Quenching of collective phenomena in combined intersubband-cyclotron resonances in GaAs

Intersubband resonances of high-mobility electron inversion layers in AlAs-GaAs heterojunctions from the ground to the first excited subband are investigated with far-infrared spectroscopy in magnetic fields tilted with respect to the sample normal. In this configuration, combined intersubband-cylcotron transitions with changes in Landau-level index ΔN are allowed. Up to three different transitions ΔN=0,±1 are observed simultaneously, and their collective shifts are found to depend on the Landau-level filling factor. Compared to the main transition ΔN=0, collective phenomena are observed to be suppressed for the combined resonances ΔN=±1. This allows one to determine the subband separation and the size of the collective shift at zero magnetic-field strength, providing a sensitive test of the interface potential in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs heterojunctions.