Direct evidence of a new type of electron-phonon interaction in charge transfer molecular conductors and superconductors

The modulation of the electron correlation term U of the Hubbard Hamiltonian by intramolecular normal mode of vibrations (U-vib interaction) is considered for a two-site two-electron model. It is shown how this interaction perturbs the Raman active modes and how these can be used for determining the corresponding coupling constants. One finds that the C=C stretching mode of TMTXF (X=T, S) molecules strongly modulates the U interaction. This suggests that a bipolaronic mechanism may be active for the pairing of electrons in the superconductors based on this type of molecules.