Non-linear magnetotransport in microwave-illuminated two-dimensional   electron systems
  We study magnetoresistivity oscillations in a high-mobility two-dimensional electron system subject to both microwave and dc electric fields. First, we observe that the oscillation amplitude is a periodic function of the inverse magnetic field and is strongly suppressed at microwave frequencies near half-integers of the cyclotron frequency. Second, we obtain a complete set of conditions for the differential resistivity extrema and saddle points. These findings indicate the importance of scattering without microwave absorption and a special role played by microwave-induced scattering events antiparallel to the electric field. 
A.T. Hatke, H.-S. Chiang, M.A. Zudov, L.N. Pfeiffer, and K.W. West