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.

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We investigate the transport properties of insulating phases in the 2D electron system of highmobility A16aAs/GaAs heterostructures of Corbino geometry at very low temperatures.We find that the nonlinear current-voltage characteristics for insulating phases in the integer and fractional quantum Hall regime and for a low-density insulating phase are very similar. The behavior of these characteristics with changing temperature and filling factor unambiguously points to the percolation metal-insulator transition as the cause for all insulating phases investigated.We propose a metalinsulator phase diagram in the (B,N, ) plane based on our experimental data. PACS numbers: 71.30.+h, 73.20.Dx, 73.40.Kp The metal-insulator transition at low filling factors in a 2D electron system of high-mobility A1GaAs/GaAs heterostructures has been investigated in a number of studies (see, e. g., [1 -5]) by using various experimental techniques. In the majority of the reports the transition into an insulating phase is attributed to the formation of a pinned Wigner crystal. However, some doubts in this interpretation were expressed, e.g. , in Ref. [6]. Optical investigations strongly suggest that in high magnetic fields the 2D electron system becomes strongly inhomogeneous; in photoluminescence spectra there coexist two lines, one of which is caused by radiative recombination of 2D electrons from metallic regions [4,7,8] and the other proves the existence of insulating islands in the electron system. The conduction of an inhomogeneous macroscopic system is determined by the coverage of the sample with metallic and insulating areas, respectively. In this case the metalinsulator transition must be discussed as a percolation problem. Here we introduce a method based on investigations of the current-voltage characteristics which makes it possible to compare the transport properties of insulating phases realized in the integer and fractional quantum Hall regime and at low electron densities in A1GaAs/GaAs heterostructure samples. Our experimental results provide strong evidence for the percolation nature of all metalinsulator transitions studied.Let us first consider the definition of insulating phases in a 2D system. As known, in the integer and fractional quantum Hall regime the Hall conductivity o-Y is quantized and the dissipative conductivity cr " tends to zero at low temperatures.The Hall conductivity is finite due to the existence of extended states below the Fermi level that are able to carry dissipationless Hall current, as has been shown in experiments on charge transfer in Corbino samples [9]. It is reasonable to call such a state of the 2D electron system an insulator because the Fermi level lies in localized states and electron transport in the direction of the electric field is absent. Obviously, a so-defined insulating phase can be characterized by the value of the Hall conductivity. The insulating phase at low electron densities corresponds to o. "Y = 0 since in this case the extended states below the Fermi level are not ...

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