  Magnetoconductivity oscillations and absolute negative conductivity induced
by nonequilibrium populations of excited subbands in a degenerate multisubband
two-dimensional electron system are studied theoretically. The displacement
from equilibrium, which can be caused by resonant microwave excitation or by
any other reason, is assumed to be such that electron distributions can no
longer be described by a single Fermi level. In this case, in addition to the
well-known conductivity peaks occurring at the Shubnikov-de Haas conditions and
small peaks of normal intersubband scattering, sign-changing oscillations with
a different shape are shown to be possible. We found also that even a small
fraction of electrons transferred to the excited subband can lead to negative
conductivity effects.
