We perform a direct study of the magnitude of the anomalous splitting in the cyclotron resonance (CR) of a two-dimensional (2D) electron system as a function of sample disorder. In a series of $\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ 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 nonzero wave vector rotonlike minimum in the dispersion of 2D magnetoplasmons comes into resonance with the CR, with the two modes being coupled via disorder.

• Received 28 February 2006

DOI:https://doi.org/10.1103/PhysRevB.73.241309