Surface Acoustic Waves:

a probe of the frequency and length scale dependent conductivity

Lisa Tracy

Surface acoustic waves (SAW's) are elastic waves which propagate along the surface of an elastic body, with most of the energy density confined to a depth of about one wavelength below the surface. Taking advantage of the piezoelectric interaction in GaAs, SAW's can be used to measure the frequency and length scale dependent conductivity of a 2D electron gas in a GaAs/AlGaAs heterostructure.

Photo of SAW transducer on GaAs (finger width ~ 10 microns)

Shown above is a SAW transducer consisting of interdigitated metal fingers. Applying an electric field between the fingers at microwave frequencies launches a SAW across the sample. After passing through a region with a 2DEG, the SAW is recieved by a second transducer. The amplitude and velocity of the recieved SAW are related to the conductivity of the 2DEG.

One application of SAW's is to observe the enhanced conductivity of a 2D electron gas near half-filling of the lowest Landau level where the electron gas may be thought of as a collection of "composite fermions". This enhanced conductivity shows up as a minimum in the SAW velocity shift when the wavelength of the SAW is less than the composite fermion mean free path. The image below shows the effect as observed in our laboratory. The half-filling point occurs near B = 7 T in this sample, where a minimum in the velocity shift develops at high frequency.