Microwave Photoresistance in dc-driven 2D Systems at Cyclotron Resonance   Subharmonics
  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. 
A. T. Hatke, H.-S. Chiang, M. A. Zudov, L. N. Pfeiffer, K. W. West