J. Shabani, Y. Liu, M. Shayegan, L. N. Pfeiffer, K. W. West, K. W. Baldwin
We report an experimental investigation of the unique fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor \nu = 1/2 in very high quality wide GaAs quantum wells, and at very high magnetic fields up to 35 T. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths W ranging from 44 to 96 nm, and have bilayer-like, symmetric charge distributions and variable densities in the range of $\simeq 4 \times 10^{11}$ to $\simeq 4 \times 10^{10}$ cm$^{-2}$. We present several experimental phase diagrams for the stability of the \nu=1/2 FQHE in these quantum wells. First, for a given W, the 1/2 FQHE is stable in a limited range of intermediate densities; it makes a transition to a compressible phase at low densities and to an insulating phase at high densities. The densities at which the \nu=1/2 FQHE is stable are larger for narrower quantum wells. Second, we present a plot of the symmetric-to-antisymmetric subband separation ($\Delta_{SAS}$), which characterizes the inter-layer tunneling, vs density for various W. This plot reveals that $\Delta_{SAS}$ at the boundary between the compressible and FQHE phases increases linearly with density. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the inter-layer and intra-layer Coulomb interactions and $\Delta_{SAS}$. We conclude that, consistent with the conclusions of previous studies, the \nu=1/2 FQHE observed in wide GaAs quantum wells with symmetric charge distribution is likely a two-component state.
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http://arxiv.org/abs/1306.5290
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