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 (View the MathML source), 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 View the MathML source, allowing us to track the extended states down to very low magnetic fields (down to View the MathML source). Our result suggests that as B→0 the extended states float up in energy as predicted by the scaling theory of localisation.