Charge imbalance and bilayer two-dimensional electron systems atνT=1

Abstract

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