An antiphased magnetoplasma (MP) mode in a two-dimensional electron gas has been studied by means of inelastic light-scattering spectroscopy. Unlike the cophased MP mode, it is purely quantum excitation which has no classic plasma analog. It is found that zero-momentum degeneracy for the antiphased and cophased modes predicted by the first-order perturbation approach in terms of the electron-electron interaction is lifted. The zero-momentum energy gap is determined by a negative correlation shift of the antiphased mode. This shift, observed experimentally and calculated theoretically within the second-order perturbation approach, is proportional to the effective Rydberg constant in a semiconductor material.

• Received 8 December 2008

DOI:https://doi.org/10.1103/PhysRevB.79.121310