Microwave photoresponse in the 2D electron system caused by intra-Landau   level transitions
  The influence of microwave radiation on the DC-magnetoresistance of 2D-electrons is studied in the regime beyond the recently discovered zero resistance states when the cyclotron frequency exceeds the radiation frequency. Radiation below 30 GHz causes a strong suppression of the resistance over a wide magnetic field range, whereas higher frequencies produce a non-monotonic behavior in the damping of the Shubnikov-de Haas oscillations. These observations are explained by the creation of a non-equilibrium electron distribution function by microwave induced intra-Landau level transitions. 
S. I. Dorozhkin, J. H. Smet, V. Umansky, K. von Klitzing