Response of the Cyclotron Harmonic Spike to an In-Plane Magnetic Field

Microwave-induced resistance oscillations (MIRO) have been commonly observed in high- mobility GaAs/AlGaAs two-dimensional electrons systems (2DES) under microwave irradiations. In ultraclean GaAs/AlGaAs quantum wells (mobility $\sim 3.0\times 10^7cm^2/Vs)$ we have recently observed an extraordinary 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$_{perp})$ and the in-plane (B$_{//})$ components can be independently applied to the sample. The experiments reveal a distinctive response of the spike to the B$_{//}$ as compared with that of the MIRO. While the major MIRO peaks show an increasing phase-shift towards 0.25 in increasing B$_{//}$, the spike position shows an essentially zero shift. This finding lends additional support for the notion that the spike is a new effect in the microwave-driven 2DES.