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 ∼0.5 K at half-integer filling factors ν=1.5 and 2.5, diminishes near ν=1 and 2, and vanishes at a critical carrier concentration ≊${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. Current-voltage characteristics show a sharp threshold at ${\mathit{E}}_{\mathit{t}}$≊2-100 mV/cm. This threshold field decreases near filling factors ν=1 and 2, and vanishes near the same critical electron concentration. We attribute this phase to the pinned electron solid.