Morphology and transport properties of Ca-doped superconducting epitaxial YBCO films

Oxygen doping enhancing the CuO plane density of states n(m) is well known to improve normal conducting and superconducting dc, ac and rf transport properties of YBa2Cu3O7-delta (YBCO) by orders of magnitude. By Ca doping Y1-xCaxBa2Cu3O7-delta, enhancing n(m) similarly, transport improvements by factors between two and ten have been found for bicrystal junctions and single crystals only. To study the effect of Ca doping on epitaxial pulsed laser deposition (PLD) films on CeO2/Al2O3 we performed systematic transport studies for nominal x = 0, 0.1, 0.15 and 0.2 combined with energy dispersive x-ray (EDX) and Rutherford back-scattering (RBS) analysis. The resulting superconducting rf properties, and therewith the mean values of the weak link critical current density j(cJ), degraded with Ca content, confirming trends found by dc measurements of PLD films on SrTiO3 (STO) and on LaAlO3 (LAO) and by rf measurements of Ca-doped films on CeO2 buffered Al2O3 (AO) and on LAO. The normal conducting weak link resistance R-bn decreased with x in line with n(m) enhancements but j(cJ) R-bn also decreased. Reasons for the j(cJ) and j(cJ)R(bn) degradations and the morphology changes and for the differences between Ca-doped YBCO bicrystal junctions and natural junctions in Ca-doped epitaxial YBCO films will be given.