Raman and microwave surface impedance measurements in the organic superconductor κ-Et2Cu[N(CN)2]BrxCl1−x

We report a Raman study of CH2 and C=C vibrations for single crystals of the organic superconductor κ-Et2Cu[N(CN)2]BrxCl1−x (nominal value x=.5) in both the normal and superconducting states (Tc=l 1.3 K). The average charge transfer ρ=0.5 is estimated from the frequency of the C=C ring stretching Raman mode ν3 (Ag). Raman results are discussed in connection with our previous study of the normal and superconducting states of Et2Cu[N(CN)2]Br. High sensitivity microwave surface impedance is also studied. The microwave resistance Rs and microwave reactance Xs are reported as a function of temperature in order to complement our Raman experiments and to determine precisely the critical temperature Tc and the type of superconductivity.