Contactless observation of microwave induced resistance oscillations in ZnO/Mg$_x$Zn$_{1-x}$O heterojunction

In high-quality ZnO/Mg$_x$Zn$_{1-x}$O heterojunctions, microwave-induced magnetoresistance oscillations have been investigated using a contactless technique. The basic detection principle relies on the measurements of a signal transmission in the radio frequency range ($f\sim 50$ MHz) between two T-shaped antennas, capacitively coupled to a two-dimensional electron system. When the sample is exposed to exciting microwave radiation with the frequency $F = 60$–$140$ GHz in low magnetic fields, at least three oscillations are well resolved in the high-frequency conductivity. The amplitude of the first oscillation is comparable in amplitude to Shubnikov–de Haas oscillations in strong magnetic fields. A significant advantage of this method is the absence of Ohmic contacts or metallization deposited on the sample surface, which provides additional information for understanding the origin of this phenomenon.