ELECTRON-TRANSFER BOAT-VIBRATION MECHANISM FOR SUPERCONDUCTIVITY IN ORGANIC MOLECULES BASED ON BEDT-TTF

摘要

The highest T-c organic superconductors all involve the organic molecule bis(ethylenedithio)tetrathiafulvalene (denoted as BEDT-TTF or ET) coupled with an appropriate acceptor. This leads to ET, ET(+), or (ET)(2)(+) species in the crystal. Using ab initio Hartree-Fock calculations (6-31G** basis set), we show that ET deforms to a boat structure with an energy 28 meV (0.65 kcal/mol) lower than that of planar ET (D-2 symmetry). On the other hand, ET(+) is planar. Thus, conduction in this system leads to a coupling between charge transfer and the boat deformation vibrational modes at 20 cm(-1) (ET) and 28 cm(-1) (ET(+)). We suggest that this electron-phonon coupling is responsible for the superconductivity and predict the isotope shifts (delta T-c) for experimental tests of the electron-transfer boat-vibration (ET-BV) mechanism. The low frequency of this boat mode and its coupling to various lattice modes could explain the sensitivity of T-c to defects, impurities, and pressure. We suggest that new higher temperature organic donors can be sought by finding modifications that change the frequency and stability of this boat distortion mode. [References: 24]