Magnetoresistance oscillations in two-dimensional electron systems induced by both AC and DC fields*

We report on magnetotransport measurements in a high-mobility two-dimentional electron system subject simultaneously to AC (microwave) and DC (Hall) fields. We find that DC excitation affects microwave photoresistance in a nontrivial way. Photoresistance maxima (minima) evolve into minima (maxima) and back, with some new minima appearing as zero-resistance states. Most of our observations are explained in terms of indirect electron transitions using a new, ``combined'' resonant condition. Strong coupling and interplay of AC- and DC-induced effects call for a theory treating both excitation types within a single framework.