Re-entrant insulator-metal-insulator transition at B=0 in a two dimensional hole gas

We report the observation of a re-entrant insulator--metal--insulator transition at B=0 in a two dimensional (2D) hole gas in GaAs at temperatures down to 30mK. At the lowest carrier densities the holes are strongly localised. As the carrier density is increased a metallic phase forms, with a clear transition at \sigma = ~5e^2/h. Further increasing the density weakens the metallic behaviour, and eventually leads to the formation of a second insulating state for \sigma > ~50e^2/h. In the limit of high carrier densities, where k_F.l is large and r_s is small, we thus recover the results of previous work on weakly interacting systems showing the absence of a metallic state in 2D.