Optical properties of radical cation salts derived from the TTF molecule: Evidence for electronic correlations and phase transitions in organic conductors

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Abstract
Following a general presentation of the organic conductors which belong to the series of radical cation salts derived from the tetrathiafulvalene molecule we analyze the characteristic charge transfer absorption bands observed in these mixed valence compounds. Using a Drude-Lorentz model we show that the energy gap present in these materials is basically of the Hubbard type. Nevertheless we observe also in specific cases additional contributions due to either a periodic lattice distortion or a counter-ion electrostatic perturbation. We conclude finally that the electronic correlations are the dominant interactions in these narrow 1d electronic band systems.