Prize Talk: Frank Isakson Prize for Optical Effects in SolidsHydrodynamic Electrodynamics and Energy Wave of Dirac Liquid in Graphene

Graphene, a single layer of carbon atoms, exhibits novel two-dimensional electronic behavior characterized by massless Dirac electrons. Close to the charge neutral points, thermally excited electrons and holes form a novel Dirac liquid with elementary excitations distinctly from a Fermi liquid. Using an on-chip terahertz spectroscopy, we directly probe the unusual electrodynamic behavior of massless Dirac liquid in ultraclean graphene encapsulated in boron nitride. We reveal a quantum critical behavior in electron-electron scattering rate characteristic of a hydrodynamic description of the Dirac liquid. In addition, we visualize for the first the propagation of an energy wave, also known as the electronic second sound, of the Dirac fluid in graphene.