Features of microwave absorption by two-dimensional electron systems at the harmonics of cyclotron resonance

Extrema of microwave absorption at the harmonics of cyclotron resonance of a two-dimensional electron system of a gated GaAs/AlGaAs heterojunction have been discovered. The extrema at the second and third harmonics exist in the entire frequency range of measurements ($38$–$145$ GHz) and transform from absorption minima to maxima with a decrease in the frequency and an increase in the temperature. The minima are explained by microwave absorption accompanied by the excitation of Bernstein modes, the spectrum of which exhibits gaps near the harmonics of cyclotron resonance. The features of these modes in field-effect transistors with two-dimensional electron systems have been discussed.