Resistivity of dilute 2D electrons in an undoped GaAs heterostructure
  We report resistivity measurements from 0.03 K to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, $0.16 \times 10^{10} {cm}^{-2}$ to $7.5 \times 10^{10} {cm}^{-2}$, are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This non-monotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured. 
M. P. Lilly, J. L. Reno, J. A. Simmons, I. B. Spielman, J. P.   Eisenstein, L. N. Pfeiffer, K. W. West, E. H. Hwang, S. Das ...