Lattice dynamics and electron-phonon coupling in theβ−(BEDT−TTF)2I3

Abstract

The crystal structure and lattice phonons of (BEDT-TTF)2I3 superconducting β-phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the Quasi Harmonic Lattice Dynamics (QHLD) method. The empirical atom-atom potential is that successfully employed for neutral BEDT-TTF and for non superconducting α-(BEDT-TTF)2I3. Whereas the crystal structure and its temperature and pressure dependence are properly reproduced within a rigid molecule approximation, this has to be removed account for the specific heat data. Such a mixing between lattice and low-frequency intra-molecular vibrations also yields good agreement with the observed Raman and infrared frequencies. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The charge transfer integrals are evaluated by the extended Hückel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function α(ω)F (ω), which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling constant λ is found to be ∼ 0.4. 74.70. Kn,74.25.Kc

Quasi-two-dimensional organic superconductors: A review

“…Support for a more exotic species of superconductivity also comes from tunnelling experiments, which give maximum energy gap values for β-(BEDT-TTF) 2 AuI 2 which are almost four times larger than expected from BCS theory [137]. Nevertheless, as in the case of the κ-phase salts, recent calculations for β-phase salts, based on optical data [138], also suggest an important role for phonons in the mechanism for superconductivity.…”

Lattice dynamics of TTF–CA across the neutral–ionic transition

“…Thus the lattice phonons correspond to translations and rotations of the rigid molecules (rigid molecule approximation). In this paper, we calculate the crystal structures and lattice phonon frequencies of TTF-CA, following a well assessed treatment [14,15], which combines ab initio methods for the isolated molecules with semiempirical calculations for the crystal structure and lattice dynamics.…”

“…To account for the coupling between lattice and intramolecular vibrations, we adopt an exciton-like model [15,23], where the interaction between different molecular coordinates is mediated by the intermolecular potential which depends directly on the atomic displacements. These correspond to the Cartesian eigenvectors of the normal modes of the isolated molecule, and we use the ab initio eigenvectors and the scaled ab initio frequencies, calculated as described above.…”

“…The A ij , B ij , C ij Buckingham parameters involving C and H atoms are from [24], whereas those for Cl, O and S atoms are from [25][26][27]. The cross-species parameters were calculated via standard ''mixing rules'' [15]. The atomic charges q i , q j in the Coulomb term are the potential derived atomic charges [16], estimated from the above ab initio calculations of the isolated species.…”

Time-Resolved Photoresponse Measurements of the Electrical Conductivity of the Quasi-Two-Dimensional Organic Superconductor β-(BEDT-TTF)2I3 Using a Nanosecond Laser Pulse

“…Theoretical investigation of low frequency phonons in β-(BEDT-TTF) 2 I 3 has suggested a considerable contribution of intermolecular vibrational modes such as librational and translational motions of BEDT-TTF molecules. 34 In addition, extensive mixing between these intermolecular vibrational modes and intramolecular vibrational modes has also been indicated. One can incontrovertibly expect the isotope effect on the frequencies of these vibrational modes and the relevant phonon density of states.…”