HIGH-FREQUENCY CONDUCTIVITY IN THE FRACTIONAL QUANTUM HALL REGIME

Surface acoustic wave (SAW) propagation on high quality AlGaAs/GaAs heterostructures is examined in the fractional quantum Hall regime. By measuring the transmitted SAW amplitude and frequency the 2D electron system (2DES) conductivity at the SAW frequency can be deduced and compared to the measured d.c. conductivity. Up to about 500MHz the SAW derived conductivity does not differ substantially from the measured d.c. conductivity. At higher frequencies some deviations are observed at FQHE and IQHE states. However, at Landau level filling nu = 1/2 SAW propagation is markedly different from that observed in the neighboring filling factor range; the response to the SAW is the opposite of that suggested by the d.c. conductivity. In addition, anomalous sound propagation is also observed in the small filling factor range nu less-than-or-similar-to 1/5, where electron solid formation is suspected. The properties of these distinct SAW anomalies are described and their possible origins discussed.