Evolution of the line shape of radiation-induced magnetoresistance oscillations under bi-chromatic microwave excitation

We examine the evolution of the line shape of radiation-induced magnetoresistance oscillations under bi-chromatic microwave excitation as the microwave power level at one frequency is held fixed and the power level at the other frequency is varied. In order to overcome signal to noise issues in the limit of small oscillations at very low magnetic fields, we applied a double modulation, double lock-in technique where the magnetic field was modulated sinusoidally while applying a small ac-current to the sample. With this approach, it became possible to measure the magnetic field derivative, dR_xx /dB, of the diagonal resistance, and realize more oscillations, especially in the low $B$-field limit. Thus, while using this double lock in measurement technique, the sample was bi-chromatically excited at various bichromatic microwave frequency combinations. Half cycle plots of dR_xx/dB reveal that the bichromatic response magneto-transport response follows mostly low frequency response at low magnetic field region and starts following the high frequency response at high magnetic field regions. Here we present the results to address the question of whether or not superposition is followed in the bichromatic microwave magnetoresistance response.