Out-of-equilibrium criticalities in graphene superlattices

In thermodynamic equilibrium, current in metallic systems is carried by electronic states near the Fermi energy, whereas the filled bands underneath contribute little to conduction. Here, we describe a very different regime in which carrier distribution in graphene and its superlattices is shifted so far from equilibrium that the filled bands start playing an essential role, leading to a critical-current behavior. The criticalities develop upon the velocity of electron flow reaching the Fermi velocity. Key signatures of the out-of-equilibrium state are current-voltage characteristics that resemble those of superconductors, sharp peaks in differential resistance, sign reversal of the Hall effect, and a marked anomaly caused by the Schwinger-like production of hot electron-hole plasma. The observed behavior is expected to be common to all graphene-based superlattices. Description Displacing the Fermi surface Electrons that contribute to electrical conduction in a metal typically occupy high energy levels near the Fermi level. To get electrons from lower bands to join the flow, extremely large electric fields would be needed. In graphene and its superlattices, Berdyugin et al. show that small, experimentally accessible fields are sufficient to achieve this regime. The researchers discerned the signatures of this highly nonequilibrium state in transport data. —JS A highly nonequilibrium transport regime is achieved in graphene and its superlattices.