Precursors to Exciton Condensation in Quantum Hall Bilayers

Bilayer two-dimensional electron systems at high magnetic field exhibit an interlayer coherent excitonic phase when the layer separation is sufficiently small, the temperature sufficiently low, and the total electron density in the bilayer equals the degeneracy of a single Landau level. This phase is well known to display exotic transport phenomena, notably Josephson-like interlayer tunneling and essentially dissipationless transport of excitons across the bulk of the 2D system. While first detected twenty years ago in GaAs-based double quantum wells, this and related interlayer correlated phases have recently been observed in graphene-based multilayer systems.

In spite of this long history, the nature of the quantum phase transition between the incoherent phase at large layer separation and the coherent excitonic phase at small separations remains poorly understood. In this talk I will report on recent experiments which shed new light on the transition. In particular, I will discuss tunneling spectroscopy measurements which reveal evidence for interlayer electron-hole correlations at layer separations near, but above, the transition to the exciton condensate at total Landau level filling ν_T=1. These correlations are manifested by a nonlinear suppression of the Coulomb pseudogap which inhibits low energy interlayer tunneling in weakly coupled bilayers. The pseudogap suppression is strongest at ν_T=1 and grows rapidly as the critical layer separation for exciton condensation is approached from above. These and other observations suggest that electron-hole pairing fluctuations exist in the incoherent phase well above the critical layer separation, in a manner reminiscent of Cooper pair fluctuations above the superconducting critical temperature.