  We have investigated the magnetophonon resonance (MPR) effect in a series of
single GaAs quantum well samples which are symmetrically modulation doped in
the adjacent short period AlAs/GaAs superlattices. Two distinct MPR series are
observed originating from the $\Gamma$ and X electrons interacting with the
GaAs and AlAs longitudinal optic (LO) phonons respectively. This confirms
unequivocally the presence of X electrons in the AlAs quantum well of the
superlattice previously invoked to explain the high electron mobility in these
structures (Friedland et al. Phys. Rev. Lett. 77,4616 (1996).
