Response of the microwave-induced cyclotron harmonic resistance spike to an in-plane magnetic field

Microwave-induced resistance oscillations (MIROs) have been commonly observed in high-mobility GaAs/AlGaAs two-dimensional electron systems (2DESs) under microwave irradiation. In ultraclean GaAs/AlGaAs quantum wells, we have recently observed a very large resistance spike at the second harmonic of cyclotron resonance. In order to elucidate its origin, we have studied the response of microwave photoresistances in a two-axis magnetic field configuration, where the perpendicular (B _z) and the in-plane (B _x) components can be independently applied to the sample. The experiments reveal a distinctive response of the spike to the B _x compared with that of the MIROs. While the major MIRO peaks show an increasing phase shift toward a quarter period in increasing B _x, the spike position shows an essentially zero shift. This finding lends additional support for the notion that the spike is a unique effect in the microwave-driven 2DES.