Enhanced Critical Current Density in Heterostructural YBCO/Ca-Doped YBCO Multilayers

We conducted experiments involving BZO-added YBCO/Ca-doped YBCO heterostructures with varying layer numbers to investigate the role of Ca doping and the mechanism behind the enhanced J(c). Our findings reveal that the inclusion of Ca-doped layers enhances the quality of the YBCO matrix within the BZO-added layer by reducing microstrain and the formation of other crystalline defects, while also optimizing the oxygen content of YBCO with the increasing layer number. These structural improvements lead to a significant increase in self-field J(c)(0), which is also observed to correspond to an increase in in-field J(c)(B) without directly impacting flux pinning. The remarkable enhancement in J(c) at 65 K can be explained by a theoretical model, where the improvement in J(c) at high temperatures is attributed to the more coherent interface between the BZO nanorods and the YBCO matrix. Therefore, we conclude that the overall enhancement of J(c) in the Ca-doped heterostructures is attributed to the improved crystalline structure rather than enhanced flux pinning.