Quantum oscillations in a two-dimensional electron system under low-frequency microwave irradiation

We study the magnetoresistance of an ultrahigh mobility GaAs/AlGaAs two-dimensional electron system (2DES) in a weak magnetic field under low-frequency (f<20 GHz) microwave (MW) irradiation. We observe that, with decreasing MW frequency, microwave induced resistance oscillations (MIRO) are damped and multiphoton processes become dominant. At very low MW frequency (f<4 GHz), MIRO disappear gradually and an SdH-like oscillation develops. Our analysis indicates that the oscillation may originate from alternating Hall-field induced resistance oscillations (ac-HIRO), due to the transition from the elastic scattering in real space. On the other hand, from the view of photon energy, the oscillations can be understood as a multiphoton process of MIRO in the low MW frequency limit. We show that the two different nonequilibrium mechanisms of MIRO and HIRO can be unified under low-frequency MW irradiation.