PHONON SPECTROSCOPY OF THE FRACTIONAL QUANTUM HALL-EFFECT

The low-lying bulk excitations of two-dimensional electron systems sub ject to a strong perpendicular magnetic held have been studied using p honon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measuremen ts have demonstrated fundamental differences in phonon absorption betw een fractional quantum Hall states, non-rational filing factors and fi lling factors close to 1/2. At non-rational filing factors the respons e is bolometric, whereas at filing factors corresponding to FQHE state s the response also depends on the spectrum of the heater pulse. Aroun d nu = 1/2 the response is largely independent of magnetic field with a weak maximum at nu = 1/2.