  We report on theoretical studies of a recently discovered strong
microwave-induced magnetoresistance peak obtained in ultra-clean
two-dimensional electron systems at low temperatures. The most striking feature
of such a peak is that it shows up on the second harmonic of the cyclotron
resonance and with an amplitude that can reach an order of magnitude larger
than the microwave-induced resistance oscillations. We apply the
microwave-driven electron orbits model in the ultra-clean scenario.
Accordingly, we calculate the elastic scattering rate (charged impurity) which
will define the unexpected resonance peak position. We also obtain the
inelastic scattering rate (phonon damping), that will be responsible of the
large peak amplitude. Calculated results are in good agreement with
experiments. These results would be of special interest in nanophotonics; they
could lead to the design of novel ultrasensitive microwave detectors.
