Charge neutral counterflow transport at filling factor 1 in GaAs hole bilayers

Interacting bilayers placed in perpendicular magnetic field exhibit a peculiar quantum Hall state (QHS) at total filling factor ν = 1, owing to the carrier-carrier interaction in the two layers. The physics of the ν = 1 QHS is similar to that of the many-particle ground state of a superconductor. Unlike conventional superconductors, however, in the ν = 1 QHS carriers in one layer pair with vacancies in the opposite layer forming charge neutral particles which flow without dissipation at the lowest temperatures. Here we review the experimental evidence supporting this picture, with an emphasis on magnetotransport in interacting GaAs hole bilayers in a configuration where equal and opposite currents are passed in the two layers.