Raman studies of BEDT-TTF molecule-based organic superconductors

The temperature dependence of the frequencies of the Raman features due to ν9 (Ag) and ν60 (B3g) modes has been measured in the BEDT-TTF (ET) molecule-based organic superconductors κ-(ET)2Cu[N(CN)2]Br, κ-(ET)2Cu[N(CN)2]Cl, αt-(ET)2I3, and β-(ET)2AuI2. The frequency of the ν9 (Ag) mode was observed to soften in κ-(ET)2Cu[N(CN)2]Br and κ-(ET)2Cu[N(CN)2]Cl at low temperatures where NMR data indicated antiferromagnetic fluctuations, providing evidence of spin-phonon interactions. The result is also compared with a very recent theory which deals with the effect of electronic correlations on the frequencies of the molecular phonon modes. We observed a peak splitting for the ν60 (B3g) mode in αt-(ET)2I3 below 190 K, where an incommensurate superstructure was reported, and a frequency decrease of 1.7±0.4 cm-1 for the upper component of the ν60 (B3g) mode in the superconducting compound, demonstrating further the unusual properties of this mode