通过Ca替代增强YBa$_2$Cu$_4$O$_8$薄膜的临界电流和不可逆场

中文摘要 AI 辅助

超导材料的内禀载流子浓度可能不是最优的，不同的超导性质可能具有不同的最优掺杂水平。在此，我们表明Ca替代增加了内禀欠掺杂的外延(Y$_{1-x}$Ca$_x$)Ba$_2$Cu$_4$O$_8$ (YCa124)薄膜中的空穴浓度，其中$x \leq 0.10$，使YCa124向最优掺杂靠近，提高了$T_c$和$J_c$。在40 K下，与未掺杂薄膜相比，10% Ca掺杂在0.03 T和6 T下分别将$J_c$提高了2.4倍和10倍。$J_c$的场依赖性遵循$J_c \propto B^{-0.5}$，与Ca含量无关，表明Ca替代不改变主导的钉扎机制。这种场依赖性与电子显微镜观察到的平面Y-125堆垛层错共生体引起的钉扎一致。此外，Ca替代减缓了$T \gtrsim 15$ K下的热激活涡旋运动（蠕变），而在较低温度下，蠕变速率$S$随磁场增加表现出异常的降低。最后，我们考虑了掺杂如何调节由退配对电流密度$J_d$设定的$J_c$上限，以及由Ginzburg参数$G_i$设定的蠕变下限。我们发现$J_c$与$J_d$之间以及$S$与$G_i$之间存在正相关，并将其与多种超导体的趋势进行了比较。

英文摘要

The intrinsic carrier concentration of superconducting materials may not be optimal, and different superconducting properties may have distinct optimal doping levels. Here, we show that Ca substitution increases the hole concentration in intrinsically underdoped epitaxial (Y$_{1-x}$Ca$_x$)Ba$_2$Cu$_4$O$_8$ (YCa124) films with $x \leq 0.10$, driving YCa124 toward optimal doping, increasing $T_c$ and $J_c$. At 40 K, 10% Ca doping increases $J_c$ by factors of 2.4 and 10 at 0.03 and 6 T, respectively, compared with the undoped film. The field dependence of $J_c$ follows $J_c \propto B^{-0.5}$, independent of Ca content, indicating that Ca substitution does not alter the dominant pinning mechanism. This field dependence is consistent with pinning by planar Y-125 stacking-fault intergrowths observed by electron microscopy. In addition, Ca substitution slows thermally activated vortex motion (creep) at $T \gtrsim 15$ K, while at lower temperatures the creep rate $S$ shows an unusual decrease with increasing field. Finally, we consider how doping tunes the upper bound on $J_c$, set by the depairing current density $J_d$, and the lower bound on creep, set by the Ginzburg parameter $G_i$. We find positive correlations between $J_c$ and $J_d$, and between $S$ and $G_i$, and compare them with trends across a broad range of superconductors.

发表机构

• University of Washington（华盛顿大学）
• Aichi Institute of technology（爱知工业大学）
• Seikei University（成蹊大学）
• Japan Fine Ceramics Center（日本精细陶瓷中心）

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