Excitonic spin instabilities and continuous phase transitions in quantum Hall magnets

Inelastic light scattering from collective excitations has emerged as a powerful tool to explore novel ground states of quantum Hall liquids. We present here a short review of light scattering experiments in coupled electron bilayers in GaAs double quantum wells. At even integer filling factors the bilayers display soft collective excitation modes that are linked to phase transformations in the spin degree of freedom. In this work light scattering emerges as a method that offers novel venues for the study of fascinating quantum phase transitions of electrons in artificially layered semiconductors. The experiments reveal that excitonic terms of electron interactions play key roles and offer remarkable insights on finite temperature physics. We focus on studies of collective mode behaviour and on states of spin created by the interplay between Zeeman energies, tunnelling gaps and electron exchange interactions. (C) 2001 Published by Elsevier Science Ltd.