AI 中文总结
本研究结合超快光电子能谱与第一性原理激子布洛赫方程，揭示了单层WSe₂中自由载流子聚合成激子的声子起源机制，为光学材料与器件设计提供了方向。
AI 中文摘要
凝聚态物理中充满了高能自由粒子聚合成低能束缚少粒子态的现象。虽然单个粒子的冷却过程已被充分理解，但冷自由载流子形成束缚态的关键步骤仍不明确，该过程涉及复杂的能量和动量弛豫路径。本文通过将单层WSe₂的超快时间分辨、动量分辨光电子能谱与第一性原理激子布洛赫方程相结合，解析了带边初始自由电子和空穴向束缚激子的转换过程。凭借前所未有的能量分辨率，我们观测到自由载流子能带与激子能带的瞬态共存，伴随两者之间的谱重转移。我们确定了激子形成的声子起源，并将这种共存归因于向最低能量激子态的级联弛豫过程，其中中间态布居数较少。该过程的效率受谷多重性、大动量声子发射和自旋翻转过程控制。通过阐明束缚态如何从其基本组分中产生，我们的研究结果为设计激子形成策略提供了方向，对使用激子或自由载流子工作的光学材料和器件具有直接意义。
英文摘要
Condensed matter physics is replete with phenomena involving high-energy free particles coalescing into low-energy bound few-particle states. While the cooling of the individual particles is well understood, the crucial step by which cold free carriers form a bound state remains elusive, involving complex energy and momentum relaxation pathways. Here, by combining ultrafast time- and momentum-resolved photoemission spectroscopy on a monolayer WSe$_2$ with the first-principles excitonic-Bloch equations, we resolve the conversion of initially free electrons and holes at the bandedges into bound excitons. With unprecedented energy resolution, we observe the transient \textit{coexistence} of free-carrier and excitonic bands, accompanied by a transfer of spectral weight between the two. We establish the phononic origin of exciton formation and ascribe this coexistence to a sequential relaxation cascade toward the lowest-energy excitonic states, wherein intermediate states remain weakly populated. The efficiency of this process is controlled by valley multiplicity, large-momentum phonon emission and spin-flip processes. By elucidating how bound states emerge from their elementary constituents, our results point to strategies for engineering exciton formation, with direct implications for optical materials and devices that operate with excitons or free carriers.
