Infrared and resonance Raman spectra of κ-(BEDT-TTF)2Cu(NCS)2 organic metals. H8-, H4D4- and D8-(BEDT-TTF) Derivatives

Abstract

We have investigated the partially and completely deuteriated 3,4,3′,4′-bis(ethylenedithio)-2,2′,5,5′, tetrathiafulvalene (BEDT-TTF) copper thiocyanate salts which exhibit a superconducting state at ca. 10 K. We have carried out both IR absorption spectra on polycrystalline samples and polarized reflectivity measurements on single crystals at 300 and 15 K. Resonant Raman scattering under visible excitation was also performed at room temperature for investigating the Raman excitation profiles associated with the charge-transfer bands. After a careful assignment of the vibrational spectra, we examined the electron–molecular vibration (EMV) coupling involving a_g vibronic modes. These EMV interactions are tentatively compared with Yamaji's model, which postulates a BCS-type model for the occurrence of a superconducting state associated with these internal molecular vibrations.