Valley-mediated singlet- and triplet-polaron interactions and quantum dynamics in a doped WSe$_2$ monolayer

TL;DR

This study investigates the quantum dynamics of singlet and triplet Fermi polarons in doped WSe$_2$ monolayers, revealing long-lived valley polarization and providing insights for controlling spin and valley degrees of freedom in 2D semiconductors.

Contribution

It demonstrates the formation and quantum decoherence of singlet and triplet polarons in doped WSe$_2$, highlighting their valley coherence dynamics on sub-picosecond timescales.

Findings

Long-lived singlet polaron valley polarization up to 200-800 ps.

Valley coherence linked to energy fluctuations of polarons.

Quantum decoherence depends on doping density.

Abstract

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 WSe$_2$, 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.