Effect of bulk Ca-substitution in YBaCuO thin films for microwave applications

Calcium diffusion into defects in the high-temperature superconductor (HTS) compound YBa2Cu3O7−x (YBCO) has been previously investigated [D. Seron, D.E. Oates, G. Hammerl, J. Mannhart, P.J. Hirst, R.G. Humphreys, A.C. Anderson, M.A. Hein, J. Derov, Phys. Rev. B 72 (2005) 104511] as a possible way to improve the dc, ac and rf transport properties and to clarify nucleation, growth, and doping of YBCO. In this paper, the microwave properties of homogeneously deposited Ca-substituted YBCO, Y1−yCayBa2Cu3O7−x (YCBCO) films are presented and compared with the previously published Ca-free YBCO films with a Y0.7Ca0.3Ba2Cu3O7−x capping layer. By using different substrates and growth process detailed in the paper, particular defects are created in the YBCO or YCBCO films. In every situation, Ca-substitution is probed and its effect is compared with very high quality epitaxial YBCO films close to the intrinsic limit of microwave nonlinearity for YBCO. Information on the percolation, doping effect, and the role of the defects obtained from the dc properties in the normal state are used to help interpret the nonlinear microwave behavior.