Direct Observation of Intravalley Phonon Scattering of 2s Excitons in MoSe$_2$ and WSe$_2$ Monolayers

TL;DR

This study uses high-resolution resonance Raman spectroscopy at cryogenic temperatures to observe intravalley phonon scattering of 2s excitons in MoSe₂ and WSe₂ monolayers, revealing complex excitonic dispersion and new scattering pathways.

Contribution

It provides the first detailed observation of intravalley phonon scattering involving 2s excitons in these monolayers, highlighting complex dispersion relations and new Raman features.

Findings

Resonances with Rydberg states despite low oscillator strength

Identification of new Raman peaks linked to intravalley scattering

Complex dispersion and peak-splitting near dark excitonic states

Abstract

We present a high-resolution resonance Raman study of hBN encapsulated MoSe$_2$ and WSe$_2$ monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe$_2$ A2s and MoSe$_2$ A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k=0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe$_2$ A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests that the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex.