Relationship between film thickness and the critical current of YBa₂Cu₃O_7-δ-coated conductors

During the development of YBa₂Cu₃O_7-δ (YBCO) coatings on flexible metal tapes, it has become evident that the achievable critical current (I_c) reaches a maximum value of about 200 A per cm of conductor width at a coating thickness of 1-2 μm. Additional YBCO beyond this thickness can actually reduce I_c. To investigate, critical current density (J_c) has been measured for samples with YBCO ranging from 0.39 to 6.3 μm in thickness. Several films were thinned by ion milling and remeasured with two significant results: almost no supercurrent is carried at thickness levels above 2 μm; and for films thicker than 3 μm, J_c is drastically reduced near the substrate as well.