  Transient magnetotransport of two-dimensional electrons with
partially-inverted distribution excited by an ultrashort optical pulse is
studied theoretically. The time-dependent photoconductivity is calculated for
GaAs-based quantum wells by taking into account the relaxation of electron
distribution caused by non-elastic electron-phonon interaction and the
retardation of the response due to momentum relaxation and due to a finite
capacitance of the sample. We predict large-amplitude transient oscillations of
the current density and Hall field (Hall oscillations) with frequencies
corresponding to magnetoplasmon range, which are initiated by the instability
owing to the absolute negative conductivity effect.
