Intramolecular phonons of the beta and beta* phases of the unlabeled and 13C-labeled (BEDT-TTF)2I3 organic superconductor above and near Tc

摘要

We present a temperature dependence investigation of the C equals C intramolecular phonon for both the normal and inter-ring C* equals C* labeled salts (BEDT-TTF)$-2$/I$-3$/ superconducting at T$-c$/ equals 1.5 K and 8 K. C equals C bond frequencies are found at 1467, 1493 cm$+$MIN@1$/ and 1420, 1488 cm$+$MIN@1$/ for the unlabeled and labeled crystals, respectively. We have also computed the infrared and Raman frequencies of the unlabeled and labeled BEDT-TTF molecules. In the low frequency range we found in special operating conditions a tri-iodide I$-3$/-line at 106 cm$+$MIN@1$/ in addition to those found at 36, 116, and 146 cm$+$MIN@1$/. At 8 K the Raman profile of the intense tri-iodide line at 116 cm$+$MIN@1$/ can hardly be fitted with a Lorentzian. On the other hand, we also did not succeed to fit correctly the low temperature profile of the inter-ring C equals C line with a single Lorentzian; however Fano line shapes gave satisfactory fits. Taking into account that recent results of Kini, Ferraro and Williams provided evidence of participation of lattice phonons in the pairing mechanism, our results show that intramolecular C equals C vibrations must also be considered as a possible mediation factor for superconductivity.