Nonlinear growth with the microwave intensity in radiation-induced magnetoresistance oscillations

We report the observation of inverse-magnetic-field-periodic, radiation-induced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs. the radiation power, $P$, in the two systems. We find that the radiation-induced oscillatory $\Delta R_{xx}$, in both materials, can be described by $\Delta R_{xx} = -A exp(-\lambda/B)sin(2 \pi F/B)$, where $A$ is the amplitude, $\lambda$ is the damping parameter, and $F$ is the oscillation frequency. Both $\lambda$ and $F$ turn out to be insensitive to $P$. On the other hand, $A$ grows nonlinearly with $P$.