  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.
