Microwave photoresistance in a two-dimensional electron gas with   separated Landau levels
  Theories of microwave-induced resistance oscillations in high-mobility two-dimensional electron gas predict that with decreasing oscillation order $n$ or with increasing frequency $\omega$ the photoresistance maxima should appear closer to the cyclotron resonance harmonics due to increased Landau level separation. In this experimental study we demonstrate that while for a given $\omega$ the peaks do move towards the harmonics with decreasing $n$, there is no corresponding movement with increasing $\omega$ for a given $n$. These findings show that the positions of the photoresistance maxima cannot be directly linked to the Landau level separation challenging our current understanding of the phenomenon. 
A. T. Hatke, M. A. Zudov, L. N. Pfeiffer, and K. W. West
cond-mat.mes-hall