**4. Conclusions**

We have systematically examined the modification of the anisotropy of *J*<sup>c</sup> in REBCO thin films by using heavy-ion irradiations: the morphology and the configuration of the irradiation defects were controlled by the irradiation conditions such as the irradiation energy and the incident direction. The direction-dispersed CDs were designed in REBCO thin films to push up the *J*<sup>c</sup> in the magnetic field angular region from *B* || *c* to *B* || *ab*, by controlling the irradiation directions. When the directions of CDs were extensively dispersed around the *c*-axis, the *J*<sup>c</sup> was enhanced over a wider magnetic field angular region centered at *B* || *c*. The *J*<sup>c</sup> at *B* || *ab*, on the other hand, was hardly affected even by CDs tilted toward the *ab*-plane, which is attributed to the strong line tension energies of flux lines around *B* || *ab* in the anisotropic superconductors. We demonstrated the improvement of *J*<sup>c</sup> at *B* || *ab* by the introduction of CDs, where the angle of CDs relative to the *ab*-plane were controlled down to Θ<sup>i</sup> = 5◦ . These results suggest that direction-dispersed CDs can provide the isotropic enhancement of *J*<sup>c</sup> over all magnetic field angular region when the angles of CDs are matched with the anisotropic line tension energy of flux lines.

Another promising morphology of CDs, i.e., discontinuous CDs, which can be introduced by heavy-ion irradiation with relatively low energy, showed large potential for the enhancement of *J*<sup>c</sup> over a wide magnetic field angular region. In particular, the combination of the discontinuity and the direction-dispersion lead to further enhancement of *J*c: the gaps in discontinuous CDs provide the suppression of the motion of flux lines, while the direction-dispersion of CDs produces the strong flux pinning over a wide magnetic field angular region.

The heavy-ion irradiation to high-*T*<sup>c</sup> superconductors can provide the flux pinning structure with higher and more isotropic *J*c, by further tuning the irradiation process: the systematic studies using the ion irradiation process may lead to the approach to the theoretical limit of *J*c, i.e., pair-breaking critical current density.

**Author Contributions:** Conceptualization, T.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by KAKENHI (25420292, 16K06269 and 19K04474) from the Japan Society for the Promotion of Society.

**Data Availability Statement:** The data presented in this study are available on request from the author.

**Acknowledgments:** The author thanks N. Ishikawa, M. Mukaida, A. Ichinose, K. Yasuda, T. Fujiyoshi, S. Semboshi, T. Ozaki, and H. Sakane for their constant support during the researches. Experimental works were partly performed under the Common-Use Facility Program of JAEA.

**Conflicts of Interest:** The authors declare no conflict of interest.
