Microwave photoresistance in a two-dimensional electron gas with separated Landau levels

Theories of microwave-induced resistance oscillations in high-mobility two-dimensional electron gas predict that with decreasing oscillation order $n$ or with increasing frequency $\omega$ the photoresistance maxima should appear closer to the cyclotron resonance harmonics due to increased Landau level separation. In this experimental study we demonstrate that while for a given $\omega$ the peaks do move towards the harmonics with decreasing $n$, there is no corresponding movement with increasing $\omega$ for a given $n$. These findings show that the positions of the photoresistance maxima cannot be directly linked to the Landau level separation challenging our current understanding of the phenomenon.