CMT Seminar: Leonid Levitov – University of Copenhagen

CMT Seminar: Leonid Levitov

Professor of Physics at MIT

Electron Fluid in Graphene: Collective Modes, Viscosity, Current
Vortices and Negative Nonlocal Resistance

It is widely believed that electrons in charge-neutral graphene form a
quantum-critical state that features universal collision-dominated
transport resembling that of relativistic viscous fluids. This talk
will discuss several new phenomena that provide striking macroscopic
signatures of the hydrodynamic regime. One is ballistic propagation of
energy in relativistic fluids, which manifests itself as a new
collective mode of energy transfer that obeys a wave equation and
transports heat rather than charge. It is a sound-like mode, however
it is electron-based rather than phonon-based, hence the velocity is
quite high. We will demonstrate that a three orders of magnitude
enhancement compared to previously investigated phonon-based energy
waves is feasible. Another new phenomenon is negative voltage response
arising due to vorticity of a viscous electron flow. We argue that the
negative voltage response may play the same role for the viscous
regime as zero electrical resistance does for superconductivity.
Besides offering a diagnostic of viscous transport which distinguishes
it from ohmic currents, the sign-changing electrical response affords
a robust tool for directly measuring the viscosity-to-resistivity
ratio. Lastly, we will discuss the subtle relation between the
negative voltage response and vortices, or whirpools, in the electron
system, and comment on the recent observation of negative nonlocal
resistance in graphene.