*Review* **Modification of Critical Current Density Anisotropy in High-***T***<sup>c</sup> Superconductors by Using Heavy-Ion Irradiations**

**Tetsuro Sueyoshi**

**Citation:** Sueyoshi, T. Modification of Critical Current Density Anisotropy in High-*T*<sup>c</sup> Superconductors by Using Heavy-Ion Irradiations. *Quantum Beam Sci.* **2021**, *5*, 16. https://doi.org/10.3390/ qubs5020016

Academic Editor: Akihiro Iwase

Received: 21 March 2021 Accepted: 18 May 2021 Published: 21 May 2021

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Department of Electrical Engineering, Kyushu Sangyo University, 2-3-1 Matsukadai Higashi-ku, Fukuoka 813-8503, Japan; s.teturo@ip.kyusan-u.ac.jp; Tel.: +81-92-673-5632; Fax: +81-92-673-5091

**Abstract:** The critical current density *J*c, which is a maximum value of zero-resistivity current density, is required to exhibit not only larger value but also lower anisotropy in a magnetic field *B* for applications of high-*T*c superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-*T*c superconducting materials, which can modify both the absolute value and the anisotropy of *J*<sup>c</sup> in a controlled manner: the unique structures of CDs, which significantly affect the *J*<sup>c</sup> properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the *J*<sup>c</sup> anisotropy in high-*T*<sup>c</sup> superconductors using CDs installed by heavy-ion irradiations. The direction-dispersion of CDs, which is tuned by the combination of the plural irradiation directions, can provide a variety of the magnetic field angular variations of *J*c in high-*T*c superconductors: CDs crossing at ±*θ*<sup>i</sup> relative to the *c*-axis of YBa2Cu3O<sup>y</sup> films induce a broad peak of *J*<sup>c</sup> centered at *B* || *c* for *θ*<sup>i</sup> < ±45◦ , whereas the crossing angle of *θ*<sup>i</sup> ≥ ±45◦ cause not a *J*<sup>c</sup> peak centered at *B* || *c* but two peaks of *J*<sup>c</sup> at the irradiation angles. The anisotropy of *J*<sup>c</sup> can also modified by tuning the continuity of CDs: short segmented CDs formed by heavy-ion irradiation with relatively low energy are more effective to improve *J*c in a wide magnetic field angular region. The modifications of the *J*c anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.

**Keywords:** high-*T*<sup>c</sup> superconductors; critical current density; flux pinning; heavy-ion irradiation; columnar defects; anisotropy
