  We performed effective mass measurements employing microwave-induced
resistance oscillation in a tunable-density GaAs/AlGaAs quantum well. Our main
result is a clear observation of an effective mass increase with decreasing
density, in general agreement with earlier studies which investigated the
density dependence of the effective mass employing Shubnikov- de Haas
oscillations. This finding provides further evidence that microwave-induced
resistance oscillations are sensitive to electron-electron interactions and
offer a convenient and accurate way to obtain the effective mass.
