Systematics of two-component superconductivity in YBa2Cu3O6.95 from microwave measurements of high-quality single crystals

A systematic set of measurements of the microwave ~10 GHz! surface impedance (Zs5Rs1iXs) of twinned YBa2Cu3O6.95 single crystals ~called YBCO/BZO! grown in BaZrO3 crucibles reveal properties that are not directly seen in similar measurements on other YBCO samples. The complex conductivity s5s12is2 obtained as a function of temperature (T) from the surface impedance data shows two key features : ~1! A conductivity peak in s1(T) around 80 K in addition to peaks at 30 K and 92 K and ~2! extra pairing below 65 K in addition to the onset of pairing below the bulk Tc of 93.4 K as revealed in s2(T). These features are present in all three YBCO/BZO crystals measured and are absent in YBCO crystals grown by other methods. These results show that in addition to pairing at Tc5 93.4 K, an additional pairing channel opens up at (;65 K!. High-pressure oxygenation of one of the crystals still yields the same results, and shows that the data cannot be due to unwanted macroscopic segregation of O-deficient regions. Systematics on three single crystals show that the height of the quasiparticle conductivity peak at 80 K in the superconducting state is correlated with the inelastic scattering rate in the normal state. Close to Tc , s2(T);(Tc2T), indicating a mean-field behavior and inconsistent with 3DXY fluctuations over a wide temperature range. A single complex order parameter cannot describe these data, and the results suggest that at least two superconducting components with corresponding pairing temperature scales (TA;65 K and TB5Tc593.4 K! are required. Comparison to model calculations considering various decoupled two-component scenarios (A1B5d1s ,s1d ,d1d) are presented. The comparison shows that the experimental data do not distinguish between these various scenarios, however, the data do require that one of the components be an order parameter with nodes in the gap, such as a d-wave order parameter. Fit parameters to the calculations using the different scenarios are presented. These components are naturally present in all YBCO samples, however, impurities appear to suppress the pair density of the low-temperature A component and lead to greatly enhanced scattering of the high-temperature B component. Overall, our results strongly suggest the presence of multiple pairing temperature scales and energies in YBa2Cu3O6 .95 . @S0163-1829~98!00513-X#