Magnetophonon resonance in high density, high mobility quantum well   systems
  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). 
C. Faugeras, D. K. Maude, G. Martinez, L. B. Rigal, C. Proust, K.-J.   Friedland, R. Hey and K. H. Ploog