Electron correlation and electron momenta in polyacetylene

Abstract

In a uniform noninteracting electron gas, (here are two nonanalytic points in the structure factor S(k), which is the Fourier transform of the electron pair correlation function g(r). One of these is the origin of k or momentum space, while the other occurs at k = 2kf, where pf= hkf is the Fermi momentum. In a Peierls insulator, attention is most often focused on the nonanalytic region around k = 2kf, which in turn is modified by the Peierls distortion. By studying here the interpretation of the linear π-plasmon dispersion observed by Ritsko et al. in polyacetylene (CH)x and also, more recently, in a specimen containing a polydiacetylene backbone, attention focuses naturally on the zero momentum non-analyticity. Using a simple model of the π-electron Fermi liquid in (CH) x paralleling that used by Feynman for the Bose liquid 4He at T = 0, the observed plasmon dispersion is related to the structure factor S(k) of the interacting π-electron liquid. Analysis of the resulting form of S(k) makes it plain that both Pauli Principle correlations between parallel spin π-electrons, and Coulomb repulsions, are essential ingredients in interpreting the linear dispersion of the π-plasmon.