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

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In a ballistic two-dimensional electron gas, the Landau damping does not lead to plasmon attenuation in a broad interval of wave vectors q << kF. Similarly, it does not contribute to the optical conductivity σ(ω, q) in a wide domain of its arguments, EF > ω> qvF, where EF, kF and vF 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.