Experimental signatures of the transition from acoustic plasmon to electronic sound in graphene

Abstract

Fermi liquids respond differently to perturbations depending on whether their frequency is higher (collisionlessregime) or lower (hydrodynamic regime) than the interparticle collision rate. This results in a different phasevelocity between the collisionless zero sound and the hydrodynamic first sound. We performed terahertz pho-tocurrent nanoscopy measurements on graphene devices, with a metallic gate close to the graphene layer, toprobe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermiliquids. We report the observation of a change in the plasmon phase velocity when the excitation frequencyapproaches the electron-electron collision rate that is compatible with the transition between the zero andthe first sound mode.