Plasmon damping and response function in doped compounds

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Abstract
We present experimental and theoretical results for the broadening of the 0.5 eV charge carrier plasmon in (A = K, Rb) compounds. The experimental width (0.5 eV) is very large and comparable to the plasmon energy. We have performed RPA calculations for a three-band model of orientationally disordered molecules. We show that it is unlikely that the width can be caused by the disorder or by a decay in single electron - hole pair excitations. Instead we have studied the decay in an electron - hole pair dressed by phonon excitations. We have calculated the response function, using Green's functions dressed by the self-energy due to the electron - phonon interaction. Vertex corrections are included to satisfy the Ward identity. We show that this leads to a width of the plasmon which is comparable to its energy, in agreement with experiment.