[论文解读] Designing high-performance superconductors with nanoparticle inclusions: comparisons to strong pinning theory

Sarah C. Jones, Masashi Miura+5 more|arXiv (Cornell University)|May 21, 2021

Physics of Superconductivity and Magnetism参考文献 90被引用 4

一句话总结

本研究探讨了在(Y0.77,Gd0.23)Ba2Cu3O7−δ超导薄膜中引入纳米颗粒后涡旋钉扎的行为，表明临界电流密度（Jc）在强钉扎理论下遵循幂律依赖关系Jc ∝ B−α。主要发现显示，α随纳米颗粒密度升高而减小，随纳米颗粒尺寸与相干长度之比（d/ξ）增大而增加；在高场区域，Jc衰减接近B−1，表明每个颗粒捕获一个涡旋，蠕变速率S的趋势与α一致。

ABSTRACT

One of the most promising routes for achieving unprecedentedly high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce $T_c$. Consequently, an optimal balance must be achieved between the nanoparticle density $n_p$ and size $d$. Determining this balance requires garnering a better understanding of vortex-nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y$_{0.77}$,Gd$_{0.23}$)Ba$_2$Cu$_3$O$_{7-\delta}$ films in magnetic fields up to 35 T, and compare the trends to recent results from time-dependent Ginzburg-Landau simulations. We identify consistencies between the field-dependent critical current $J_c(B)$ and expectations from strong pinning theory. Specifically, we find that that $J_c \propto B^{-\alpha}$, where $\alpha$ decreases from $0.66$ to $0.2$ with increasing density of nanoparticles and increases roughly linearly with nanoparticle size $d/\xi$ (normalized to the coherence length). At high fields, the critical current decays faster ($\sim B^{-1}$), suggestive that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in $J_c(B)$. Due to a spread in defect sizes, this novel peak effect remains unresolved here. Lastly, we reveal that the dependence of the vortex creep rate $S$ on nanoparticle size and density roughly mirrors that of $\alpha$, and compare our results to low-$T$ nonlinearities in $S(T)$ that are predicted by strong pinning theory.

研究动机与目标

• 理解纳米颗粒尺寸与密度如何影响高温超导体中涡旋钉扎与临界电流密度（Jc）的行为。
• 在强磁场（最高达35 T）和低温（T/Tc ≈ 0.05–0.5）条件下，检验强钉扎理论的预测。
• 阐明在含纳米颗粒的YBCO薄膜中观测到的非单调蠕变速率S(T)的起源。
• 确定在不降低Tc的前提下，使Jc最大化的纳米颗粒密度与尺寸之间的最优平衡。

提出的方法

• 制备了具有可控纳米颗粒掺杂物尺寸（d）和密度（np）的(Y0.77,Gd0.23)Ba2Cu3O7−δ外延薄膜。
• 在低温（T/Tc ≈ 0.05–0.5）下测量高达35 T的磁场依赖临界电流密度Jc(B)，以隔离强钉扎效应。
• 将Jc(B)数据拟合为幂律形式Jc ∝ B−α，以提取α随np和d/ξ（归一化至相干长度ξ）的变化关系。
• 通过磁弛豫测量分析涡旋蠕变速率S，并将其与np和d的依赖关系与理论预测进行比较。
• 将实验获得的Jc(B)和S(np, d)趋势与时间依赖的Ginzburg-Landau（TDGL）模拟及强钉扎理论形式进行对比。
• 评估Jc(B)在高场区域出现峰值的现象，作为每个纳米颗粒捕获多个涡旋的标志。

实验结果

研究问题

• RQ1 在强钉扎条件下，纳米颗粒密度（np）与尺寸（d）如何影响YBCO薄膜中Jc(B)的磁场依赖性？
• RQ2 实验获得的Jc(B)趋势在多大程度上符合强钉扎理论的预测，特别是幂律行为Jc ∝ B−α？
• RQ3 每个纳米颗粒捕获多个涡旋是否会在高场区域导致Jc(B)出现可测量的峰值？为何在此研究中该峰值未被解析？
• RQ4 涡旋蠕变速率S如何依赖于纳米颗粒的尺寸与密度？其行为是否与幂律指数α一致？

主要发现

• 在低场区域，Jc随纳米颗粒密度（np）近似线性增加，但在高密度下趋于饱和，表明通过增加缺陷密度提升Jc存在上限。
• 在Jc ∝ B−α关系中，幂律指数α随纳米颗粒密度增加从0.66降至0.2，与强钉扎理论一致。
• α随纳米颗粒尺寸与相干长度之比（d/ξ）线性增加，证实理论预期：更大的缺陷可增强钉扎强度。
• 在高场区域（B > 10 T），Jc近似按B−1衰减，表明每个纳米颗粒可能仅捕获一个涡旋，与涡旋饱和现象一致。
• 由于缺陷尺寸分布的存在，未观测到明显的高场区域Jc(B)峰值，这抑制了理论预测的集体多涡旋捕获效应。
• 涡旋蠕变速率S随纳米颗粒密度增加而降低，随d/ξ增大而升高，其对np和d/ξ的依赖关系与指数α的趋势高度一致，支持强钉扎理论的预测。