Donor impurities as a probe of electron correlations in a two-dimensional electron gas in high magnetic fields

Many-electron effects have been observed in far-infrared magnetospectroscopic studies of GaAs/AlGaAs multiple quantum wells (QWs), delta doped with Si donors in both well and barrier centers. With increasing excess electron densities in the QWs the negative donor ion (D-) singlet and triplet transitions are substantially blueshifted, exhibiting cusps at integer and fractional filling factors. At high magnetic fields the many-electron system appears to approach a collection of isolated two-electron D- ions. Exact diagonalization studies of donor and confined electrons show the importance of electron correlations and localization at high magnetic fields in understanding this behavior.