Tracking Quasiparticle Energies in Graphene with Near Field Optics

TL;DR

This paper demonstrates how near-field optics can be used to track quasiparticle energies in graphene by analyzing finite momentum optical conductivity, revealing effects of mass gaps and strain on the Dirac fermion dispersion.

Contribution

It introduces a method to probe quasiparticle energies in graphene via finite momentum optical conductivity measurements, accounting for effects of mass gaps and anisotropic strain.

Findings

Peak in conductivity remains robust at finite temperature and residual scattering.

Mass gap causes the peak to shift to lower energies and broaden.

Strain along armchair direction shifts the peak to lower q values.

Abstract

Advances in infrared nanoscopy have enabled access to the finite momentum optical conductivity $\sigma(\vec{q},\omega)$. The finite momentum optical conductivity in graphene has a peak at the Dirac fermion quasiparticle energy $\epsilon(k_F-q)$, i.e. at the Fermi momentum minus the incident photon momentum. We find that the peak remains robust even at finite temperature as well as with residual scattering. It can be used to trace out the fermion dispersion curves. However, this effect depends strongly on the linearity of the Dirac dispersion. Should the Dirac fermions acquire a mass, the peak in $\sigma(q,w)$ shifts to lower energies and broadens as optical spectral weight is redistributed over an energy range of the order of the mass gap energy. Even in this case structures remain in the conductivity which can be used to describe the excitation spectrum. By contrast, in graphene strained along the armchair direction, the peak remains intact, but shifts to a lower value of $q$ determined by the anisotropy induced by the deformation.