Raman evidence for nonadiabatic effects in optical phonon self-energies of transition metals

TL;DR

This study uses Raman spectroscopy and ab initio calculations to reveal nonadiabatic electron-phonon interactions affecting optical phonon energies and linewidths in transition metals, highlighting electron spectrum renormalization effects.

Contribution

It provides experimental and theoretical evidence of nonadiabatic effects on optical phonons in transition metals, emphasizing electron-phonon interactions over anharmonicity.

Findings

Temperature causes anisotropic phonon softening and linewidth narrowing.

Calculations reproduce experimental phonon spectral changes.

Anomalies linked to electron spectrum renormalization.

Abstract

We report a Raman study of the effect of temperature on the self-energies of optical phonons in a number of transition metals with hexagonal-close-packed structure. Anisotropic softening of phonon energies and narrowing of phonon linewidths with increasing temperature are observed. These effects are reproduced in the calculations of phonon spectral functions based on \textit{ab initio} electronic structures and with carrier scattering by phonons taken into account. The combined observations and results of simulations indicate a relation between observed anomalies and the renormalization of the electron spectrum due to electron-phonon interaction. It is emphasized that the temperature dependence of the phonon energies resembles anharmonic behavior but is actually an electron-induced effect.