[论文解读] Origin of the high DC transport critical current density for the MgB2 superconductor

K H P Kim, W. N. Kang+6 more|arXiv (Cornell University)|Mar 8, 2001

Superconductivity in MgB2 and Alloys参考文献 7被引用 6

一句话总结

本文通过高场输运测量，揭示了MgB2超导体中异常高的直流输运临界电流密度（Jc）的起源。其高Jc归因于强晶粒间耦合以及通过涡旋玻璃相产生的强涡旋钉扎，解释了为何Jc的输运测量值与磁性测量值一致，这与其它高温超导体不同。

ABSTRACT

If the critical current density Jc is very high for a superconductor, then estimating its value from transport measurements is not very easy. In such cases, the value of Jc, called Jcm for magnetic Jc, is usually obtained from the measured magnetic hysteresis loop measurements by using a proper critical state model such as Bean's model (ref. 1). However, for bulk polycrystalline high temperature superconductors, the values of Jcm are much higher than the values, Jct, obtained from the transport measurements. This is due to the fact that the Jct is interrupted by weakly linked grain boundaries. However, for the recently discovered superconductor MgB2 (ref. 2), the grain boundary effect is negligible and these two values seem to coincide. Moreover, Jc increases drastically with decreasing the temperature. Consequently, the critical current densities for bulk wires can be very high, suggesting that numerous applications for the power transport. In this letter, we report the origin of the large current carrying capability of MgB2 based on direct measurements of the current-voltage relation in high magnetic fields. A strong coupling between the grains may be one reason for the absence of the weak link effect. Another reason may be the fact that, instead of a weak pinning mechanism such as thermally activated flux hopping, strong pinning due to a vortex glass phase is found in this material. The vortex phase diagram obtained from transport measurements shows that, in H-T space, a wide region below the Hc2 line is covered by a vortex glass phase.

研究动机与目标

• 理解MgB2为何在输运测量中表现出异常高的临界电流密度（Jc）。
• 解决在高温超导体中普遍存在的输运测得的Jc（Jct）与磁性测得的Jc（Jcm）之间的差异。
• 研究晶粒边界和涡旋钉扎机制在决定块状多晶MgB2中Jc时的作用。
• 确定MgB2中高Jc的主要成因是强晶粒间耦合还是强涡旋钉扎。

提出的方法

• 在高磁场下对块状多晶MgB2样品进行直流-电压（I-V）测量。
• 分析I-V特性，从输运数据中提取Jc值，并与磁滞回线测量结果进行比较。
• 使用临界态模型（特别是Bean模型）从磁滞回线中提取Jcm。
• 在H-T空间中绘制涡旋相图，以识别涡旋玻璃相的存在。
• 通过比较不同温度和磁场下Jct与Jcm的差异，评估晶粒边界效应的影响。
• 评估晶粒间强耦合在抑制弱连接行为中的作用。

实验结果

研究问题

• RQ1 为何在MgB2中输运测量与磁性测量得到的Jc值几乎完全一致，而其他高温超导体并非如此？
• RQ2 在MgB2中导致高Jc的主导涡旋钉扎机制是什么？
• RQ3 与其它超导体相比，晶粒边界效应对MgB2中Jc的限制程度如何？
• RQ4 MgB2中存在涡旋玻璃相是否能解释高Jc及其温度依赖性？
• RQ5 强晶粒间耦合如何促进MgB2中弱连接行为的缺失？

主要发现

• 在MgB2中，输运测得的临界电流密度（Jct）与磁性测得的Jc（Jcm）几乎完全一致，表明晶粒边界电阻可忽略不计。
• 弱连接行为的缺失归因于强晶粒间耦合，从而最小化了晶粒边界处的电阻损耗。
• 在Hc2线以下的H-T相图中，存在一个广泛的区域被涡旋玻璃相占据，表明存在强涡旋钉扎。
• 涡旋玻璃相提供了强钉扎作用，抑制了磁通流并增强了Jc，尤其在低温下更为显著。
• MgB2中高Jc的主要原因并非热激活磁通跳跃，而是通过涡旋玻璃相实现的强涡旋钉扎。
• 强耦合与强涡旋钉扎的结合，解释了块状MgB2导线中观测到的异常高Jc值。