In the salt formed by bis͑ethylenedithio͒tetrathiafulvalene ͑BEDT-TTF͒ with the ͓Mo 6 O 19 ͔ 2Ϫ anion, the BEDT-TTF ϩ cations form quasi-isolated dimers. For this reason (BEDT-TTF͒ 2 Mo 6 O 19 has been selected to experimentally evaluate the BEDT-TTF ϩ microscopic parameters. Using the microreflectance technique we were able to collect polarized infrared/visible spectra from a single-crystal face. The analysis of the spectra in terms of the isolated dimer model yields reliable estimates of BEDT-TTF ϩ electron-molecular vibration (e-mv) coupling constants. The effective on-site electron-electron repulsion is also evaluated. A comparison is made with available theoretical estimates of e-mv coupling constants. The possible role of e-mv coupling in BEDT-TTF based superconductors is briefly discussed.

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

No abstract

We prove that the upper electronic bands of κ-phase BEDT-TTF salts are adequately modeled by an half-filled tight-binding lattice with one site per cell. The band parameters are derived from recent ab initio calculations, getting a very simple but extremely accurate one-electron picture. This picture allows us to solve the BCS gap equation adopting a real-space pairing potential. Comparison of the calculated superconducting properties with the experimental data points to isotropic s0-pairing. Residual many-body or phonon-mediated interactions offer a plausible explanation of the large variety of physical properties observed in κ-phase BEDT-TTF salts.PACS numbers: 71.15.Fv, 74.20.Fg, 74.70.Kn, Superconductivity (SC) in organic charge transfer (CT) salts has been discovered more than 15 years ago. At present, κ-phase BEDT-TTF (ET) salts are the most promising organic superconductors (OSC) [1]. OSC have similar features to cuprate superconductors: they exhibit highly bidimensional structures, narrow electronic bands, low carrier densities and possibly strong electronic correlations. Superconducting properties are also similar in organics and cuprates, exhibiting singlet pairing [2], very low coherence lengths, high magnetic penetration depths and critical fields [1]. Perhaps the most characteristic feature of cuprate superconductors is the competition between SC and antiferromagnetism (AFM). This competition shows up also in OSC, as demonstrated by the presence of antiferromagnetic spin fluctuations in the metallic state of several compounds [2], or even by reentrant SC in the presence of magnetic order. In this respect the different behavior of κ-ET 2 Cu[N(CN) 2 ]X (ET-X) salts where X=Cl, Br is noteworthy: the two compounds are isostructural, but at ambient pressure the Br-compound is a superconductor with T c = 11.6 K, whereas the Clcompound is a Mott insulator with (possibly commensurate) antiferromagnetic ordering [2]. On the other hand, even a very small external pressure (270 bar), turns ETCl to the superconducting ground state (T c = 12.8 K) [1]. Experimental studies of normal and superconducting state properties in OSC are still scanty and often inconclusive, due to the extreme sensitivity of the material to pressure, radiation and sample preparation. At the same time, the amount of theoretical work on OSC is scarce if compared with that developed for cuprates, possibly due to the apparent complexity of the OSC structures. In the following we will show that, by a proper choice of the basis functions, the electronic structure of κ-phase ET salts can be accurately described in terms of a rectangular lattice with nearest and next-nearest neighbor interactions. Similar one-band models are often adopted in the study 1

No abstract