Gupta Garima, Watanabe Kenji, Taniguchi Takashi, Majumdar Kausik
Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore 560012, India.
Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
Nano Lett. 2024 May 8;24(18):5413-5419. doi: 10.1021/acs.nanolett.3c04808. Epub 2024 Apr 26.
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.
动态展宽意味着由运动引起的展宽；例如，在核磁共振中，在不均匀环境中热诱导的原子核随机运动会导致共振线出现违反直觉的展宽。同样，单层半导体中的激子也会经历磁场不均匀性：电子 - 空穴自旋交换相互作用表现为激子带内具有周期性变化取向的面内赝磁场。在这种不均匀的磁环境中，激子会反复经历随机动量散射和赝自旋进动，通常会导致激子快速退极化。相反，我们表明，由于动态展宽，这种磁不均匀性在高散射率下会平均掉。从物理角度来看，激子散射越快，在布洛赫球上的赝自旋分布就越窄，这意味着激子极化有显著改善。赝磁场的面内性质导致圆偏振激子和线偏振激子之间存在相反的散射依赖性，为评估样品质量提供了一种光谱方法。
