Studies of Zero-resistance States by Dichromatic Microwaves

$^1$Dept. of Physics, University of Utah, Salt Lake City, UT 84112, $^2$School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, $^3$Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 --- We have explored experimentally dichromatic (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 have found that dichromatic resistance is closely replicated by a linear superposition of $\omega _1$ and $\omega _2$ components, provided that both monochromatic resistances are positive. In contrast, if a zero-resistance state is to be formed by one of the frequencies, such superposition relation becomes invalid. More specifically, dichromatic resistance is suppressed in this regime. This finding can be explained by taking into account the absolute negative resistance and the formation of domains, as suggested by current theoretical models.