Motion of 2D exciton in momentum space leads to pseudospin distribution narrowing on the Bloch Sphere

TL;DR

This paper demonstrates that rapid momentum scattering of excitons in monolayer semiconductors causes motional narrowing, leading to a more uniform pseudospin distribution and improved polarization, revealing insights into sample quality.

Contribution

It introduces the concept that motional narrowing occurs in exciton pseudospin dynamics, showing how increased scattering rate narrows pseudospin distribution on the Bloch sphere, which is a novel insight.

Findings

Faster exciton scattering results in narrower pseudospin distribution.

Motional narrowing improves exciton polarization.

Spectroscopic differences between circular and linear polarization relate to sample quality.

Abstract

Motional narrowing implies narrowing induced by motion, for example, in nuclear resonance, the thermally induced random motion of the nuclei in an inhomogeneous environment leads to counter-intuitive 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 pseudo-magnetic 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 rate 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 pseudo-magnetic field enforces a contrasting scattering dependence between the circularly and linearly polarized excitons - providing a spectroscopic way to gauge the sample quality.