Ultra-low temperature study of the ν=52 even-denominator FQHE state

We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at View the MathML source. At a bath temperature Tb=8 mK, we observe a Hall plateau quantized to a value of 2h/5e2 with an uncertainty smaller than 2 parts in 106 and a vanishing Rxx (View the MathML source). The thermal activation energy gaps Δ at Landau level filling factors View the MathML source, and View the MathML source are 0.11, 0.10, and 0.055 K, respectively. Adding a disorder broadening (typically ∼2 K) to these values, we deduce that all three FQHE states have probably very similar energy gaps. The electron heating experiment shows that the 2D electrons are efficiently cooled to the bath temperature for Tb⩾8 mK. We also explore the density dependence of the activation gap at View the MathML source. Preliminary results at Tb⩾25 mK show that the View the MathML source state is very sensitive to disorder.