Relationship between film thickness and the critical current of YBa2Cu3O7−δ-coated conductors

During the development of ${\mathrm{YBa}}_{\mathrm{2}}{\mathrm{Cu}}_{\mathrm{3}}{\mathrm{O}}_{\text{7−δ}}$ (YBCO) coatings on flexible metal tapes, it has become evident that the achievable critical current ${\mathrm{(I}}_c)$ 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 ${\mathrm{(J}}_c)$ 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.