Physical origin of relativistic plasmons in a two-dimensional electron system

Microwave absorption of a two-dimensional electron system (2DES) with a disk-shaped metallic gate has been investigated. We have found that relativistic plasmons are excited in the system when the gate is electrically connected to the 2DES. As a result, the 2DES is no longer electrically neutral during the plasma oscillation. This property significantly distinguishes ``charged'' relativistic plasmons from all other types of quasineutral plasma excitations. We have shown that the anomalously weak damping of relativistic plasmons is attributed to the hybridization of the plasmon with the photon mode of the external circuit. We have developed a theoretical basis for the relativistic plasmon phenomenon. The experimental data provide conclusive evidence in support of the theory.