The fate of quantum Hall extended states as B -> 0 and the possibility of a 2D metal

We report the study of two-dimensional `metallic' behaviour at B = 0 in ultra-high-quality n-GaAs (mu(peak) > 10(7) cm(2) V-1 s(-1)). and in particular address the question of what happens to the quantum Hall extended states at the centre of Landau levels as B --> 0. In these high-quality electron samples we can reach extremely low carrier densities (2 x 10(9) cm(-2)), allowing us to track the extended states down to very low magnetic fields (down to similar to0.05 T). Our result suggests that as B --> 0 the extended states float up in energy as predicted by the scaling theory of localisation. (C) 2002 Elsevier Science B.V. All rights reserved.