Optical phonons in a quarter-filled one-dimensional Hubbard model

The influence of dispersionless quantum optical phonons on the phase diagra m of a quarter-filled Hubbard chain is studied using the density-matrix ren ormalization group technique. The ground state phase diagram is obtained fo r frequencies corresponding to the intramolecular vibrations in organic con ductors. For high vibrational modes the system is only slightly affected by the electron-phonons coupling. It remains in a Luttinger liquid phase as l ong as the electronic repulsion is larger than the polaronic binding energy . For low vibrational modes the phase diagram is very rich. A noticeable po int is the existence of a 4k(F) charge-density wave phase for small values of the correlation strength. For realistic values of the electron repulsion and of the electron-phonons coupling constant a large phonons-mediated red uction of the Luttinger liquid parameter K-rho was found compared to the pu re electronic model.