Charge-order fluctuations and superconductivity in two-dimensional organic metals

We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organicmetal beta ''-(BEDT-TTF)(2)SF5CHFSO3 and superconductor beta ''-(BEDT-TTF)(2)SF5CH2CF2SO3. The chargesensitive vibrational bands are analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about 6 x 10(11) s(-1). The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor, the exchange velocity starts to decrease below 200 K, and a frozen charge-ordered state emerges and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other spectroscopic experiments, and a tentative phase diagram is proposed for the beta '' BEDT-TTF quarter-filled salts.