The 0.5 eV charger carrier plasmon has a relatively strong coupling to the electrons ( Phys. Rev. B 48, 7651 (1993)), and it should therefore be important for the properties. For instance, it is believed to cause a large part of the anomously large width of the t1u band in photoemission (M. Knupfer et. al, Phys. Rev. B 47, 13944 (1993)).

The plasmon has no dispersion within the experimental accuracy. This is found to be due to local-field effects, which tend to give a negative dispersion, and interband effects, which tend to give a positive dispersion (J. Phys.: Condens. Matter 8, 2557 (1996)).

The plasmon has a very large broadening, although the one-particle band width should be too small to allow a substantial decay of the plasmon in electron-hole pairs. The effects of orientational disorder (not important) and the electron-phonon interaction on the width have been studied (J. Phys.: Condens. Matter 8, 4001 (1996)). It is found that due to the fact that the phonon energies and the band widths are comparable, the electronic structure is modified in an unconventional way by the electron-phonon interaction. Thus spectral weight is pushed out of the non-interacting band, substantially increasing the band width and allowing for a decay of the plasmon in strongly modified electron-hole pair excitations.

The effects of the t1u plasmon on the electron self-energy (J. Phys.: Cond. Matt. 9, 5635 (1997)). has been studied in the so-called GW approximation. The t1u band width is found to be moderately reduced.