Phonon Absorption at the Magnetoroton Minimum in the Fractional Quantum Hall Effect

We have made the first phonon absorption measurements in the fractional quantum Hall regime. Experiments have been conducted on two samples which have similar electron densities but greatly differing mobilities. The energy gaps as measured by activation studies of the longitudinal resistance differ by a factor of 2. Phonon absorption measurements give almost identical values for the energy gap, demonstrating that the gap measured in this way is rather insensitive to disorder. The value of this gap is in agreement with the activation gap measured in the high mobility sample. Values obtained at $\nu =2\mathrm{}/3$ are in good agreement with theory.