Semiclassical analysis of intraband collective excitations in a two-dimensional electron gas with Dirac spectrum

Solving the initial value problem for semiclassical equations that describe two-dimensional electrons with the Dirac spectrum we found that collective excitations of the electrons are composed by a few distinct components of the oscillations. There always exist sustained plasma oscillations with well known plasmon frequency $\omega_{pl}(k)$. Additionally there are oscillations with the 'carrier frequency' $\omega = v_Fk$ slowly decaying in time according to a power law ($v_F$ and $k$ are the Fermi velocity and wavevector). The reason for onset of these oscillations has a fundamental character related to branching of the polarization function of the Dirac electrons. A strongly anisotropic initial disturbance of the electron distribution generates additional component of undamped oscillations in the form of an electron unidirectional beam which are van Kampen's modes in the Dirac plasma.