  Recent experiments on microwave-irradiated high-mobility two-dimensional
electron systems revealed a novel photoresistivity peak in the vicinity of the
second cyclotron resonance harmonic. Here we report on the nonlinear transport
measurements and demonstrate that the peak can be induced by modest dc fields
and that its position is not affected even by strong dc fields, in contrast to
microwave-induced resistance oscillations that shift to higher magnetic fields.
These findings reinforce the notion that the peak cannot be described by
existing models and provides important constraints for further theoretical
considerations.
