They offer avenues for research on out-of-equilibrium phenomena in other two-dimensional fermion systems. All three quantum phenomena can be unified in a single resonance equation. 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. 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. These phenomena are typically studied at low currents. Oscillatory magnetoresistance measurements on graphene have revealed a wealth of novel physics.

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