  The fractional quantum Hall effect is observed at low field, in a regime
where the cyclotron energy is smaller than the Coulomb interaction. The nu=5/2
excitation gap is measured to be 262+/-15 mK at ~2.6 T, in good agreement with
previous measurements performed on samples with similar mobility, but with
electronic density larger by a factor of two. The role of disorder on the
nu=5/2 gap is examined. Comparison between experiment and theory indicates that
a large discrepancy remains for the intrinsic gap extrapolated from the
infinite mobility (zero disorder) limit. In contrast, no such large discrepancy
is found for the nu=1/3 Laughlin state. The observation of the nu=5/2 state in
the low-field regime implies that inclusion of non-perturbative Landau level
mixing may be necessary to better understand the energetics of half-filled
fractional quantum hall liquids.
