In doped transition metal dichalcogenides, optically created excitons (bound electron-hole pairs) can strongly interact with a Fermi sea of electrons to form Fermi polaron quasiparticles. When there are two distinct Fermi seas, as is the case in WSe2, there are two flavors of lowest-energy (attractive) polarons -- singlet and triplet -- where the exciton is coupled to the Fermi sea in the same or opposite valley, respectively. Using two-dimensional coherent electronic spectroscopy, we analyze how their quantum decoherence evolves with doping density and determine the condition under which stable Fermi polarons form. Because of the large oscillator strength associated with these resonances, intrinsic quantum dynamics of polarons as well as valley coherence between coupled singlet- and triplet polarons occur on sub-picosecond time scales. Surprisingly, we find that a dark-to-bright state conversion process leads to a particularly long-lived singlet polaron valley polarization, persisting up to 200-800 ps. Valley coherence between the singlet- and triplet polaron is correlated with their energy fluctuations. Our finding provides valuable guidance for the electrical and optical control of spin and valley indexes in atomically thin semiconductors.
@article{arxiv.2501.02372,
title = {Valley-mediated singlet- and triplet-polaron interactions and quantum dynamics in a doped WSe$_2$ monolayer},
author = {Yue Ni and Di Huang and Danfu Liang and Albert Liu and Xiaohui Liu and Kevin Sampson and Zhida Liu and Jianmin Quan and Kenji Watanabe and Takashi Taniguchi and Dmitry K. Efimkin and Jesper Levinsen and Meera M. Parish and Xiaoqin Li},
journal= {arXiv preprint arXiv:2501.02372},
year = {2025}
}