Observations of nascent superfluidity in a bilayer two-dimensional electron system at ν T = 1

Single-layer longitudinal and Hall resistances have been measured in a bilayer two-dimensional electron system at ν T = 1 with equal but oppositely directed currents flowing in the two layers. At small effective layer separation and low temperature, the bilayer system enters an interlayer coherent state expected to exhibit superfluid properties. We detect this nascent superfluidity through the vanishing of both resistances as the temperature is reduced. This corresponds to the counterflow conductivity rising rapidly as the temperature falls, reaching σ xx CF = 580 ( e 2 / h ) by T = 35 mK . This supports the prediction that the ground state of this system is an excitonic superfluid.