Dominant Phonon Wavevectors and Strain-induced Splitting of the 2D Graphene Raman Mode

TL;DR

This paper investigates the anisotropic phonon wavevectors involved in the 2D Raman mode of graphene, revealing how strain causes splitting through phonon dispersion distortion, independent of electronic band structure changes.

Contribution

It demonstrates that the 2D Raman mode splitting under strain is due to phonon dispersion distortion, challenging previous notions of electronic effects being dominant.

Findings

Phonon wavevectors $q^{*}$ are highly anisotropic and orientation-dependent.

Strain induces splitting of the 2D mode via phonon dispersion distortion.

Electronic band structure changes are negligible for the 2D Raman spectrum.

Abstract

The dominant phonon wavevectors $q^{*}$ probed by the 2D Raman mode of graphene are highly anisotropic and rotate with the orientation of the polarizer:analyzer direction relative to the lattice. The corresponding electronic transitions connect the electronic equibandgap contours where the product of the ingoing and outgoing optical matrix elements is strongest, showing a finite component along $\bm{K}-\bm{\Gamma}$ that sensitively determines $q^{*}$. We revoke the notion of 'inner' and 'outer' processes. Our findings explain the splitting of the 2D mode of graphene under uniaxial tensile strain. The splitting originates from a strain-induced distortion of the phonon dispersion; changes in the electronic band structure and resonance conditions are negligeable for the 2D Raman spectrum.