EXCITONIC INSTABILITIES OF 2D ELECTRON-SYSTEMS IN QUANTUM HALL STATES

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