Isotopic shifts and Raman line shapes of the organic superconductor β-(BEDT-TTF)2I3

The behavior vs. temperature of the vibrational modes of normal and 13C enriched β-(BEDT-TTF)2I3 single crystals is reported. The Raman isotopic shift evidences that the inter-ring CC stretching mainly participates to the lowest component of the CC band. Comparison of this line with spectra of ET crystals evinces that the charge transfer comes from the central CC bond. The line shape was fitted at low temperatures using Fano functions with negative asymmetry parameters q. This observation is to be linked to the occurrence of a strong electron-phonon coupling. We thus assume, near Tc, that the phonons couple with a continuum of electronic origin. We conclude that internal CC vibrations may contribute for superconductivity in these systems.