Optical investigations of the superconducting energy gap in β″‐(BEDT‐TTF) 2 SF 5 CH 2 CF 2 SO 3

The organic salt β″‐(BEDT‐TTF) 2 SF 5 CH 2 CF 2 SO 3 is a two‐dimensional metal with a quarter‐filled conduction band. In the metallic state the optical conductivity evidences interaction of the charge carriers with charge‐order fluctuations that become stronger as temperature decreases. In the superconducting phase below T c  ≈ 5 K, indications of the superconducting gap with $2{\rm \Delta }\approx 12\;{\rm cm}^{- 1} $ are observed in the optical spectrum, corresponding to $2{\rm \Delta }/k_{{\rm B}} T_{{\rm c}} \approx 3.3$ . Its temperature and magnetic field dependences are also consistent with predictions by the BCS theory of a weakly coupled superconductor.The conductivity ratio $\sigma _{1} (T = 1.75\;{\rm K})/\sigma _{1} (10\;{\rm K})$ indicates the opening of the superconducting gap in β″‐(BEDT‐TTF) 2 SF 5 CH 2 CF 2 SO 3 .