Cyclotron-resonance linewidth oscillations in the integer and fractional quantum Hall regimes

The cyclotron-resonance linewidth was studied in GaAs/GaAlAs heterostructures as a function of filling factor, magnetic field, and frequency. Maxima in the cyclotron-resonance linewidth are found for integer and for fractional Landau-level fillings of (5/3, (4/3, and (2/3, giving evidence for a reduced screening due to density-of-states gaps. For partly filled levels a strong reduction of the linewidth due to resonant screening is found. The oscillations in the linewidth are highest for high-mobility and low-density samples.