Speaker
Description
In addition to naturally induced pinning centers in $RE$Ba$_{2}$Cu$_{3}$O$_{7−\delta}$ coated conductors ($RE$BCO-CCs), such as blocking layers and stacking faults act as planer pins, minor-phase precipitates and oxygen defects act as coarse and fine spherical pins, an introduction of fine pinning centers plays a crucial role for improving the critical current density Jc characteristics of $RE$BCO-CCs. Regarding fine spherical pins, $RE$$_{2}$O$_{3}$ nano-particles with a PLD technique [1] and Ba(Hf/Zr)O$_{3}$ (BHO/BZO) nano-particles with TFA-MOD [2] and F-free MOD [3] methods have been introduced, and an anomalous depression of $J_{\rm c}(\theta\sim B\parallel ab)$ at low fields and a crossover to an usual effective mass like $J_{\rm c}(\theta)$ at high fields, have been observed in some cases [2, 4]. As for columnar defects, Fujikura successfully introduced short BHO nano-rods via their Hot-wall PLD with a fast growth rate and reported $J_{\rm c}(T, B, \theta)$ with a smaller anisotropy, namely with a less remarkable $J_{\rm c}(B\parallel c)$ peak, compared with $J_{\rm c}(T, B, \theta)$ caused by well-aligned nano-rods [5].
For further improving the $J_{\rm c}(T, B, \theta)$ characteristics of $RE$BCO-CCs, it is important and beneficial to understand such novel $J_{\rm c}(T, B, \theta)$ characteristics due to artificial pinning centers. In this study, we calculated the angular dependence of the elementary pinning force $f_{\rm p}(T, \theta)$ due to spherical pins imitating BHO nano-particles and inclined short columnar pins imitating BHO nano-rods by evaluating the dimensions of pinning centers and vortex cores rigorously within a normal-core approximation. We found that $f_{\rm p}(T, \theta)$ tends to decrease with $\theta$ approaching the $B\parallel ab$ direction in some conditions and confirmed that the combination of the angular dependence of $f_{\rm p}(T, \theta)$ and $B_{\rm c2}(\theta)$ leads the anomalous $J_{\rm c}(\theta)$ depression at low $B$ and its crossover to the usual effective mass like $J_{\rm c}(\theta)$ at high $B$. As for columnar pins, we confirmed that the $f_{\rm cp}(\theta\simeq B\parallel c)$ peak becomes broad with shorter columns and successfully reproduced experimentally observed broader $J_{\rm c}(\theta)$ by taking account of the distribution of the inclination and azimuth of short BHO nano-rods.
These results suggest that experimentally observed $J_{\rm c}(T, B, \theta)$ can be basically explained by numerically evaluated $f_{\rm p}(T, \theta)$ and that a prediction of $J_{\rm c}(T, B, \theta)$ based on $f_{\rm p}(T, \theta)$ due pinning centers may be helpful for tailoring the pinning properties of $RE$BCO-CCs depending on applications.
At the conference, we would like to explain our calculations and discuss vortex pinning properties of $RE$BCO-CCs due to spherical pins and columnar pins.
[1] A. Molodyk $et\ al$., Sci. Rep., 11 (2022) 2084.
[2] For example, K. Nakaoka $et\ al$., SuST, 30 (2017) 055008., M. Miura $et\ al$., NPG Asia Mat., 9 (2017) 197.
[3] T. Yoshihara $et\ al$., IEEE-TAS, 33 (2023) 6600205.
[4] T. Okada $et\ al$., IEEE-TAS, 29 (2019) 8002705., T. Okada and S. Awaji, $to\ be\ submitted$.
[5] S. Fujita $et\ al$., IEEE-TAS, 28 (2020) 6600604.
