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In doped semiconductors, optically excited electron-hole pairs (i.e. excitons) can be treated as an impurity coupling to a Fermi sea. Atomically thin semiconductors provide a rich playground to explore the Fermi polaron problems where the electrons and excitons further acquire a valley index. The attractive interaction between the exciton and Fermi sea leads to an energetically favorable state --- the attractive polaron --- as well as a higher energy repulsive polaron, a metastable state that eventually decays into attractive polarons. Here, we study the emergence and evolution of attractive polarons and repulsive polarons in MoSe₂ and WSe₂ monolayers as the electron doping density increases. Using two-dimensional coherent electronic spectroscopy (2DCES), we follow the changes in resonant energy, oscillator strength, and quantum decoherence of the AP and RP branches. Because of their different band structures, polarons in MoSe₂ and WSe₂ monolayers exhibit distinct quantum quantum dynamics and coupling mediated by valley index. Because of the large oscillator strength associated with quasi-particles, their intrinsic dephasing dynamics occur on sub-picosecond time scales. On the other hand, long-lived population and valley dynamics have been reported in previous experiments. We attribute such dynamics from a few hundred picoseconds to a few nanoseconds to dark-to-bright exciton conversion processes.

We gratefully acknowledge funding the Department of Energy, Office of Basic Energy Sciences under grant DE-SC0019398 and the Welch Foundation grant F-1662 for sample preparation. Collaborations are enabled by National Science Foundation via MRSEC grants DMR-1720595 and DMR-2308817.

Xiaoqin Li

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