Lattice dynamics and electron-phonon coupling in the beta-(BEDT-TTF)(2)I-3organic superconductor

The crystal structure and lattice phonons of the (BEDT-TTF)(2)I-3 supercond ucting beta phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the quasiharmonic lattice dynamics (QHLD) met hod. The empirical atom-atom potential is that successfully employed for ne utral BEDT-TTF and for nonsuperconducting alpha-(BEDT-TTF)(2)I-3. The rigid molecule approximation has to be relaxed by allowing mixing between lattic e and low-frequency intramolecular vibrations. Such a mixing is found to be essential to account for the specific heat measurements, and also yields g ood agreement with the observed Raman and infrared frequencies. The crystal structure and its temperature and pressure dependence are also properly re produced, though the effect of the mixing is less important in this case. F rom the eigenvectors of the low-frequency phonons we calculate the electron -phonon coupling constants due to the modulation of charge transfer (hoppin g) integrals. The charge transfer integrals are evaluated by the extended H uckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab cryst al plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function alpha (2)(omega )F(w), which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling const ant lambda is found to be similar to0.4.