Many-body interactions, the quantum Hall effect, and insulating phases in bilayer two-dimensional hole-gas systems

We have studied the quantum Hall effect in a high mobility double-layer 2D hole gas in which we are able to control the carrier density in the two layers independently. We find that both the v = 1 and the v = 3/2 bilayer quantum Hall (QH) states are destroyed with increasing carrier density. The collapse of v = 1 is extremely abrupt, occurring at a carrier density that corresponds to an inter-layer separation of 1.8l(B). However, although the v = 3/2 QH state disappears without trace, the zero in rho(xx) at v = 1 is replaced by a broad minimum similar to that observed at v = 1/2 when only one layer is occupied. This suggests that we are observing a transition from a correlated incompressible QH state with total filling factor v = 1 to a compressible state consisting of two (possibly uncorrelated) layers of 'composite fermions'. At very low densities and filling factors below v = 1, an insulating phase is observed, which is not present when only one layer is occupied. We demonstrate that this state is entirely due to inter-layer interactions and is strong evidence for the existence of a bilayer Wigner crystal. (C) 1998 Elsevier Science B.V. All rights reserved.