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), $\ensuremath{\delta}$ doped with Si donors in both well and barrier centers. With increasing excess electron densities in the QWs the negative donor ion $({D}^{\ensuremath{-}})$ 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 ${D}^{\ensuremath{-}}$ 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.