Linking insulator-to-metal transitions at zero and finite magnetic fields

For many years, it was widely accepted(1) that electrons confined to two dimensions would adopt an insulating ground state at zero temperature and in zero magnetic field. Application of a strong perpendicular magnetic field changes this picture, resulting(2,3) in a transition from the insulating phase to a metallic quantum Hall state. Unexpectedly, an insulator-to-metal transition was recently observed(4) in high-quality two-dimensional systems at zero magnetic field on changing the charge carrier density. The mechanism underlying this transition remains unknown(5-9). Here we investigate the magnetic-field-driven transition in a two-dimensional gallium arsenide system, which also exhibits(10-12) the poorly understood zero-field transition. We find that, on increasing the carrier density, the critical magnetic field needed to produce an insulator-to-metal transition decreases continuously and becomes zero at the carrier density appropriate to the zero-field transition. Our results suggest that both the finite- and zero-magnetic field transitions share a common physical origin.