Hydrodynamic Terahertz Plasmons and Electron Sound in Graphene with Spatial Dispersion

The intrinsic transverse-magnetic modes of graphene with spatial dispersion are investigated theoretically in the hydrodynamic regime in a terahertz frequency range. It is revealed that spatial dispersion is caused by the diffusion electron-transport mechanism due to the spreading of electron-concentration gradients under the effect of pressure in an electron liquid. The cases of a screened and unscreened plasmon as well as electron sound are considered.