Surface Acoustic Wave Propagation and Inhomogeneities in Low Density   Two-Dimensional Electron Systems Near the Metal-Insulator Transition
  We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime ($<$ $10^{10}$ cm$^{-2}$) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES which become very important at low density. This has implications for the putative metal-insulator transition in two dimensions. 
L.A. Tracy, J.P. Eisenstein, M.P. Lilly, L.N. Pfeiffer, and K.W. West