Metal-insulator transition at B=0 in an ultra-low density ($r_{s}=23$) two dimensional GaAs/AlGaAs 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 ($p_{s}=2.2x10^{10} cm^{-2}, r_{s}=16$) the hole gas is strongly metallic, with an exceptional mobility of $425,000 cm^{2}V^{-1}s^{-1}$. 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 ($\nu < 1/3$) and integer ($2 > \nu > 1$) 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 $p_{s}=1.3x10^{10} cm^{-2}$ ($r_{s}=21$), becoming insulating at $p_{s}{\simeq}1x10^{10} cm^{-2}$. 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 is approaching the value calculated for the condensation of a pure Wigner crystal.