PHONON MEASUREMENTS OF THE ENERGY-GAP IN THE FRACTIONAL QUANTUM HALL STATE

We have made the first measurements of the energy gap responsible for the fractional quantum Hall effect (FQHE) using ballistic acoustic pho nons. This novel technique allows us to measure the FQHE energy gap at the magnetoroton minimum of the excitation spectrum. The gaps measure d by this technique are comparable to those deduced for high-quality s amples from the temperature dependence of the activated magnetoresista nce and by optical luminescence studies. At a filling factor of 2/3 at 9 T we find an energy gap of 6.1 K close to the theoretical value.