Effect of density on the amplitude of microwave-induced resistance oscillations*

We have measured microwave-induced resistance oscillations (MIRO) in a tunable-density 30 nm-wide GaAs/AlGaAs quantum well and observed that the MIRO amplitude increases dramatically with carrier density. The analysis shows that the increase in the effective microwave power and quantum lifetime with density are not sufficient to explain the observed growth of the oscillation amplitude. We further found that the fundamental MIRO extrema move towards cyclotron resonance with increasing density, in contrast to theoretical predictions. These unexpected findings reveal that the density dependence is not captured by existing theory.