  We show that the existing theory does not give correct in-plane spectrum of
superlattices at small in-plane momentum. Magneto-absorption experiments
demonstrate that the energy range of the parabolic region of the spectrum near
the electron subband bottom is by the order of magnitude lower than the value
predicted by the traditional approach. We developed a modified theory according
to which the energy range of the parabolic region and carrier in-plane
effective masses are determined by the effective bandgap of the superlattice
rather than by the bulk bandgaps of the superlattice layers. The results of the
new theory are consistent with the experiment.
