Experimental observation of edge magnetoplasma excitations in a two-dimensional electron system in the quantum-hall-effect regime

New low-frequency modes corresponding to acoustic edge magnetoplasma excitations have been observed in the resonance microwave absorption spectra of a two-dimensional electron system in a transverse magnetic field. The additional excitation modes have been shown to appear only in the quantum-Hall-effect regime (in narrow magnetic-field regions near the integer values of the filling factor), when the resonance microwave absorption lines exhibit sharp narrowing. The absolute values of the resonance absorption frequencies and their dependence on the parameters of the electron system coincide (without any fitting parameters) with the respective theoretical predictions of the formula describing the properties of the acoustic modes of edge magnetoplasma excitations.