Exciton-polaron Rydberg states in monolayer MoSe₂ and WSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Exciton polarons are hypothetical quasiparticles formed by excitons interacting with electron-hole clouds in Fermi seas.
• Previous studies were limited to the ground state, leaving excited states unexplored.

Purpose of the Study:

• To investigate the nature of excitonic states in charged monolayer transition metal dichalcogenides, specifically MoSe2 and WSe2.
• To experimentally verify the existence and characteristics of exciton polarons in excited Rydberg states.

Main Methods:

• Utilized reflection and photoluminescence spectroscopy on gated monolayer MoSe2 and WSe2 devices encapsulated with boron nitride.
• Applied gate voltage to tune the Fermi sea and observe changes in excitonic spectra.

Main Results:

• Observed gate-tunable exciton polarons associated with 1s-3s exciton Rydberg states.
• Measured comparable energy redshifts (15-30 meV) for both ground and excited state exciton polarons.
• Demonstrated that excited states persist, contradicting the trion dissociation picture.
• Observed suppression and energy shifts in higher Rydberg states with increasing Fermi sea density, consistent with exciton polaron theory.

Conclusions:

• The study confirms the exciton polaron nature of both ground and excited excitonic states in charged monolayer MoSe2 and WSe2.
• Experimental results and theoretical models align, providing strong evidence for the exciton polaron quasiparticle in excited states.
• Findings challenge the trion model for excited states and offer a more comprehensive understanding of excitonic behavior in these materials.