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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 {Δ}N are allowed. Up to three different transitions {Δ}N=0,±{}1 are observed simultaneously, and their collective shifts are found to depend on the Landau-level filling factor. Compared to the main transition {Δ}N=0, collective phenomena are observed to be suppressed for the combined resonances {Δ}N=±{}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 Alₓ{Ga}_{1{{-}}x}$As-GaAs heterojunctions.
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Batke et al. (1991) studied this question.
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