Temperature dependence of the in-plane penetration depth of Y'BazCu3O6 9s and YBa2 (Cuo 998/ Zno po15) s 0 6 95 crystals from T to T The low-temperature dependence of the microwave complex surface impedance (17 GHz) of pure and Zn-doped high-quality YBCO single crystals is reported. The surface resistance of the pure crystal shows a peaked structure around 40 K which disappears with Zn doping. The in-plane penetration depth varies linearly with temperature below 25 K; a T contribution appears at the lowest temperature when Zn impurities are added. This suggests that the former temperature dependence is intrinsic to pure crystals whereas the T behavior, also observed in thin films, would be due to defects. The linear temperature dependence is expected for superconducting gaps with lines of nodes.
We report a systematic Raman study of intramolecular vibrations for single crystals of the organic su-perconductor~-(BEDT-TTF)2Cu(N(CN)2)Br in both the normal and superconducting states. The study has focused on the C=C stretching vibrations and the CH2 scissoring modes. From the frequency of the C = C-ring-stretching Raman mode v3( Ag ) we estimate the average charge transfer p =0.4 on the BEDT-TTF donor at 295 K while a metal-insulator transition region around 190 K is confirmed by a frequency discontinuity of the C = C ring vibrations. Frequency of CH& vibrational modes show a discontinuity at T, that indicates a selective change in the interaction between the ethylene groups of the donor molecule BEDT-TTF and the anion (CU (N(CN)2)Br) while the frequency of the C =C-ring-stretching mode is blue shifted just above T"suggesting a possible renormalization of the electron-molecular interaction. Also, in order to complement our Raman experiments and to appreciate both the crystal quality and the nature of the superconducting state in our studied samples, we present measurements, as a function of temperature, of the microwave surface impedance.
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