Plasmon attenuation and optical conductivity of a two-dimensional electron gas

In a ballistic two-dimensional electron gas, the Landau damping does not lead to plasmon attenuation in a broad interval of wave vectors q << k_F. Similarly, it does not contribute to the optical conductivity \sigma (\omega, q) in a wide domain of its arguments, E_F > \omega > qv_F, where E_F, k_F and v_F are, respectively, the Fermi energy, wavevector and velocity of the electrons. We identify processes that result in the plasmon attenuation in the absence of Landau damping. These processes are: the excitation of two electron-hole pairs, phonon-assisted excitation of one pair, and a direct plasmon-phonon conversion. We evaluate the corresponding contributions to the plasmon linewidth and to the optical conductivity.