Electron-phonon Cerenkov instability in graphene revealed by global and local noise measurements*

Understanding and controlling non-equilibrium electronic phenomena is an outstanding challenge in science and engineering. Graphene presents a particularly interesting system for studying such effects because its high mobility means electrons may be accelerated very far out of equilibrium. We find that when fully encapsulated graphene is driven with a sufficiently high DC current, the drift velocity of the electrons exceeds the phase velocity of sound. This results in population inversion of the electronic system and stimulated emission of phonons becomes the dominant scattering process. This phonon Cerenkov effect leads to a runaway growth of certain phonon modes in the direction of momentum flow. Using NV centers as local noise probes, we map the exponential spatial growth of the phonon population by measuring the resulting local current fluctuations. Additionally, we measure a significantly altered global noise spectrum and AC conductivity at GHz frequencies, both of which are well described by stimulated emission of acoustic phonons. Since the amplified phonons are primarily in the THz range, the observation of a phonon Cerenkov effect in graphene is a first step towards realizing an on-chip generator of THz sound and radiation.