Saw attenuation by the localised states of a 2D carrier system in a magnetic field

The surface acoustic wave attenuation and dispersion have been measured as a function of magnetic field for both a 2D electron system and a 2D hole sy stem at frequencies between 100 and 1100 MHz. The experiments were made at 350 mK in the quantum Hall regime. In the region of filling factor one wher e the quasi-DC conductivity becomes very small, an attenuation and dispersi on is found which we attribute to localised carrier states. We estimate tha t the relaxation time of the localised electrons is about 60 ps while the r elaxation time for localised holes is about 20 ps. (C) 1999 Elsevier Scienc e B.V. All rights reserved.