Graphene's non-equilibrium fermions reveal Doppler-shifted magnetophonon resonances accompanied by Mach supersonic and Landau velocity effects

TL;DR

This paper uncovers non-equilibrium phenomena in graphene at high currents, including Doppler-like phonon shifts, Mach effects, and superfluid-like Landau velocity, revealing new quantum behaviors in 2D fermion systems.

Contribution

It reports three novel high-current non-equilibrium effects in graphene, unifying them in a single resonance framework, advancing understanding of out-of-equilibrium quantum phenomena.

Findings

Doppler-like shift and splitting of TA phonons

Observation of intra-LL Mach effect with TA phonons

Onset of inter-LL transitions at critical drift velocity

Abstract

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.