Experiments on a Quantum Hall Bilayer Excitonic Condensate

Double layer 2D electron systems support a remarkable low temperature collective phase when the total number of electrons in the system precisely fills the lowest spin-resolved Landau level and the separation between the layers is less than a critical value. This strongly correlated state may be viewed as a Bose condensate of interlayer excitons or, equivalently, as an easy-plane pseudo-ferromagnet. In this talk I will review our experimental studies of this phase. Interlayer tunneling measurements, which are reminiscent of the dc Josephson effect and demonstrate the spontaneous onset of interlayer phase coherence, will be discussed first. These experiments have allowed for a direct observation of the linearly dispersing Goldstone collective mode of the system and have shown that the stability of the excitonic phase is enhanced by small antisymmetric layer density imbalances. Second, I will summarize transport experiments in which independent control over the currents flowing in each 2D layer is exercised. Such measurements have already demonstrated the precise quantization of the Hall component of the interlayer drag in the system. More recently, measurements in which equal, but oppositely directed, currents flow in the two layers have revealed that both the longitudinal and the Hall resistances of the individual layers vanish in the low temperature limit. This result proves that the conductivity in the counter-flow, or pseudo-spin channel, is vastly larger than that in the conventional, or parallel flow, channel. Finally, I will comment on the degree to which these findings support the theoretical prediction of excitonic superfluidity in the bilayer system.