Spectroscopic determination of the order parameter of coupled bilayers at νT=1

We report inelastic light scattering measurements of spin excitations on coupled electron bilayers with relatively large tunneling gaps at total filling factor nu(T) = 1. We show that the pseudospin polarization order parameter, where the pseudospin labels the occupation of symmetric and antisymmetric levels, can be determined from the energy of long wavelength spin excitations. Our experiments indicate that the order parameter in the quantum Hall ground state collapses at the incompressible-compressible phase transition. The latter is driven by decreasing the tunneling gap through the application of an in-plane magnetic field. (c) 2006 Elsevier B.V. All rights reserved.