Charge imbalance and bilayer two-dimensional electron systems at νT=1

We use interlayer tunneling to study bilayer two-dimensional electron systems at nu(T)=1 over a wide range of charge-density imbalance Delta nu=nu(1)-nu(2) between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic nu(T)=1 phase at small layer separation can survive at least up to an imbalance of Delta nu=0.5, i.e., (nu(1),nu(2))=(3/4,1/4). Phase transitions between the excitonic nu(T)=1 state and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing d/center dot, the temperature T, or the charge imbalance Delta nu. We observe that close to the phase boundary the coherent nu(T)=1 phase can be absent at Delta nu=0, present at intermediate Delta nu, and then absent again at large Delta nu, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical d/center dot shifts linearly with temperature, while at Delta nu=1/3 the critical d/center dot is only weakly dependent on T. At Delta nu=1/3 we report on an observation of a direct phase transition between the coherent excitonic nu(T)=1 bilayer integer quantum Hall phase and the pair of single-layer fractional quantized Hall states at nu(1)=2/3 and nu(2)=1/3.