COLLECTIVE EXCITATIONS OF THE FRACTIONAL QUANTUM HALL-EFFECT

We have made the first phonon absorption measurements in the fractiona l quantum Hall regime. Experiments have been conducted on both high mo bility n and p-type GaAs/AlGaAs heterojunctions. In the n-type samples used the electron densities were similar whilst the mobilities differ ed greatly. The energy gaps determined by phonon absorption measuremen ts give almost identical values, demonstrating that this method of mea surement is rather insensitive to disorder. The values obtained at v = 2/3 are in good agreement with theoretical estimates of the magnetoro ton energy gap. The results on p-type samples again demonstrate the re latively small effect of disorder on the values obtained. The values a re much lower than simple theoretical estimates. It is believed this i s due to the larger influence of Landau level mixing in these samples.