Intrinsic gap of the ν=5/2 fractional quantum hall state

The fractional quantum Hall effect is observed at low magnetic field where the cyclotron energy is smaller than the Coulomb interaction energy. The ν=52 excitation gap at 2.63 T is measured to be 262±15mK, similar to values obtained in samples with twice the electronic density. Examining the role of disorder on the 52 state, we find that a large discrepancy remains between theory and experiment for the intrinsic gap extrapolated from the infinite mobility limit. The observation of a 52 state in the low-field regime suggests that inclusion of nonperturbative Landau level mixing may be necessary to fully understand the energetics of half-filled fractional quantum Hall liquids.