Comparison of different routes for improving vortex pinning in YBa ₂Cu₃O₇ thin films and coated conductors

Achieving high critical currents in YBa₂Cu₃O ₇ (YBCO) films on metallic polycrystalline substrates (coated conductors) is an essential step to make coated conductors a technological reality. Therefore it is extremely important to identify the vortex pinning centers responsible for the high critical current density (J_c), to find ways to further improve the vortex pinning in these superconducting films. We present a comparative study of the angular and field dependences of J _c for two successful approaches used to improve vortex pinning, one that introduces correlated defects and another one that increases the random pinning contribution. The correlated defects are obtained by the introduction of BaZrO₃ nano-particles into the superconducting film matrix, which in turn induce c-axis correlated defects. The enhanced random pinning contribution is achieved by mixing different rare earth. The presence of additional correlated defects results in a slower J_c field decay together with a J_c enhancement around the c-axis region. The rare earth substitutions yield J_c enhancements at self-field and at low fields. We show that even though at 75.5 K and intermediate field range both methods produce comparable results, at lower temperatures the correlated defects become more effective pinning the vortices, inducing higher enhancements of J _c.