Correlaction Effects on the Band Gap of Conducting Polymers

摘要

By applying the projection technique to the computation of excitationenergies, we study the correlation effects on the band gap of conductingpolymers. In the presence of an additional electron or hole, the correlationinduces a polarization cloud around the addi- tional particle, which forms apolaron. For the excitation energy of a polaron,there is a competition betweena {\it loss} of the correla- tion energy in the ground state and a {\it gain}of polarization energy. For the Hubbard interaction, the {\it loss} ofcorrelation energy is dominant and correlations increase the bandgap.However,for long-range interactions, the {\it gain} of polarization energyis dominant and correlations decrease the band gap. Screening the long- rangeinteraction suppresses the {\it gain}of the polarization energy so thatcorrelations again increase the band gap.A small dimerization is alwaysfavorable to the correlation effects. For {\it trans}-poly- acetylene, weobtain the on-site repulsion $U=4.4$eV and the nearest- neighbor interaction$V=0.8$ eV. The screening of $\pi$ electrons due to the polarizability of$\sigma$ electrons is quite strong. ** to be published in Phys. Rev. B