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 ${\mathit{n}}_{\mathit{s}}$ (but higher than some sample-dependent critical value ${\mathit{n}}_{\mathrm{cr}}$∼${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$), 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 ${\mathit{n}}_{\mathit{s}}$<${\mathit{n}}_{\mathrm{cr}}$, the sample is insulating. The resistance is empirically found to scale with temperature both below and above ${\mathit{n}}_{\mathrm{cr}}$ with a single parameter that approaches zero at ${\mathit{n}}_{\mathit{s}}$=${\mathit{n}}_{\mathrm{cr}}$ suggesting a metal-insulator phase transition.