The even denominator fractional quantum Hall states at large Landau level mixing

We present a study of the energy gaps of the even denominator fractional quantum Hall states of the second Landau level in a two-dimensional electron gas with a record low density of n = 8.2x10$^{10}$ cm$^{-2}$. These measurements are motivated by the expectation that Landau level mixing present in samples of low densities breaks the degeneracy of the Pfaffian and its particle-hole conjugate anti-Pfaffian. Cooling the electron gas in our Helium-3 immersion cell to 5mK reveals at filling factor 5/2 a fully quantized Hall plateau and a vanishingly small magnetoresistance. Because of the low density of our sample, the 5/2 fractional state is observed at the highest degree of Landau level mixing reported to date. We have measured the energy gaps of the 5/2 and 7/2 fractional quantum Hall states. The intrinsic gap deduced in the limit of no disorder will be compared to previously reported values for samples with higher densities.