OPTICAL SPECTROSCOPY OF NEUTRAL AND CHARGED EXCITONS IN GAAS ALGAAS QUANTUM-WELLS IN HIGH MAGNETIC-FIELDS/

We implement optical spectroscopy to study charged excitons in modulat ion-doped GaAs/AlGaAs quantum wells. We report the first observation o f the positively charged exciton and of the triplet state of the negat ively charged exciton. Applying a gate voltage at high magnetic fields , we investigate the transition between metallic and insulating states . We find that while the photoluminescence line of the metallic two-di mensional electron gas transforms smoothly into a negatively charged e xciton, the Zeeman splitting of this line exhibits an abrupt change at the metal-insulator transition.