Possible metal-insulator transition at B=0 in two dimensions.

We have studied the zero magnetic field resistivity, ρ, of unique high-mobility two-dimensional electron systems in silicon. At very low electron density, n s (but higher than some sample-dependent critical value, n cr ∼ 10 11 cm −2), conventional weak localization is overpowered by a sharp drop of ρ by an order of magnitude with decreasing temperature below ∼ 1 − 2 K. No further evidence for electron localization is seen down to at least 20 mK. For n s < n cr , the sample is insulating. The resistance is empirically found to scale with temperature both below and above n cr with a single parameter which approaches zero at n s = n cr suggesting a metal/insulator phase transition.