Chirality-induced Dynamic Kohn Anomalies in Graphene

TL;DR

This paper presents a theoretical analysis of phonon energy renormalization in graphene, revealing unique Kohn anomalies caused by dynamical screening and chirality effects, which differ from those in ordinary metals.

Contribution

The study introduces an exact analytic derivation of phonon self-energy in graphene, identifying new Kohn anomalies linked to dynamical screening and chirality effects.

Findings

Three distinct Kohn anomalies for LO phonons at specific wavevectors

One Kohn anomaly for TO phonons at a specific wavevector

Identification of anomalies arising from dynamical screening and chirality

Abstract

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wavevector $q = \omega/v, 2k_F\pm\omega/v$ for LO phonons and one at $q = \omega/v$ for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA $q = 2k_F$ in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling.