Quantum phase transition of a quantum hall excitonic state in electron bilayers

We review recent inelastic light scattering results that shed light on the interplay between incompressible and compressible quantum phases of electron bilayers at total filling factor ν_T = 1. Our analysis and previous data suggests a first-order transition and phase coexistence of incompressible and compressible regions close to the phase boundary. We focus here on the origin of phase coexistence and argue that density fluctuations due to residual disorder in the quantum Hall regime, previously invoked as source of compressible puddles, play a marginal role. The sharpness of the phase boundary in the presence of inhomogeneous broadening and finite temperature effects remains to be explained.