Double layer two-dimensional electron systems: Probing the transition   from weak to strong coupling with Coulomb drag
  Frictional drag measurements revealing anomalously large dissipation at the transition between the weakly- and strongly-coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor $\nu_T =1$ are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling. 
M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West