Excitonic instabilities of 2D electron systems in quantum hall states

We present a short summary of recent resonant inelastic light scattering research that revealed remarkable softenings of long wavelength intersubband spin-excitations of electron double-layers in even integer quantum Hall states. The mode softenings arise from enhanced exchange vertex-corrections, or excitonic bindings, in the quantum Hall states. The collapse of spin-excitations could be associated with a quantum phase transition to a broken symmetry phase in which the order parameter involves spin correlations between the layers.