  Oscillatory magnetoresistance measurements on graphene have revealed a wealth
of novel physics. These phenomena are typically studied at low currents. At
high currents, electrons are driven far from equilibrium with the atomic
lattice vibrations so that their kinetic energy can exceed the thermal energy
of the phonons. Here, we report three non-equilibrium phenomena in monolayer
graphene at high currents: (i) a "Doppler-like" shift and splitting of the
frequencies of the transverse acoustic (TA) phonons emitted when the electrons
undergo inter-Landau level (LL) transitions; (ii) an intra-LL Mach effect with
the emission of TA phonons when the electrons approach supersonic speed, and
(iii) the onset of elastic inter-LL transitions at a critical carrier drift
velocity, analogous to the superfluid Landau velocity. All three quantum
phenomena can be unified in a single resonance equation. They offer avenues for
research on out-of-equilibrium phenomena in other two-dimensional fermion
systems.
