Far infrared magnetospectroscopic studies of negative donor ions (D-), and
donors in the presence of many excess electrons in high:magnetic fields in
GaAs/AlGaAs quantum wells are reviewed. Both singlet and triplet transition
s of well-center D--ions were observed and are in good agreement with recen
t theoretical calculations. For off-well-center D--ions evidence for a pred
icted magnetic-field-induced "unbinding" of the second electron was found.
In the presence of many excess electrons the D--singlet and -triplet transi
tions are blue-shifted substantially and evolve into bound magnetoplasmon e
xcitations. Cusps are observed at integral and fractional Landau-level fill
ing factors (v) in a plot of normalized blue-shift of the D--singlet-like b
ound magnetoplasmon transition vs, v. For v < 1, the singlet-like bound mag
netoplasmon transition continuously approaches the isolated D--singlet tran
sition with increasing magnetic field, while the triplet-like transition lo
ses strength, irrespective of the electron density. Exact diagonalization s
tudies of a donor ion with a few electrons in a parabolic lateral confining
potential show the importance of electron-electron interactions and locali
zation due to the long-range fluctuating potential in explaining this behav
ior. High pressure studies in a specially designed diamond anvil cell exhib
it a continuous evolution from bound magnetoplasmon transitions to isolated
D--transitions to neutral donor transitions in a single sample as the pres
sure is increased and the electron density in the wells is decreased.