Multiple and virtual photon processes in radiation-induced   magnetoresistance oscillations in two-dimensional electron systems
  Recently discovered new structures and zero-resistance states outside the well-known oscillations are demonstrated to arise from multiphoton assisted processes, by a detailed analysis of microwave photoresistance in two-dimensional electron systems under enhanced radiation. The concomitant resistance dropping and peak narrowing observed in the experiments are also reproduced. We show that the radiation-induced suppression of average resistance comes from virtual photon effect and exists throughout the whole magnetic field range. 
X.L. Lei and S.Y. Liu