We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at ν = 5/2. At a bath temperature Tb= 8 mK, we observe a Hall plateau quantized to a value of 2h/5e2with an uncertainty smaller than 2 parts in 106and a vanishing Rxx(Rxx= 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 Tb≥8 mK. We also explore the density dependence of the activation gap at ν = 5/2. Preliminary results at Tb≥25 mK show that the 5/2 state is very sensitive to disorder.
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