  The microwave response of a two-dimensional electron system (2DES) covered by
a conducting top gate is investigated in the relativistic regime for which the
2D conductivity $\sigma_{2 \rm{D}} > c/2\pi$. Weakly damped plasma waves are
excited in the gated region of the 2DES. The frequency and amplitude of the
resulting plasma excitations show a very unusual dependence on the magnetic
field, conductivity, gate geometry and separation from the 2DES. We show that
such relativistic plasmons survive for temperatures up to 300 K, allowing for
new room-temperature microwave and terahertz applications.
