We study microwave photoresistivity oscillations in a high mobility
two-dimensional electron system subject to strong dc electric fields. We find
that near the second subharmonic of the cyclotron resonance the frequency of
the resistivity oscillations with dc electric field is twice the frequency of
the oscillations at the cyclotron resonance, its harmonics, or in the absence
of microwave radiation. This observation is discussed in terms of the
microwave-induced sidebands in the density of states and the interplay between
different scattering processes in the separated Landau level regime.Comment: 4 pages, 4 figure