Metal-insulator transition at B=0 in an ultra-low density two-dimensional hole gas

We have observed a metal-insulator transition in an ultra-low density two-dimensional hole gas formed in a high-quality GaAs-AlGaAs heterostructure at B = 0. At the highest carrier density studied (p(s) = 2.2 x 10(10) cm(-2), r(s) = 16)the hole gas is strongly metallic, with an exceptional mobility of 425000 cm(2) V-1 s(-1). The low disorder and strength of the many-body interactions in this sample are highlighted by the observation of re-entrant metal-insulator transitions in both the fractional(v < 1\3) and integer (2 > v > 1) quantum Hall regimes. On reducing the carrier density the temperature and electric field dependence of the resistivity show that the sample is still metallic at p(s) = 1.3 x 10(10) cm(-2) (r(s) = 21), becoming insulating at p(s) similar or equal to 1 x 10(10) cm(-2). Our results indicate that electron-electron interactions are dominant at these low densities, pointing to the many-body origins of this metal-insulator transition. We note that the value of r(s) at the transition (r(s) = 23 +/- 2) is large enough to allow the formation of a weakly pinned Wigner crystal, and approaches the value calculated for the condensation of a pure Wigner crystal. (C) 1998 Elsevier Science B.V. All rights reserved.