Exciton-polaron Rydberg states in monolayer MoSe2 and WSe2

Erfu Liu,Zhengguang Lu,Chun Hung Lui,Kenji Watanabe,Dmitry Smirnov,Jeremiah Van Baren,Yia-Chung Chang,Takashi Taniguchi

doi:10.1038/s41467-021-26304-w

Abstract

Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea. It has been evoked to explain the excitonic spectra of charged monolayer transition metal dichalcogenides, but the studies were limited to the ground state. Here we measure the reflection and photoluminescence of monolayer MoSe2 and WSe2 gating devices encapsulated by boron nitride. We observe gate-tunable exciton polarons associated with the 1 s–3 s exciton Rydberg states. The ground and excited exciton polarons exhibit comparable energy redshift (15~30 meV) from their respective bare excitons. The robust excited states contradict the trion picture because the trions are expected to dissociate in the excited states. When the Fermi sea expands, we observe increasingly severe suppression and steep energy shift from low to high exciton-polaron Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Our experiment and theory demonstrate the exciton-polaron nature of both the ground and excited excitonic states in charged monolayer MoSe2 and WSe2.

Highlights

Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea
As the solid-state counterpart of hydrogen atoms, excitons can exhibit Rydberg-like optical spectra, which are crucial to revealing the exciton quantum structure
When the exciton is coupled to the Fermi sea (FS) electron–hole pairs, it can form a complex quasiparticle called exciton polaron[14,15,16,17,18]

Summary

Introduction

Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea. There have been ongoing debates to distinguish the trion and exciton-polaron pictures, because they give similar energy in the low-charge-density or weak-coupling regime. We apply both approaches to investigate the ground and excited excitonic Rydberg states coupled to the gatetunable FS in monolayer MoSe2 and WSe2 devices.

Results

Conclusion