Multiphoton processes in microwave photoresistance of two-dimensional electron systems

We extend our studies of microwave photoresistance of ultrahigh mobility two-dimensional electron systems (2DES) into the high-intensity, nonlinear regime employing both monochromatic and bichromatic radiation. Under high-intensity monochromatic radiation omega we observe zero-resistance states (ZRSs) that correspond to the rational values of epsilon=omega/omega(C) (omega(C) is the cyclotron frequency) and can be associated with multiphoton processes. Under bichromatic radiation omega(1),omega(2) we discover a resistance minimum, possibly a precursor of bichromatic ZRSs, which seems to originate from a frequency mixing process, omega(1)+omega(2). These findings indicate that multiphoton processes play important roles in the physics of nonequilibrium transport of microwave-driven 2DES, and suggest new directions for theoretical and experimental studies.