Tightly bound excitons in monolayer semiconductors represent a versatile platform to study two-dimensional propagation of neutral quasiparticles. Their intrinsic properties, however, can be severely obscured by spatial energy fluctuations due to a high sensitivity to the immediate environment. Here, we take advantage of the encapsulation of individual layers in hexagonal boron nitride to strongly suppress environmental disorder. Diffusion of excitons is then directly monitored using time and spatially resolved emission microscopy at ambient conditions. We consistently find very efficient propagation with linear diffusion coefficients up to $10{\mathrm{cm}}^2/\mathrm{s}$, corresponding to room-temperature effective mobilities as high as $400{\mathrm{cm}}^2/\mathrm{Vs}$ as well as a correlation between rapid diffusion and short population lifetime. At elevated densities we detect distinct signatures of many-particle interactions and consequences of strongly suppressed Auger-type exciton-exciton annihilation. A combination of analytical and numerical theoretical approaches is employed to provide pathways toward comprehensive understanding of the observed linear and nonlinear propagation phenomena. We emphasize the role of dark exciton states and present a mechanism for diffusion facilitated by free-electron hole plasma from entropy-ionized excitons.

中文翻译：

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抑制紊乱的单层半导体中的激子扩散

单层半导体中紧密结合的激子代表了研究中性准粒子二维传播的通用平台。但是，由于对周围环境的高度敏感性，空间能量的波动会严重掩盖其内在特性。在这里，我们利用六方氮化硼中各个层的封装来强烈抑制环境混乱。然后在环境条件下使用时间和空间分辨发射显微镜直接监测激子的扩散。我们一直发现线性扩散系数高达$10{\mathrm{厘米}}^2/\mathrm{s}$，对应的室温有效迁移率高达 $400{\mathrm{厘米}}^2/\mathrm{VS}$以及快速扩散与人口寿命短的相关性。在较高的密度下，我们检测到许多粒子相互作用的独特特征以及强烈抑制的俄歇型激子-激子an灭的后果。分析和数值理论方法相结合可为全面了解所观察到的线性和非线性传播现象提供途径。我们强调暗激子态的作用，并提出了一种由熵电离激子的自由电子空穴等离子体促进扩散的机制。