Charged magnetoexcitons in two dimensions: Isolated X^- and many-electron effects

The states of charged magnetoexcitons in two-dimensional systems are considered. Exact optical selection rules for intra-and inter-band processes are discussed. The effect of excess electrons on internal transitions of negatively charged excitons X in quantum wells is studied experimentally and theoretically. An experimentally observed blue-shift with excess electron density is explained in terms of collective excitations, magnetoplasmons bound to a valence band hole.