REENTRANT INSULATING PHASE IN SI INVERSION-LAYERS IN LOW MAGNETIC-FIELDS

We have studied the transport properties of an insulating phase confined between metallic states with integer-quantized Hall resistance in a dilute two-dimensional electron system in high-mobility Si metal-oxide-semiconductor field-effect transistors at temperatures below 0.5 K and at low magnetic fields H < 5 T. The longitudinal resistance was found to be thermally activated; the activation energy reaches its maximum value of approximately 0.5 K at half-integer filling factors nu=1.5 and 2.5, diminishes near nu=1 and 2, and vanishes at a critical carrier concentration almost-equal-to 10(11) cm-2. Current-voltage characteristics shaw a sharp threshold at E(t) almost-equal-to 2 - 100 mV/cm. This threshold field decreases near filling factors nu=1 and 2, and vanishes near the same critical electron concentration. We attribute this phase to the pinned electron solid.