Surface acoustic wave attenuation by localized electrons in a 2DEG at a GaAs/AlGaAs heterojunction

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Abstract
The surface acoustic wave (SAW) transmission has been measured at 4.2 K, as a function of magnetic field up to 5 T, for a GaAs wafer on which a heterojunction has been grown by MBE. Frequencies up to 1920 MHz were generated by interdigital SAW transducers deposited directly on the GaAs surface. At all frequencies, oscillations in the SAW attenuation, analogous to the Shubnikov-de Haas oscillations in sigma xx, can be observed due to piezoelectric coupling to the non-localized electrons of the two-dimensional electron gas (2DEG) of the heterojunction. The authors find, at frequencies of about 900 MHz and above, an additional attenuation at all magnetic fields, which increases with magnetic field. At half-integer filling factor, the increase in attenuation is initially linear with increase of field and then, at higher fields, the attenuation increases as the square root of the field. They interpret this as due to deformation potential coupling to the localized electrons of the 2DEG. A small SAW attenuation is observed at zero magnetic field which may be due to a very low density of three-dimensional electrons in the GaAs buffer layer or remaining in the doped AlGaAs layer.