the brief window preceding the complete loss of valley coherence. Further advances could include engineering alternative exciton-type states or employing photonic structures to fine-tune exciton-photon coupling for expanded coherence manipulation. These avenues portend exciting developments in the domain of TMD-based valleytronics, motivating efforts to extend exciton lifetime and optimize control over valley degrees of freedom.