DC and high-frequency conductivity of the organic metals beta''-(BEDT-TTF)_2SF_5RSO_3 (R = CH_2CF_2 and CHF)

摘要

The temperature dependences of the electric-transport properties of the two-dimensional organic conductors beta"-(BEDT-TTF)_2SF_5CH_2CF_2SO_3, beta''-(d_8-BEDT-TTF)_2SF_5CH_2CF_2SO_3, and beta"-(d_8-BEDT-TTF)_2SF_5CH_2CF_2SO_3 are measured by dc methods in and perpendicular to the highly conducting plane. Microwave measurements are performed at 24 and 33.5 GHz to probe the high-frequency behavior from room temperature down to 2K. Superconductivity is observed in beta"-(BEDT-TTF)_2SF_5CH_2CF_2SO_3 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 beta"-(BEDT-TTF)_2SF_5CH_2CF_2SO_3, for instance, charge fluctuations cause an increase in microwave resistivity for T < 20 K which is not detected in dc measurements. beta"-(BEDT-TTF)_2SF_5CHFSO_3 exhibits a simple metallic behavior at all frequencies. In the dc transport, however, we observe indications of localization in the perpendicular direction.