  Inelastic light scattering measurements of low-lying collective excitations
of electron double layers in the quantum Hall state at total filling nu_T=1
reveal a deep magnetoroton in the dispersion of charge-density excitations
across the tunneling gap. The roton softens and sharpens markedly when the
phase boundary for transitions to highly correlated compressible states is
approached. The findings are interpreted with Hartree-Fock evaluations that
link soft magnetorotons to enhanced excitonic Coulomb interactions and to
quantum phase transitions in the ferromagnetic bilayers.
