Strong Coulomb drag and broken symmetry in double-layer graphene

TL;DR

This study investigates Coulomb drag in double-layer graphene with extremely close layers, revealing unexpected strong interactions and symmetry-breaking effects that differ from previous weaker coupling regimes.

Contribution

It demonstrates the observation of strong Coulomb drag and broken symmetry in double-layer graphene at nanometer-scale separation, a regime previously unexplored experimentally.

Findings

Drag vanishes at neutrality due to electron-hole symmetry

Drag is strongest when both layers are neutral

Drag sign changes and increases with magnetic field

Abstract

Spatially separated electron systems remain strongly coupled by electron-electron interactions even when they cannot exchange particles, provided that the layer separation d is comparable to a characteristic distance l between charge carriers within layers. One of the consequences of this remote coupling is a phenomenon called Coulomb drag, in which an electric current passed through one of the layers causes frictional charge flow in the other layer. Previously, only the regime of weak (d>>l) to intermediate (d ~ l) coupling could be studied experimentally. Here we use graphene-BN heterostructures with d down to 1 nm to probe interlayer interactions and Coulomb drag in the limit d<<l where the two Dirac liquids effectively nest within the same plane, but still can be tuned and measured independently. The strongly interacting regime reveals many unpredicted features that are qualitatively different from those observed previously. In particular, although drag vanishes because of electron-hole symmetry when either layer is neutral, we find that drag is at its strongest when /both/ layers are neutral. Under this circumstance, drag is positive in zero magnetic field but changes its sign and rapidly grows in strength with field. The drag remains strong at room temperature. The broken electron-hole symmetry is attributed to mutual polarization of closely-spaced interacting layers. The two-fluid Dirac system offers a large parameter space for further investigation of strong interlayer interaction phenomena.