To better pin the vortex at external magnetic fields, the HTS films must contain APCs with desired morphology, dimension, orientation, and concentration. Nanoscale APCs with lateral dimension approaching 2ξ (coherence length) on the order of a few nanometers in HTSs must be generated to suppress the dissipation of vortex motion. This has prompted extensive efforts in the past few decades or so and exciting results have been obtained in generating nanoscale APCs in HTS films. In this section, we introduced the research progress of different types and dimensions APCs in detail, and summarized the impact on superconducting performance.

The effect of ionic radii on the T_c of REBCO has been documented in previous work [34]. It is well known that varied rare-earths have different ionic radii. The phenomenon that T_c varies linearly with ionic radius of RE ions has been detected and was attributed to strain-induced charge redistribution between the CuO₂ planes and the charge reservoir (CuO-chains). Several rare-earth elements, including Sm, Eu, and Nd, have been doped in place of Y with various molar cationic ratios to enhance the vortex-pinning capabilities of YBCO films [35]. The Y atom in Y-Ba-Cu-O has been totally replaced in certain studies [35,36,37,38] by another rare-earth atom or a mixture of two or more rare-earth atoms, which has improved vortex pinning. Several combinations, including (Gd_0.8Er_0.2) [37] and (Nd_1/3Gd_1/3Eu_1/3) [38], were published to determine whether the strain caused by lattice mismatch increased when mixtures of rare-earth elements were used instead of a single rare-earth element. Except for the situation when defects were random and unrelated, the enhancement was not notable in any circumstances. There have been attempts to substitute Tb, Ce, Pr, Nd, La, Co, Dy, and Eu at the Y site of YBCO and the RE site of REBCO films [24,39,40,41,42]. The increased density of these substituent nanoprecipitates in doped REBCO films compared to pristine REBCO film led to elevated J_c and F_p values across a wide range of applied magnetic fields, which in turn led to stress field due to lattice mismatch between the phases in the resulting REBCO films.

The idea of strain engineering has been applied to generate and control the morphology and dimension of APCs embedded in HTS films. According to the elastic strain energy model, the appropriate level of interfacial strain can act as a driving force for the self-assembly of 1D vortex pinning, controlling the morphology [43,44], dimensionality [45,46], orientation [47], and concentration [48]. Numerous studies have shown that 1D columnar APCs grown along the c-axis of REBa₂Cu₃O_7-δ films exhibit strong vortex pinning ability, resulting in high J_c when the applied magnetic field is along the c-axis direction [43,48,49,50].

MacManus-Driscoll et al. [51] first reported the introduction of BaZrO₃ secondary phase into YBCO films using the PLD (Pulsed Laser Deposition) technique to enhance the performance. It was found that BaZrO₃ nanoparticles and nanocolumns produced significant c-axis orientation-related enhancement of J_c despite its random distribution in the YBCO matrix. Following the work of MacManus-Driscoll et al., 1D BaZrO₃ APCs have been intensively investigated. In the subsequent report by Yamada et al. [52], the addition of YSZ (yttrium oxide stabilized zirconium oxide) to YBCO targets resulted in the formation of columnar BaZrO₃ nanostructures in YBCO films and would leave a YBCO film matrix containing Ba defects. Self-assembly of vertical arrays of BaZrO₃ phases is observed in this composite film. The vertical alignment of these self-assembled BaZrO₃ columnar phases was hypothesized to be due to the preferential nucleation of impurity islands in the strain field above the impurity particles [53]. Physical property measurements showed that these self-assembled vertical BaZrO₃ phase arrays resulted in strong pinning of vortices, especially when the applied magnetic field was along the c-axis direction. Goyal et al. [54] also reported enhanced pinning of BaZrO₃/YBCO nanocomposite films along the c-axis direction. The BaZrO₃/YBCO interface is strongly strained due to the high lattice mismatch of 7.7% between BaZrO₃ and YBCO, which leads to the formation of a high defect density semi-coherent BaZrO₃/YBCO heterointerface [55,56]. This defect is considered the source of the high pinning efficiency achieved at the 1D BaZrO₃ magnetic flux pinning centers.