Evidence for charge order in organic superconductors obtained by vibrational spectroscopy

We study charge disproportionation in few quasi-two-dimensional BEDT-TTF-based compounds by following the temperature dependence of a charge-sensitive vibration ν27(B1u) of BEDT-TTF molecule. While in a charge ordered insulator θ-(BEDT-TTF)2RbZn(SCN)4 a difference between charge on the lattice sites is as high as 0.6e, a small charge disproportionation of 0.15-0.2e is found in two metallic compounds that become superconducting at low temperatures β″-(BEDT-TTF)2SF5CH2CF2SO3 and β-(EDT-TTF)4[Hg3I8](1-x). In contrast to these, a pure metallic β″-(BEDT-TTF)2SO3CHFSF5 does not show any presence of charge disproportionation. This study suggests a correlation between a slight charge disproportionation in the metallic state and superconductivity.