Ultra-low temperature study of the ν = 5/2 even-denominator FQHE state

We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at ν = 5/2. At a bath temperature T_b = 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 R_xx (R_xx = 1.7±1.7 Ω). The thermal activation energy gaps Δ at Landau level filling factors ν = 5/2,7/3, and 8/3 are 0.11, 0.10, and 0.055 K, respectively. Adding a disorder broadening (typically approximately 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 T_b≥8 mK. We also explore the density dependence of the activation gap at ν = 5/2. Preliminary results at T_b≥25 mK show that the 5/2 state is very sensitive to disorder.