Microwave induced magnetoresistance oscillations at the subharmonics of   the cyclotron resonance
  The magnetoresistance oscillations, which occur in a two-dimensional electron system exposed to strong microwave radiation when the microwave frequency $\omega$ coincides with the n-th subharmonic of the cyclotron frequency $\omega_c$ have been investigated for n = 2, 3 and 4. It is shown that these subharmonic features can be explained within a non-equilibrium energy distribution function picture without invoking multi-photon absorption processes. The existence of a frequency threshold above which such oscillations disappear lends further support to this explanation. 
S. I. Dorozhkin, J. H. Smet, K. von Klitzing, L. N. Pfeiffer, and K.   W. West