Experimental studies of the fractional quantum Hall effect in the first excited Landau level

We present a spectrum of experimental data on the fractional quantum Hall effect (FQHE) states in the first excited Landau level, obtained in an ultrahigh mobility two-dimensional electron system and at very low temperatures, and report the following results. For the even-denominator FQHE states, the sample dependence of the ν=5/2 state clearly shows that disorder plays an important role in determining the energy gap at ν=5/2. For the developing ν=19/8 FQHE state, the temperature dependence of the Rxx minimum implies an energy gap of ∼5 mK. The energy gaps of the odd-denominator FQHE states at ν=7/3 and 8/3 also increase with decreasing disorder, similar to the gap at 5/2 state. Unexpectedly and contrary to earlier data on lower mobility samples, in this ultrahigh quality specimen, the ν=13/5 state is missing, while its particle-hole conjugate state, the ν=12/5 state, is a fully developed FQHE state. We speculate that this disappearance might indicate a spin polarization of the ν=13/5 state. Finally, the temperature dependence is studied for the two-reentrant integer quantum Hall states around ν=5/2 and is found to show a very narrow temperature range for the transition from quantized to classical value.