Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-basedorganic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different e lectrical properties, namely, superconductors with different transition tem peratures and conductors, which transfer to insulating state with decreasin g temperature, have been studied by micro-optic spectroscopy. Polarized ref lectance spectra of microcrystals have been measured for the three principa l crystallographic directions within the 700-40000 cm(-1) region, and the c orresponding spectra of the optical functions obtained. The anisotropy of t he electronic system in the crystals has been established as two-dimensiona l. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. I t is concluded that the conductors have smaller vibronic coupling constants , more narrow allowed electronic bands, and stronger electron-electron inte raction compared to those of the superconductors, and that vibronic couplin g is the necessary condition for the onset of superconductivity in the supe rconductors studied. (C) 1999 American Institute of Physics. [S1063-7834(99 )03705-3].