Electron correlations in polyacetylene.

Abstract

The influence of electronic correlations on structural and electronic properties of polyacetylene is studied by adding a Hubbard term (on-site Coulomb coupling U) to the Su-Schrieffer-Heeger Hamiltonian. We use the Gutzwiller wave function as an ansatz for the electronic ground state. For weak electron-phonon coupling (\ensuremath{\lambda}\ensuremath{\ll}1) and U/(4${t}_{0}$)\ensuremath{\ll}1 (where 4${t}_{0}$ is the \ensuremath{\pi} bandwidth) we obtain analytical results for the ground-state energy, the amplitude of bond alternation, the effective force constant for the bond-stretching mode, and a mean-field gap parameter \ensuremath{\Delta}. For intermediate couplings the energy is minimalized numerically. Our results are applicable for U/(4${t}_{0}$)\ensuremath{\lesssim}1 where they agree surprisingly well with the Monte Carlo calculations of Hirsch. We find strong indications that everywhere in this region the ground state is dimerized if \ensuremath{\lambda}>0. For larger U we expect a smooth crossover to the spin-Peierls phase. For \ensuremath{\lambda}<0.37 the Hubbard term initially enhances bond alternation and the dimerization amplitude exhibits a maximum at a finite value of U. For larger values of \ensuremath{\lambda} the maximum occurs at U=0 and electronic correlations decrease bond alternation. Using single-particle parameters as deduced from properties of small organic molecules we compare our results with empirical data on polyacetylene. Our analysis consistently reproduces the measured properties if U is chosen between 7 and 9 eV.