Optical properties of beta ''-(ET)(2)SF5CH2CF2SO3: A layered molecular superconductor with large discrete counterions

We report the polarized infrared and optical reflectance of beta "-(ET)(2)S F5CH2CF2SO3 as a function of temperature. This fully organic superconductor displays weakly metallic behavior over the entire temperature range of our investigation. The electronic properties present several unusual features including the lack of a conventional free carrier (Drude) response at low t emperature, infrared localization phenomena, and an unexpected temperature dependence of the oscillator strength. Similar behavior is observed in othe r beta and beta " superconductors above T-c, suggesting that this class of compounds is very close to a metal --> insulator transition where electroni c correlations are important and the Drude response carries very little spe ctral weight. Vibronic features increase in intensity with decreasing tempe rature in the infrared, as expected. The temperature dependence of key anio n vibrational modes are discussed in terms of intermolecular hydrogen bondi ng within the anion pocket. [S0163-1829(99)04330-1].