Nonadiabatic coupling effects in MgB$_2$ reexamined

TL;DR

This paper presents an ab initio nonadiabatic theory explaining MgB$_2$'s Raman spectrum and temperature dependence by incorporating electron-hole pair scattering and dynamical electron-phonon interactions.

Contribution

It introduces a parameter-free nonadiabatic framework that accurately reproduces MgB$_2$'s Raman features and highlights the importance of scattering effects in phonon renormalization.

Findings

Nonadiabatic effects significantly influence MgB$_2$'s Raman spectrum.

Electron-hole pair scattering partially mitigates phonon frequency renormalization.

Dynamical electron-phonon coupling is crucial for temperature-dependent spectral features.

Abstract

The unusual Raman spectrum of MgB$_2$ and its formidable temperature dependence are successfully reproduced by means of a parameter-free \emph{ab initio} nonadiabatic theory that accounts for the electron-hole pair scattering mechanisms with the system phonons. This example turns out to be a prototypical case where a strong nonadiabatic renormalization of the phonon frequency is partially washed out by the aforementioned scattering events, bringing along a characteristic temperature dependence. Both electron-hole pair lifetime and energy renormalization effects due to dynamical electron-phonon coupling turn out to play a crucial role. This theory could aid in comprehending other Raman spectra characterized with unconventionally strong electron-phonon interaction.