Zero-magnetic-field collective insulator phase in a dilute 2D electron system

We report on a collective low temperature metal-insulator transition which develops in a dilute 2D electron system in Si at zero magnetic field, below a critical carrier density nsc{}10¹¹ cm^-2. In the insulator phase, the dc conduction is thermally activated and exhibits a sharp threshold as a function of electric field. The collective insulator state at zero field shows many of the features attributed to the pinned Wigner solid. We have also observed a trend from a collective to a single-particle insulator state with decreasing electron density and/or increasing disorder.