INSULATING PHASES IN A 2-DIMENSIONAL ELECTRON-SYSTEM OF HIGH-MOBILITY SI MOSFETS

We have investigated the transport properties of insulating phases in Si MOSFET's at extremely low temperatures. It has been found that insulating phases in the quantum Hall regime behave in a way that is similar to a low-density insulating phase: for all phases we find similar behavior of both the activation energy for the resistance in the linear part of the current-voltage characteristics and the critical electric field corresponding to the onset of nonlinearity. A characteristic length extracted from measurements of the activation energy and critical electric field has been found to diverge near each metal-insulator phase boundary. These results support the localization and reject the Wigner crystal as the origin for any insulating phase. Nonlinear current-voltage characteristics of the insulating phases can be explained by electric-field-induced electron delocalization. We have obtained the critical index for the localization length s almost-equal-to 1, which is close to the value s = 1.3 for classical percolation. We have tested that the effect of temperature can also be treated in terms of delocalization so that the temperature dependence of the width of peaks in the conductivity is explained by the thermal shift of the effective mobility edge of a Landau level. The experimental results point out the existence of the mobility edge in two-dimensional systems in zero magnetic field, in contrast with the predictions of scaling theory.