R Properties of High-Temperature Superconducting Materials

Systematic studies of the RF properties of various high temperature superconductors were carried out. The experimental technique employed was a parallel plate resonator technique adapted for studying superconducting thin films. The penetration depth and the surface resistance of the samples were measured simultaneously. The YBa_2Cu_3O_ {7-delta} films studied have a wide range of material properties: transition temperature (Tc), normal state resistivity, etc.. At low temperatures, their penetration depths were found to have an essentially quadratic temperature dependence with systematic deviations that were correlated with the Tc of the samples. For the dirtiest films (low Tc and low normal channel conductivity), possible exponential behavior was observed for the penetration depth at the lowest temperatures. At higher temperatures (above Tc/2), all the films exhibited the same temperature dependence as that of YBa_2Cu_3O_ {6.95} single crystals. Further analysis revealed that the data are qualitatively consistent with the dirty d-wave model of the high Tc superconductors and that there exists a correlation between the Tc of the films and the amount of disorder present. A strong correlation between Tc and the normal channel conductivity was also observed in these films. The temperature dependence of the normal channel conductivity strongly suggests a rapidly decreasing scattering rate for the quasi-particles below Tc. Preliminary studies on Bi_2Sr _2CaCu_2O_8 single crystals revealed some interesting temperature dependencies for their penetration depths. However we suspect the data are contaminated by c-axis contributions to the transport due to the large anisotropy of the material. In Tl_2Ba_2CaCu _2O_8 thin films, the penetration depth was found to be nearly linear with temperature at low temperatures. One pair of the films exhibited the same temperature dependence as that of YBa_2Cu_3O_{6.95} single crystals over the whole temperature range below Tc. More work on this material is needed to establish the implications of this correlation.