  We report on microwave-induced resistance oscillations (MIROs) in a
tunable-density 30-nm-wide GaAs/AlGaAs quantum well. We find that the MIRO
amplitude increases dramatically with carrier density. Our analysis shows that
the anticipated increase in the effective microwave power and quantum lifetime
with density is not sufficient to explain the observed growth of the amplitude.
We further observe that the fundamental oscillation extrema move towards
cyclotron resonance with increasing density, which also contradicts theoretical
predictions. These findings reveal that the density dependence is not properly
captured by existing theories, calling for further studies.

Mesoscale and Nanoscale Physics