Counterflow measurements in GaAs hole bilayers: Possible evidence for excitonic condensation

We study interacting GaAs bilayer hole systems, with very small interlayer tunneling, in a counterflow geometry where equal currents are passed in opposite directions in the two, independently contacted layers. In the quantum Hall state at total bilayer filling ν = 1 both the longitudinal and Hall counterflow resistances tend to vanish in the limit of zero temperature. This observation demonstrates the pairing of oppositely charged carriers in opposite layers (excitons), and implies that the ground state of the system is an excitonic condensate.