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 $\nu <2\mathrm{}$ the $X^{-}$-like transitions are blueshifted; they are absent for $\nu >2\mathrm{}$. The blueshifted transitions are explained in terms of a new type of collective excitation—magnetoplasmons bound to a mobile valence band hole.