DC and high-frequency conductivity of the organic metals β"-(BEDT-TT)₂SF₅RSO₃ (R=CH₂CF₂ and CHF)

The temperature dependences of the electric-transport properties of the two-dimensional organic conductors β"--(BEDT-TTF)₂SF₅CH₂CF₂SO₃, β"-(d₈-BEDT-TTF)₂SF₅CH₂CF₂SO₃, and β"-(d₈-BEDTTTF)₂SF₅CH₂CF₂SO₃ are measured by dc methods in and perpendicular to the highly conducting plane. Microwave measurements are performed at 24 and 33.5GHz to probe the high-frequency behavior from room temperature down to 2 K. Superconductivity is observed in β"-(BEDT-TTF)₂SF₅CH₂CF₂SO₃ and its deuterated analogue. Although all the compounds remain metallic down to low-temperatures, they are close to a charge-order transition. This leads to deviations from a simple Drude behavior of the optical conductivity which become obvious already in the microwave range. In β"-(BEDT-TTF)₂SF₅CH₂CF₂SO₃, for instance, charge fluctuations cause an increase in microwave resistivity for T < 20K which is not detected in dc measurements. β"-(BEDT-TTF)₂SF₅CHFSO₃ exhibits a simple metallic behavior at all frequencies. In the dc transport, however, we observe indications of localization in the perpendicular direction.