Uniaxial strain-induced Kohn anomaly and electron-phonon coupling in acoustic phonons of graphene

TL;DR

This study investigates how uniaxial strain affects electron-phonon interactions and Kohn anomalies in graphene, revealing strain-induced anomalies in acoustic phonons and challenging previous assumptions about phonon-specific e-ph coupling.

Contribution

The paper demonstrates that uniaxial strain induces a Kohn anomaly in the longitudinal acoustic phonon branch of graphene, expanding understanding of strain effects on electron-phonon coupling.

Findings

Strain induces a discontinuity in LA phonon frequency derivative.

Kohn anomaly appears in LA branch under strain.

E-ph coupling is enhanced by uniaxial strain.

Abstract

Recent advances in strain engineering at the nanoscale have shown the feasibility to modulate the properties of graphene. Although the electron-phonon (e-ph) coupling and Kohn anomalies in graphene define the phonon branches contributing to the resonance Raman scattering, and is relevant to the electronic and thermal transport as a scattering source, the evolution of the e-ph coupling as a function of strain has been less studied. In this work, the Kohn anomalies and the e-ph coupling in uniaxially strained graphene along armchair (AC) and zigzag (ZZ) directions were studied by means of density functional perturbation theory calculations. In addition to the phonon anomaly at the transversal optical (TO) phonon branch in the K point for pristine graphene, we found that uniaxial strain induces a discontinuity in the frequency derivative of the longitudinal acoustic (LA) phonon branch. This behavior corresponds to the emergence of a Kohn anomaly, as a consequence of a strain-enhanced e-ph coupling. Thus, the present results for uniaxially strained graphene contrast with the commonly assumed view that the e-ph coupling around the K point is only present in the TO phonon branch.