Emergence of large non-adiabatic effects induced by the electron-phonon interaction on the complex vibrational quasi-particle spectrum of the doped monolayer MoS$_{2}$

TL;DR

This paper provides a first-principles analysis of how electron-phonon interactions induce large non-adiabatic effects in the vibrational spectrum of doped monolayer MoS₂, revealing complex spectral structures and mode instabilities.

Contribution

It introduces a detailed theoretical framework and analytical model to understand non-adiabatic effects and complex vibrational spectra caused by electron doping in monolayer MoS₂.

Findings

Doping causes linewidth broadening and instability of certain phonon modes.

Non-adiabatic effects significantly alter phonon dispersion upon valley population.

A multiple-phonon quasi-particle picture explains the complex vibrational spectral features.

Abstract

We present a comprehensive first-principles analysis of the non-adiabatic effects due to the electron-phonon interaction on the vibrational spectrum of the electron-doped monolayer MoS$_{2}$. Deep changes in the Fermi surface upon doping cause the linewidth broadening of the normal modes governing the spin-conserving inter-valley electronic scattering, which become unstable with the population of all the spin-split conduction valleys. We find that the non-adiabatic spectral effects modify dramatically the adiabatic dispersion of the long-wavelength optical phonon modes, responsible for intra-valley scattering, as soon as inequivalent valleys get populated. These results are illustrated by means of a simple analytical model. Finally, we explain the emergence of an intricate dynamical structure for the strongly interacting out-of-plane polarized A 0 1 optical vibrational mode spectrum by means of a multiple-phonon quasi-particle picture defined in the full complex frequency plane, showing that this intriguing spectral structure originates from the splitting of the original adiabatic branch induced by the electron-phonon coupling.