Ultra-Low Temperature Study of the nu=5/2 Even-Denominator FQHE State

We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at nu=5/2. At a bath temperature T sub b = 8mK, we observe a Hall plateau quantized to a value of 2h/5e sup 2 with an uncertainty smaller than 2 parts in 10 sup 6 and a vanishing R sub (xx) (R sub (xx) = 1.7+-1.7 OMEGA). The thermal activation energy gaps DELTA sub (nu) at Landau level filling factors nu=5/2, 7/3, and 8/3 and 0.11, 0.10 and 0.055K, respectively. Adding a disorder broadening (typically ~2K) 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 sub b >= 8mK. We also explore the density dependence of the activation gap at nu=5/2. Preliminary results at T sub b >= 25mK show that the 5/2 state is very sensitive to disorder.