Bichromatic Microwave Photoresistance of Two-Dimensional Electron System
  We explore experimentally bichromatic (frequencies $\omega_1$ and $\omega _2$) photoresistance of a two-dimensional electron system in the regimes of microwave-induced resistance oscillations and zero-resistance states. We find bichromatic resistance to be well described by a superposition of $\omega_1$ and $\omega_2$ components, provided that both monochromatic resistances are positive. This relation holds even when the oscillation amplitudes are small and one could expect additive contributions from monochromatic photoresistances. In contrast, whenever a zero-resistance state is formed by one of the frequencies, such superposition relation breaks down and the bichromatic resistance is strongly suppressed. 
M.A. Zudov, R.R. Du, L.N. Pfeiffer, and K.W. West