Charge Imbalance and Bilayer 2D Electron Systems at $ν_T = 1$

Abstract

We use interlayer tunneling to study bilayer 2D electron systems at $ν_T = 1$ over a wide range of charge density imbalance, $Δν=ν_1-ν_2$, 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 $(ν_1, ν_2)$ = (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 $d/\ell$, 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 $Δν$, but 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 $d/\ell$ shifts linearly with temperature, while at $Δν$ = 1/3 the critical $d/\ell$ is only weakly dependent on $T$. At $Δν$ = 1/3 we report the first 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 $ν_1$ = 2/3 and $ν_2=1/3$.

13 pages, 8 postscript figures. Final published version