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

We report inelastic light scattering measurements of spin excitations on coupled electron bilayers with relatively large tunneling gaps at total filling factor ν_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.