Bilayer 2D electron systems at νtot=1: phase boundary between weak and strong coupling

The collapse of the excitonic quantized Hall state at νtot=1 in double layer two-dimensional electron systems is studied via interlayer tunneling and Coulomb drag. We find that spontaneous interlayer phase coherence, perhaps the most important hallmark of the excitonic phase and directly detected via a strong resonant enhancement of the zero-bias tunneling conductance, collapses very rapidly above a critical layer separation. In contrast, a related anomaly in the longitudinal component of Coulomb drag at νtot=1 subsides much more slowly as the layer separation is increased beyond the critical point.