We studied the flux pinning properties of BaZrO 3 -doped YBa 2 Cu 3 O 7−x and BaSnO 3 -doped YBa 2 Cu 3 O 7−x films. We found that BaSnO 3 -doped films showed very high global pinning forces, F p , of 28.3 GN m −3 (77 K, B c) and 103 GN m −3 (65 K, B c), twice that of BaZrO 3 -doped films. Transmission electron microscopy analysis showed that, in both films, nanorods of the dopant phase were incorporated. The BaSnO 3 nanorods were nearly straight but the BaZrO 3 nanorods became curved with the increasing film thickness.

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

Intriguing properties of self-organized nanocomposites of perovskite oxides are usually derived from the complex interface of constituent material phases. A sophisticated control of such a system is required for a broad range of energy and device applications, which demand a comprehensive understanding of the interface at the atomic scale. Here, we visualized and theoretically modeled the highly elastically strained nanorod, the interface region with misfit dislocations and heterointerface distortion, and the matrix with strain-induced oxygen vacancies in the self-organized YBaCuO nanocomposite films with Ba perovskite nanorods. Large misfit strain was elastically accommodated in the nanocomposites, but since the elastic strain was mainly accommodated by the nanorods, the concentration of strain-induced oxygen vacancies was small enough for the matrix to keep high critical temperature (>85 K). The interfacial bonding distorted the atomic structure of YBaCuO, but the thickness of distortion was limited to a few unit cells (less than the coherence length) due to the electron screening. The effect of volume fraction on elastic strain and the electron screening are crucial for strong vortex pinning without significant degradation of both the elementary pinning force and critical temperature in the nanocomposites. Thus, we comprehensively clarified the self-organized nanocomposite structure for on-demand control of superconductivity and oxide functionality in the nanocomposite engineering of perovskite oxides.

The stress-elongation (R-A) curve has been experimentally characterized as follows. The initial quasi-elastic region (stage I) was very narrow where the pure Ag component had already yielded due to the thermal contraction. In the following stage II, the slope was reduced owing to the yielding of the Ag alloy. Stage III was defined as the macroscopic multiple fracture taking place in the oxide layer. The R-A behaviour at 77 K has been theoretically simulated based on the rule of mixture. The oxide layer was exerted by a large compressive residual stress due to the thermal contraction. Applying tensile elongation, its compressive stress reduced and became zero at a specific elongation A r1 , after which the sign of stress changed to tensile and its quantity increased. When the tensile stress of the oxide layer reached its fracture strength, macroscopic yielding was expected. Such a result obtained from the simulation can describe well the experimental R-A curve as well as the following elongation dependence of critical current. The critical current decreased linearly with increasing tensile elongation from the beginning of stage I and its gradual decrease followed in stages II and II . This elongation dependence of critical current was reversible in stages I and II, but irreversible in substage II . In stage III corresponding to the plastic deformation region, the critical current was reduced largely due to the macroscopic crack formation.