Non-adiabatic phonons within the doped graphene layers of XC$_6$ compounds

TL;DR

This study uses Raman scattering to analyze phonons and electron-phonon interactions in doped graphene layers of XC$_6$ compounds, revealing non-adiabatic effects and phonon softening related to charge transfer.

Contribution

First systematic Raman analysis of phonons and electron-phonon interactions in doped graphene layers of XC$_6$ compounds, highlighting non-adiabatic effects and charge-dependent phonon behavior.

Findings

Out-of-plane carbon phonon softens with reduced layer spacing.

Non-adiabatic renormalization affects in-plane carbon modes.

Increased charge in the $ ext{π}^*$ band enhances non-adiabatic effects.

Abstract

We report the first Raman scattering measurements of BaC$_6$, SrC$_6$, and YbC$_6$, which along with new data on CaC$_6$, permits a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC$_6$, SrC$_6$, YbC$_6$, and CaC$_6$. This is due to increasing charge in the $\pi^*$ electronic band. Electron-phonon interaction effects between the in-plane carbon modes at $\approx1500$\invcm and the $\pi^*$ electrons cause a strong non-adiabatic renormalization. As charge is transferred into the $\pi^*$ band these non-adiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.