Lattice and Charge Fluctuations in a Molecular Superconductor

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We report IR and Raman spectra for a two-dimensional superconductor, β′′-(BEDT-TTF)4[Ga(C2O4)3(H3O)]C6H5NO2, below the superconducting transition temperature. The behavior of the lattice-sensitive vibrational modes indicated that unidirectional tetramers are transiently formed in parts of the conducting layer. The behavior of the charge-sensitive vibrational modes revealed that molecular charges were inhomogeneous in both tetramerized and non-tetramerized molecules, although the charges in non-tetramerized molecules were more delocalized than those in tetramerized molecules. These results suggest that simultaneous fluctuations of lattice and charge are relevant to the pairing mechanism of the superconductivity.