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

We use interlayer tunneling to study bilayer two-dimensional electron systems at ν_T=1 over a wide range of charge-density imbalance Δν=ν₁−ν₂ between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic ν_T=1 phase at small layer separation can survive at least up to an imbalance of Δν=0.5, i.e., (ν₁,ν₂)=(3/4,1/4). Phase transitions between the excitonic ν_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/ℓ, the temperature T, or the charge imbalance Δν. We observe that close to the phase boundary the coherent ν_T=1 phase can be absent at Δν=0, present at intermediate Δν, and then absent again at large Δν, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical d/ℓ shifts linearly with temperature, while at Δν=1/3 the critical d/ℓ is only weakly dependent on T. At Δν=1/3 we report on an observation of a direct phase transition between the coherent excitonic ν_T=1 bilayer integer quantum Hall phase and the pair of single-layer fractional quantized Hall states at ν₁=2/3 and ν₂=1/3.