Photoemitted electrons reveal large-momentum (“dark”) excitons in monolayer WSe2 Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenides, may enable new optoelectronic technologies (1). The optical excitation in these atomically thin materials creates tightly bound excitons composed of an excited electron and a valence-state hole (2), as well as a plethora of exciton complexes due to the reduced screening in Coulomb attraction (3–5). So far, excitons with large momenta have not been directly probed because photons only carry very small momenta and cannot directly interact with large-momentum excitons, but these dark excitons are predicted to exist in certain 2D semiconductors (6, 7). On page 1199 of this issue, Madéo et al. (8) used time- and angle-resolved photoemission spectroscopy (TR-ARPES) to directly probe dark excitons in monolayer tungsten diselenide (WSe2). By tracking the dynamics of electrons that constitute both bright and dark excitons, the authors reveal how both are formed and show that the latter outnumber the former at steady state.

中文翻译：

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探索激子的阴暗面

光发射电子显示单层 WSe2 中的大动量（“暗”）激子 二维 (2D) 半导体，例如过渡金属二硫属化物，可能会启用新的光电技术 (1)。这些原子级薄材料中的光学激发会产生由受激电子和价态空穴组成的紧密结合激子 (2)，以及由于库仑引力的筛选减少而产生的大量激子复合物 (3-5)。到目前为止，还没有直接探测到大动量的激子，因为光子只携带非常小的动量，不能直接与大动量的激子相互作用，但预测这些暗激子存在于某些二维半导体中 (6, 7)。在本期第 1199 页，Madeo 等人。(8) 使用时间和角度分辨光电子能谱 (TR-ARPES) 直接探测单层二硒化钨 (WSe2) 中的暗激子。通过跟踪构成亮激子和暗激子的电子的动力学，作者揭示了两者是如何形成的，并表明后者在稳态下的数量超过前者。