Giant non-adiabatic effects in layer metals: Raman spectra of intercalated graphite explained

TL;DR

This paper presents the first ab initio calculations confirming large non-adiabatic effects in the vibrational spectra of layer metals, significantly impacting their Raman spectra and offering a framework to estimate non-adiabaticity from experimental data.

Contribution

It provides the first ab initio analysis of non-adiabatic phonon frequencies in metals and introduces a method to estimate non-adiabatic effects from Raman spectra.

Findings

Non-adiabatic effects can be large in layer metals, up to 30% of phonon frequencies.

Non-adiabatic effects are likely present in the Raman spectra of all bulk metals.

A framework to estimate electron momentum-relaxation time from vibrational spectra is developed.

Abstract

The occurrence of non-adiabatic effects in the vibrational properties of metals have been predicted since the 60's, but hardly confirmed experimentally. We report the first fully \emph{ab initio} calculations of non-adiabatic frequencies of a number of layer and conventional metals. We suggest that non-adiabatic effects can be a feature of the vibrational Raman spectra of any bulk metal, and show that they are spectacularly large (up to 30% of the phonon frequencies) in the case of layer metals, such as superconducting $MgB_2$, $CaC_6$ and other graphite intercalated compounds. We develop a framework capable to estimate the electron momentum-relaxation time of a given system, and thus its degree of non-adiabaticity, in terms of the experimentally observed frequencies and linewidths.