Effect of swift heavy ions irradiation on the microstructure and current-carrying capability in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub> high temperature superconductor films
Authors: Liu Li , Liu Jie , Zeng Jian , Zhai Peng-Fei , Zhang Sheng-Xia , Xu Li-Jun , Hu Pei-Pei , Li Zong-Zhen , Ai Wen-Si ,   Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China,   School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−<i>δ</i></sub> (YBCO) high temperature superconductor materials have many promising applications in energy, transportation and so on. Nonetheless, the application of YBCO in high magnetic field was limited because of low critical current. One-dimensional latent tracks produced by swift heavy ions irradiation can be effective pinning centers, thus enhancing superconductivity in external field. YBCO high temperature superconducting films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum condition. Structure damages in irradiated samples were characterized by transmission electron microscopy (TEM). Continuous amorphous latent tracks, with diameter from 5 nm to 15 nm, throughout the whole superconducting layer can be observed from TEM images. Physical property measurement system (PPMS) was used to measure superconducting properties of samples before and after irradiation. When irradiated at optimal fluence of 8 × 10<sup>10</sup> ions/cm<sup>2</sup>, critical current reaches its maximum value and pinning force was twice of unirradiated sample, while critical temperature almost unchanged. The analysis of experimental results shows that latent tracks produced by swift heavy ions irradiation can enhance in-field current-carrying capability, without decreasing critical temperature. In the power-law regime <inline-formula><tex-math id="M2">\begin{document}${J_c} \propto {B^{ - \alpha }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20191914_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20191914_M2.png"/></alternatives></inline-formula> values of <i>ɑ</i> decreased with the increasing of fluence, indicating a weaker magnetic field dependence of critical current. <i>ɑ</i> reaches its lowest value 0.375 when irradiated at a fluence of 5.0 × 10<sup>11</sup> ions/cm<sup>2</sup>, corresponding to a lowest in-field <i>J<sub>c</sub></i>. This result may be a combination of increasing pinning centers and decreasing superconductor volumes that work together. Normalized pinning force <i>f</i><sub>p</sub> = <i>F</i><sub>p</sub>/<i>F</i><sub>p,max</sub> of sample irradiated with different fluence as a function of magnetic field <i>h</i> = <i>H</i>/<i>H</i><sub>max</sub> was analyzed using Higuchi model. Fitting results show that planar defects are main source of pinning when <i>h</i> > 1, independent of irradiation. Whereas, dominate pinning centers shifting from surface pinning to isotropic normal point pinning with increasing fluence when <i>h</i> < 1. Given that latent tracks produced by Ta ions irradiation act as strong anisotropic pinning centers, the reason of the dominate pinning centers change with increasing fluence remains to be further studied.