Internal transitions of negatively charged magnetoexcitons and many body effects in a two-dimensional electron gas

Internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons (X-) and their evolution with excess electron density have been studied. in GaAs/AlGaAs quantum wells. In the dilute electron limit, due to magnetic translational invariance, the optically detected resonance spectra are dominated by bound-to-continuum bands in contrast to the negatively charged donor system D-, which exhibits strictly bound-to-bound transitions. With increasing excess electron density Landau-level filling factors nu < 2 the X--like transitions are blueshifted; they are absent for nu > 2. The blueshifted transitions are explained in terms of a new type of collective excitation-magnetoplasmons bound to a mobile valence band hole.