Anomalies in the cyclotron resonance in high-mobility GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions

Frequency-dependent cyclotron-resonance measurements are reported on a series of GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions with some of the highest mobilities achieved. Both the effective mass and resonance linewidth show anomalies which appear to be related to a level-filling factor of ν=$n_s$h/eB close to 2. A modified Kramers-Kronig analysis demonstrates that the two are directly linked and have a common origin. At high fields where only one Landau level is occupied (ν<2), the resonance position is shifted to higher frequencies (lower mass values) than would be expected for simple free-carrier behavior. This is thought to be due to potential fluctuations giving rise to weakly bound states for electrons in the lowest Landau level, implying that ‘‘extrinsic’’ effects are significant even in these very-high-mobility samples.