Motion of Two-Dimensional Excitons in Momentum Space Leads to Pseudospin Distribution Narrowing on the Bloch Sphere

Motional narrowing implies narrowing induced by motion;
for example,
in nuclear magnetic resonance, the thermally induced random motion
of the nuclei in an inhomogeneous environment leads to a counterintuitive
narrowing of the resonance line. Similarly, the excitons in monolayer
semiconductors experience magnetic inhomogeneity: the electron–hole
spin-exchange interaction manifests as an in-plane pseudomagnetic
field with a periodically varying orientation inside the exciton band.
The excitons undergo random momentum scattering and pseudospin precession
repeatedly in this inhomogeneous magnetic environment, typically resulting
in fast exciton depolarization. On the contrary, we show that such
magnetic inhomogeneity averages out at high scattering rates due to
motional narrowing. Physically, a faster exciton scattering leads
to a narrower pseudospin distribution on the Bloch sphere, implying
a nontrivial improvement in exciton polarization. The in-plane nature
of the pseudomagnetic field enforces a contrasting scattering dependence
between the circularly and linearly polarized excitons, providing
a spectroscopic way to gauge the sample quality.