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Abstract

Recent experiments on quantum Hall bilayers in the vicinity of total
filling factor 1 (nu(T) = 1) have revealed many exciting observations
characteristic of a superfluidic exciton condensate. We report on our
experimental work involving the nu(T) = 1 exciton condensate in
independently contacted bilayer two-dimensional electron systems. We
observe previously reported phenomena as a zero-bias resonant tunneling
peak, a quantized Hall drag resistivity, and in counter-flow
configuration, the near vanishing of both rho(xx) and rho(xy)
resistivity components. At balanced electron densities in the layers,
we find for both drag and counter-flow current configurations,
thermally activated transport with a monotonic increase of the
activation energy for d/l(B) < 1.65 with activation energies up to 0.4
K. In the imbalanced system the activation energies show a striking
asymmetry around the balance point, implying that the gap to charge
excitations is considerably different in the separate layers that form
the bilayer condensate. This indicates that the measured activation
energy is neither the binding energy of the excitons, nor their
condensation energy. (c) 2006 Published by Elsevier B.V.