Valley phonons and exciton complexes in a monolayer semiconductor

Minhao He,Min Yang,Jiaqiang Yan,Takashi Taniguchi,David G Mandrus,Kenji Watanabe,Xiaodong Xu,Wang Yao,Nathan P Wilson,Hongyi Yu,Dinh Van Tuan,Hanan Dery,Pasqual Rivera

doi:10.1038/s41467-020-14472-0

Abstract

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects. Here, we report the observation of multiple valley phonons – phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone – and the resulting exciton complexes in the monolayer semiconductor WSe2. We find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncover an intervalley exciton near charge neutrality. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.

Highlights

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena
We found that three valley phonons facilitate efficient spin-conserving intervalley scattering, which results in a series of dark exciton and dark trion phonon-replicas in the low-temperature photoluminescence (PL) spectrum
This results in a surprising finding: the single electron in both the positive dark trion and intervalley exciton resides in the valley opposite to that which the optical pump is coupled to

Summary

Introduction

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. We found that three valley phonons facilitate efficient spin-conserving intervalley scattering, which results in a series of dark exciton and dark trion phonon-replicas in the low-temperature photoluminescence (PL) spectrum. The sign and magnitude of Landé effective g-factors of the various replicas, together with their PL helicity under optical pumping, reveals that the spin-preserving intervalley scattering of the electron is more efficient than its intravalley spin-flip during the dark exciton/trion formation process.

Results

Conclusion