Abstract
The critical current density Jc, 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-Tc superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-Tc superconducting materials, which can modify both the absolute value and the anisotropy of Jc in a controlled manner: the unique structures of CDs, which significantly affect the Jc properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the Jc anisotropy in high-Tc 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 Jc in high-Tc superconductors: CDs crossing at ±θi relative to the c-axis of YBa2Cu3Oy films induce a broad peak of Jc centered at B || c for θi < ±45°, whereas the crossing angle of θi ≥ ±45° cause not a Jc peak centered at B || c but two peaks of Jc at the irradiation angles. The anisotropy of Jc 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 Jc in a wide magnetic field angular region. The modifications of the Jc anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.
Highlights
We have systematically examined the modification of the anisotropy of Jc 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
When the directions of columnar defects (CDs) were extensively dispersed around the c-axis, the Jc was enhanced over a wider magnetic field angular region centered at B || c
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The critical current density Jc in magnetic field (in-field Jc ), which is a maximum current density with zero-resistivity, is the most important parameter in REBCO-coated conductors for the practical applications. The absolute values of Jc for REBCO-coated conductors, have still remained below the practical level for high magnetic field applications [3]. The in-field Jc can be controlled by immobilization of nano-sized quantized-magneticflux-lines (flux lines) penetrating into superconductors in a magnetic field. The motion of flux lines is suppressed by crystalline defects and impurities in the specimen, which areiscalled pinningbycenters (PCs).defects. Artificially embedding crystallinewhich defectsare as flux lines suppressed crystalline and impurities in the specimen, effective. PCs in REBCO thin films [8,9,10,11,12]
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