  We report on theoretical studies of a recently discovered strong
radiation-induced magnetoresistance spike obtained in ultraclean
two-dimensional electron systems at low temperatures. The most striking feature
of this spike 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 radiation-induced resistance oscillations. We apply the
radiation-driven electron orbits model in the ultraclean scenario. Accordingly,
we calculate the elastic scattering rate (charged impurity) which will define
the unexpected resonance spike position. We also obtain the inelastic
scattering rate (phonon damping), that will be responsible of the large spike
amplitude. We present a microscopical model to explain the dependence of the
Landau level width on the magnetic field for ultraclean samples. We find that
this dependence explains the experimental shift of the resistance oscillations
with respect to the magnetic field found in this kind of samples. We study also
recent results on the influence of an in-plane magnetic field on the spike. We
are able to reconcile the obtained different experimental response of both
spike and resistance oscillations versus an increasing in-plane field. The same
model on the variation of the LL width, allows us to explain such surprising
results based in the increasing disorder in the sample caused by the in-planed
magnetic field. 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.
