  We perform a direct study of the magnitude of the anomalous splitting in the
cyclotron resonance (CR) of a two-dimensional electron system (2DES) as a
function of sample disorder. In a series of AlGaAs/GaAs quantum wells,
identical except for a range of carbon doping in the well, we find the CR
splitting to vanish at high sample mobilities but to increase dramatically with
increasing impurity density and electron scattering rates. This observation
lends strong support to the conjecture that the non-zero wavevector, roton-like
minimum in the dispersion of 2D magnetoplasmons comes into resonance with the
CR, with the two modes being coupled via disorder.
