Atomically thin semiconductors provide an excellent platform to study intriguing many-particle physics of tightly-bound excitons. In particular, the properties of tungsten-based transition metal dichalcogenides are determined by a complex manifold of bright and dark exciton states. While dark excitons are known to dominate the relaxation dynamics and low-temperature photoluminescence, their impact on the spatial propagation of excitons has remained elusive. In our joint theory-experiment study, we address this intriguing regime of dark state transport by resolving the spatio-temporal exciton dynamics in hBN-encapsulated WSe₂ monolayers after resonant excitation. We find clear evidence of an unconventional, time-dependent diffusion during the first tens of picoseconds, exhibiting strong deviation from the steady-state propagation. Dark exciton states are initially populated by phonon emission from the bright states, resulting in creation of hot (unequilibrated) excitons whose rapid expansion leads to a transient increase of the diffusion coefficient by more than one order of magnitude. These findings are relevant for both fundamental understanding of the spatio-temporal exciton dynamics in atomically thin materials as well as their technological application by enabling rapid diffusion.

中文翻译：

---

暗激子在原子级薄半导体中的非平衡扩散

原子级薄的半导体为研究紧束缚激子的有趣多粒子物理学提供了一个极好的平台。特别是，钨基过渡金属二硫属元素化物的特性是由明暗激子态的复杂流形决定的。虽然已知暗激子主导弛豫动力学和低温光致发光，但它们对激子空间传播的影响仍然难以捉摸。在我们的联合理论实验研究中，我们通过解决 hBN 封装的 WSe _{2 中}的时空激子动力学来解决这种有趣的暗态传输机制共振激发后的单层。我们发现在前几十皮秒内出现了非常规的、依赖于时间的扩散的明确证据，表现出与稳态传播的强烈偏差。暗激子态最初由亮态的声子发射填充，导致产生热（不平衡）激子，其快速膨胀导致扩散系数瞬时增加一个数量级以上。这些发现与原子级薄材料中时空激子动力学的基本理解以及它们通过实现快速扩散的技术应用相关。