INFRARED AND RESONANCE RAMAN-SPECTRA OF KAPPA-(BEDT-TTF)(2) CU(NCS)2 ORGANIC METALS - H8-(BEDT-TTF), H4D4-(BEDT-TTF), AND D8-(BEDT-TTF) DERIVATIVES

We have carefully investigated the partially and completely deuterated bisethylenedithiolotetrathiafulvalene (BEDT-TTF) copper thiocyanate kappa-phase salts (H8-(BEDTTTF)2 Cu(SCN)2, H4D4-(BEDTTTF)2Cu(SCN)2 and D8-(BEDTTTF)2 Cu(SCN)2 which exhibit a superconducting state around 10K as shown by a.c. magnetic susceptibility measurements. We have carried out IR absorption spectra on polycristalline samples at different temperatures and Resonant Raman scattering under visible excitations for investigating the Raman excitation profiles associated with the charge transfer bands. We have examined with a peculiar attention the electron-molecular vibration (e-mv) coupling involving the a(g) vibronic modes. A final discussion concerns the role played by these intramolecular modes (Yamaji's model) for the onset of the superconducting state which exhibits a large and inverse isotopic effect in these materials.