Many-body integer quantum Hall effect: Evidence for new phase transitions

The =1 quantum Hall effect in bilayer 2D electron systems is shown to continuously evolve from a regime dominated by single-particle tunneling into one where interlayer Coulomb interactions stabilize the state. This many-body integer quantum Hall state exhibits a phase transition to a compressible state at large layer separation. We also find evidence for an intriguing and unexpected second transition to a new incompressible state, driven by an in-plane magnetic field. While the origin of this last result is unclear, we discuss a recent model of a pseudospin textural phase transition.