Magnetic-field-induced unbinding of the off-well-center $D^{-}$ singlet state in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ multiple quantum wells

Results of far-infrared Fourier-transform magnetotransmission from two ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ multiple quantum well samples δ-doped with Si-donor on- and off-well centers are compared. In contrast to well-center $D^{-}$ ions, the off-well-center $D^{-}$ singlet binding energy decreases with increasing magnetic field between 5.5 and 15 T, as reflected by the decreasing strength of the $D^{-}$ singlet transition and a consequent increase of the neutral donor ${1s-2p}^{+}$ transition. This observation represents a verification of the predicted magnetic-field-induced unbinding (magnetic evaporation) of shallow impurity states for the off-well-center $D^{-}$-ion system.