Anomalous sound propagation at =1/2 in a 2D electron gas: Observation of a spontaneously broken translational symmetry?

Surface-acoustic-wave propagation on high-quality AlGaAs/GaAs heterostructures containing a two-dimensional electron system has been examined in the fractional quantum Hall effect (FQHE) regime. The response of the electron system to the sound wave is found to be similar in the FQHE states to that previously studied in the integral quantum Hall states. However, a striking disparity is observed at Landau-level filling =1/2, where sound propagation is distinctly different from that in the neighboring filling-factor range. We propose that phase separation of the 2DEG may be responsible for the feature at =1/2.