Strain-Driven "Sinusoidal" Valley Control of Hybridized $\Gamma -\mathrm{K}$ Excitons

TL;DR

This paper develops a unified theoretical framework explaining strain-induced photoluminescence in monolayer WS$_2$, revealing a novel exciton transfer pathway and predicting a sinusoidal valley response for advanced valleytronic applications.

Contribution

It introduces a new microscopic understanding of strain-driven exciton dynamics, including a previously unrecognized transfer pathway and a sinusoidal valley control mechanism.

Findings

Reproduces experimental PL data of $ m extGamma - K$ excitons.

Identifies a two-step exciton transfer process involving exchange interaction and spin flip.

Predicts a sinusoidal valley response enabling tunable valley control.

Abstract

The photoluminescence (PL) of momentum-indirect $\rm \Gamma- K$ excitons in monolayer WS$_2$ under biaxial strain was recently observed by Blundo et al. [Phys. Rev. Lett. 129, 067402 (2022)], yet its microscopic origin remains elusive. Here we develop a unified framework that reproduces the measured PL and reveals its fundamental excitonic mechanism. We reveal that: (i) the PL originates from genuinely hybridized direct-indirect excitonic eigenstates, rather than nominally mixed species with fixed dominant character; (ii) the direct exciton converts into the indirect one via a previously unrecognized two-step pathway -- exchange-interaction-driven exciton transfer followed by a spin flip; and (iii) a higher-energy indirect exciton, absent from prior studies, acts as a crucial intermediate mediating this conversion. Beyond explaining experiment, our theory predicts a striking strain-driven "sinusoidal'' valley response, furnishing a continuously tunable valley dial that far exceeds binary control schemes. This unified picture of strain-engineered direct-indirect exciton dynamics introduces a new paradigm for manipulating long-lived valley degrees of freedom, opening a pathway toward programmable valley pseudospin engineering and next-generation valleytronic quantum technologies.