Inelastic light scattering spectroscopy of collective spin excitations in low-dimensional semiconductors: Evidence for excitonic instabilities

The talk presents recent resonant inelastic light scattering experiments that succeeded in accessing these soft modes and investigate their evolution at ultra-low temperatures. The focus of the presentation is on the excitations of electron bilayers in quantum Hall states with total Landau level filling factors nuT = 2 and nuT = 1. The experiments probed soft spin and pseudo-spin (associated to changes in the layer-index degree of freedom) collective modes at both zero and finite wavevectors, when the states of the bilayers are in close proximity to transitions to broken symmetry states with interlayer many-body coherence.