Electron-hole collision limited transport in charge-neutral bilayer graphene

TL;DR

This study reveals a new electron-hole collision limited transport regime in charge-neutral bilayer graphene, where ballistic transport is hindered by electron-hole interactions rather than impurities or phonons, confirmed through experiments and theoretical modeling.

Contribution

It demonstrates that electron-hole collisions can dominate charge transport in bilayer graphene at charge neutrality, a previously unobserved regime, supported by experimental and theoretical analysis.

Findings

Negative four-terminal resistance observed near charge neutrality

Transport at low densities is governed by electron-hole scattering

The phenomenon is robust across different device structures

Abstract

Ballistic transport occurs whenever electrons propagate without collisions deflecting their trajectory. It is normally observed in conductors with a negligible concentration of impurities, at low temperature, to avoid electron-phonon scattering. Here, we use suspended bilayer graphene devices to reveal a new regime, in which ballistic transport is not limited by scattering with phonons or impurities, but by electron-hole collisions. The phenomenon manifests itself in a negative four-terminal resistance that becomes visible when the density of holes (electrons) is suppressed by gate-shifting the Fermi level in the conduction (valence) band, above the thermal energy. For smaller densities transport is diffusive, and the measured conductivity is reproduced quantitatively, with no fitting parameters, by including electron-hole scattering as the only process causing velocity relaxation. Experiments on a trilayer device show that the phenomenon is robust and that transport at charge neutrality is governed by the same physics. Our results provide a textbook illustration of a transport regime that had not been observed previously and clarify the nature of conduction through charge-neutral graphene under conditions in which carrier density inhomogeneity is immaterial. They also demonstrate that transport can be limited by a fully electronic mechanism, originating from the same microscopic processes that govern the physics of Dirac-like plasmas.