Transition from acoustic plasmon to electronic sound in graphene
Abstract
Doped graphene samples host an electronic Fermi liquid that shows hydrodynamic behavior in a certain range of densities and temperatures [1]. As is typical for all fluids, Fermi liquids respond differently to perturbations depending on whether their frequency is larger (collisionless regime) or smaller (hydrodynamic regime) than the inter-particle collision rate. Specifically, in the collisionless regime, the electron liquid responds elastically to shear stresses. This results in a different phase velocity between the collisionless zero sound and the hydrodynamic first sound, the collisionless mode being stiffer and with higher velocity [2]. We performed terahertz photocurrent nanoscopy measurements [3] on graphene devices, with a metallic gate close to the sample, to probe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermi liquids. We report [4] the observation of a change in the plasmon phase velocity when the excitation frequency approaches the electron-electron collision rate. This first observation of the first sound mode in an electronic Fermi liquid is of fundamental interest and can enable novel terahertz emitter and detection implementations.
References
[1] D.A. Bandurin et al. Science 351, 1055 (2016).
[2] I. Torre et al. Physical Review B 99, 144307 (2018).
[3] P. Alonso Gonzalez et al. Nat. Nanotech. 12, 31 (2017).
[4] D. Barcons Ruiz et al. arXiv:2301.07399, (2023).
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