Optimizing Flux Pinning of $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{\rm O}_{7\hbox{-}{\delta}} $ (YBCO) Thin Films With Unique Large Nanoparticle Size and High Concentration of $\hbox{\rm Y}_{2}\hbox{BaCuO}_{5}$ (Y211) Additions

Addition of second-phase nanosize defects to YBa 2 Cu 3 O 7-δ (YBCO) superconductor thin films is known to enhance flux pinning and increase current densities (Jc). The addition of Y 2 BaCuO 5 (Y211) was previously studied in (Y211/ YBCO)N multilayer structures and in Y211 + YBCO films deposited from pie-shaped targets. This research systematically studies the effect of Y211 addition in thin films deposited by pulsed laser deposition from YBCO 1-x Y211 x (x = 0-15 vol.%) single targets, at temperatures of 785°C-840°C. Interestingly, the resulting size of Y211 particles is 20-40 nm, in contrast to 10-15 nm in previous studies of Y211 and 5-10 nm for other second-phase defect additions, and the number density is reduced. A slight increase of J c (H, T) was achieved, compared with previous optimization studies. Results and comparisons of flux pinning, intrinsic stresses imaged by TEM, current densities, critical temperatures, and microstructures will be presented. The overall low intrinsic stress on YBCO from Y211 lattice mismatch is smaller than previously studied second-phase defect additions known, which is hypothesized to be the driving force in achieving the unusually large second-phase nanoparticle size and volume fraction thus far in YBCO thin films.