Magnetoplasma resonances in two-dimensional electron rings

Employing millimeter-wave spectroscopy, we have studied the magnetoplasma excitations of a two-dimensional electron gas confined in a ring geometry. The observed spectrum decomposes into several high- and two low-frequency branches. For large magnetic fields the latter are identified as edge magnetoplasmons localized at the inner and outer boundary of the rings. With decreasing magnetic field the inner edge mode exhibits a crossover from antidotlike to dotlike behavior. The resonances can also be analyzed in terms of plasmons in a strip with periodic boundary conditions.