Enhancement of the critical current density of a film due to high activity of small-grain CeO-20 substrate at 77 K. (a) Open symbols correspond to the film grown on the large grain ceria buffer, CeO-100, while closed symbols represent film on a small-grain buffer, CeO-20. The oxygen partial pressure values, optimized for each sample, are 100 mTorr for the CeO-100 sample and 65 mTorr for the CeO-20 sample. The measurements were performed at 77 K, the external magnetic field, is normal to the film -axis . (b) Development of a long-period phase after oxygen annealing at (closed symbols) of as-grown CeO-20 film sample (open symbols). The diffraction spectrum of the annealed sample (closed symbols) shows two additional superstructure peaks, labeled SSa(002) and SSa(004), corresponding to d-spacing of 1.748(1) nm and 0.879(2) nm, respectively. The formation of the new phase is related to the high performance of CeO-20 samples. The solid lines are Pearson7 approximations used in the line profile analysis. The spectrum was acquired using 10 kV synchrotron radiation with values converted to their equivalent.
Phase formation in CeO-20 films processed at various levels. Peaks of the SSa phase are labeled by “o” symbol and those corresponding to Y248 are labeled by “.” (a) spectra of as-grown CeO-20 samples, the numbers correspond to the level. After the level is reduced from 220 to 160 mTorr, a phase identified as Y248 completely displaces superconducting YBCO. Reduction in to 110 mTorr results in the formation of re-entrant YBCO, which appears to have a structural signature identical to that of standard YBCO. (b), Effect of 30 min annealing in oxygen at , the numbers correspond to . Re-entrant YBCO transforms into the SSa phase, and subsequently the sample becomes superconducting at 90 K. Notably, the sample processed at 160 mTorr remains nonsuperconducting upon oxygen annealing.
Effect of decomposition of the SSa phase after the high-temperature annealing of CeO-20 samples. (a) Evolution of scans showing transformation of long-period SSa (reflections marked by “x” symbol) into shorter period SSb (reflections marked by “o” symbol) after annealing at . Note the development of multiple satellites around the YBCO (003) peak at . One of the satellites transforms into a strong superstructure peak at . Solid lines are approximations of the disordered crystal model (Refs. 5 and 26) used to determine the SF frequency presented on the right of each plot along with the annealing temperatures. (b) Change of the in-field critical current density of CeO-20 sample after high-temperature annealing. We relate the reduction in the performance to the relaxation of A-type superstructure and formation of B-type structure, (c) reciprocal space maps in the vicinity of the YBCO (001) reflection, showing collapse of the long-period superstructure after annealing at . This event correlates with the threefold reduction in , panel b). The horizontal line marks the position of the YBCO (001) reflection. The maps were recorded using 10 kV synchrotron radiation.
TEM analysis and structure model of SSa phase, (a) TEM cross-section of the SSa precipitate embedded in the matrix viewed along the  direction. The white rectangle outlines the SSa inclusion. [(b)–(d)] Structure models of (b) , , (c) , , and (d) a model of SSa phase, . The SSa model cell is formed by insertion of two -CuO double layers into a Y248 cell.
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