TEMPERATURE DEPENDENCE AND ISOTOPIC SHIFTS OF THE VIBRATIONAL SPECTRA IN #beta#-(BEDT-TTF _2I_3 ORGANIC SUPERCONDUCTORS

摘要

We present a temperature-dependence study of Raman spectra of both normal and ~(13)C labeled #beta#-(BEDT-TTF)_2I_3 single crystals. Electron spin resonance has been used to check that all crystals had pure #beta# structure. The spectra of triiodides moieties, similar to those given earlier by Sugai and Saito, exhibit well defined features at 26,40,91,108, 118, 146 cm~(-1). The line at 108 cm~(-1) was found to depend on the temperature and did not appear systematically. The mixing of the three C=C bonds has been evaluated from experimental data and shows that the charge transfer comes from the central C=C bond. This conclusion is supported when comparing the frequencies of the C=C lines with those given by the spectra of BEDT-TTF crystals. The low-frequency component of the C=C doublet has strong isotopic shift. Low temperature profiles of this line were adjusted with Fano line shapes. The electron-phonon coupling parameter s was found to be negative over a wide temperature range characteristic of a coupling with a continuum of possible electronic origin. Intramolecular C=C vibrations could therefore also be considered as a possible mediation factor for superconductivity in these Systems.