Electron states and electron-phonon coupling in the BEDT-TTF-based organic superconductors

Abstract

The electronic structure and the coupling of electrons to the vibrational modes of a single bis (ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecule, the building block of a class of organic superconductors, are studied from quantum chemical calculations. We find that the molecular orbital structure near the Fermi energy can be described with a simple tight-binding model involving only the out-of-plane p orbitals on the carbon and sulfur atoms in the central part of the molecule. Just three of the vibrational modes of Ag symmetry couple strongly to the electron states resulting in a frequency shift of typically 10% for these modes. The high-frequency Ag(11) mode is predicted to be broadened the most by the coupling, by an amount of ∼8%. The calculated electron-phonon coupling constant in the solid, λ∼0.4, is strong enough to produce a superconducting transition temperature Tc∼3-10 K as estimated from McMillan's formula, indicating that the superconductivity in the BEDT-TTF compounds can be explained within the standard BCS electron-phonon coupling theory.