Inelastic light scattering spectroscopy of collective spin excitations inlow-dimensional semiconductors: evidence for excitonic instabilities

摘要

The study of electronic spin excitations offers venues for the understanding of fundamental many-body phenomena, quantum phase transitions and broken-symmetry electronic phases in artificial semiconductor low-dimensional systems. In two-dimensional electron gases in a magnetic field, in particular, such collective modes are built from electron-hole pairs (magneto-excitons) excited among quantized Landau levels. Minima in the dispersion of these modes at zero or finite in-plane wavevectors can be induced by excitonic interaction terms between the pairs. These excitonic contributions can induce instabilities that trigger new ground states with peculiar magnetic properties and/or ordering. This is particularly relevant for spin excitation across the tunneling gap in coupled bilayers confined in double quantum wells. Transformations associated to such tunneling instabilities are intriguing and can lead to the creation of a condensate of magneto-excitons. This sets the motivation of a large body of current experimental and theoretical efforts that mainly concentrate on the magneto-transport manifestations of such phenomena. Observation of such soft and unstable spin modes, however, would allow probing directly the impact of manybody excitonic interactions and represents a major challenge of current and future research in this field.