Charge Imbalance and Bilayer 2D Electron Systems at ${\nu }_T=1$

We use interlayer tunneling to study bilayer 2D electron systems at $\nu_T = 1$overawiderangeofchargedensityimbalance,$\Delta \nu =\nu_1-\nu_2$, 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 $\mathrm{\Delta }\nu$ = 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 $d\mathrm{/}\mathrm{\ell }$, the temperature $T$, or the charge imbalance, $\mathrm{\Delta }\nu$. We observe that close to the phase boundary the coherent ${\nu }_T=1$ phase can be absent at $\mathrm{\Delta }\nu$ = 0, present at intermediate $\mathrm{\Delta }\nu$, but then absent again at large $\mathrm{\Delta }\nu$, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical $d\mathrm{/}\mathrm{\ell }$ shifts linearly with temperature, while at $\mathrm{\Delta }\nu$ = 1/3 the critical $d\mathrm{/}\mathrm{\ell }$ is only weakly dependent on $T$. At $\mathrm{\Delta }\nu$ = 1/3 we report the first 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 ${\nu }_2=1\mathrm{/}3$.