[论文解读] Two band superconductivity in MgB2: basic anisotropic properties and phase diagram

Manuel Angst, R. Puźniak|arXiv (Cornell University)|May 2, 2003

Superconductivity in MgB2 and Alloys参考文献 1被引用 5

一句话总结

本文通过单晶的扭矩磁测量研究了MgB2中的双带超导性，揭示了上临界场Hc2的强场依赖性和温度依赖性各向异性，其行为偏离了各向异性金兹堡-朗道理论。关键发现是涡旋物质在约0.85Hc2处表现出峰值效应，表明存在有序-无序转变，场依赖的有效各向异性源于各向异性σ带与各向同性π带中超导性的差异抑制。

ABSTRACT

Magnesium diboride MgB2 has been an extraordinarily hot research topic since the discovery of superconductivity below 40 K, due to the vast amount of unusual properties originating from the involvement in superconductivity of two sets of bands that are of a different dimensionality. We review, with a strong focus on results obtained by torque magnetometry, how this leads to a complex behavior of the anisotropic superconducting state properties. The different dimensionality of the two sets of bands is manifested in the upper critical field Hc2 anisotropy, which was found to strongly decrease with increasing temperature. While the angular dependence of Hc2 follows roughly the predictions of anisotropic Ginzburg-Landau theory AGLT, small, but systematic deviations were observed near Tc. This, and the temperature dependent anisotropy, witness a breakdown of the AGLT description of MgB2, even close to Tc. Theoretical calculations are in qualitative agreement with the observed angle dependence, but suggest a much lower penetration depth anisotropy at low temperatures. Reversible torque vs angle curves in the mixed state indicate that the penetration depth anisotropy in intermediate fields has to be higher than currently available theoretical estimates. The observed field dependent effective anisotropies can be accounted for qualitatively by the faster depression of superconductivity in the more isotropic bands by the applied field. The vortex matter phase diagram is drawn, based on measurements of the reversible and irreversible torque in the mixed state. In the irreversible torque, a peak effect (PE) was observed in fields of about 0.85 Hc2. History effects of the critical current density in the PE region suggest that the PE signifies an order-disorder transition of vortex matter.

研究动机与目标

• 理解MgB2中双带超导性与各向异性混合态特性之间的相互作用。
• 通过角度和温度依赖的Hc2测量，研究MgB2在Tc附近各向异性金兹堡-朗道理论的失效机制。
• 通过可逆与不可逆扭矩测量，确定涡旋物质相变的本质，包括峰值效应。
• 将实验测得的各向异性数据与双带超导性的理论模型及涡旋芯重叠模型相协调。
• 评估对MgB2在高场应用中临界电流密度和材料制备的影响。

提出的方法

• 在高质量MgB2单晶上进行扭矩磁测量，以测量随磁场和温度变化的可逆与不可逆扭矩。
• 通过分析混合态中扭矩信号的角度依赖性，绘制出上临界场Hc2(θ,T)。
• 以各向异性金兹堡-朗道理论（AGLT）作为理论框架进行对比，识别出在Tc附近出现的偏离。
• 通过观察磁场取向引起的扭矩响应，分析场依赖的有效各向异性，将其与σ带和π带中超导性的差异抑制联系起来。
• 基于扭矩数据构建涡旋物质相图，识别出峰值效应和历史依赖的临界电流行为。
• 将实验结果与γH(T)和γλ(T)的理论预测进行比较，突出渗透深度各向异性中的差异。

实验结果

研究问题

• RQ1 MgB2中的双带超导性如何影响Hc2的角度和温度依赖性？其与各向异性金兹堡-朗道理论的偏离程度如何？
• RQ2 混合态中观察到的场依赖性和温度依赖性的有效各向异性由何引起？其与σ带和π带不同行为的关系是什么？
• RQ3 在约0.85Hc2处临界电流密度中观察到的峰值效应的起源是什么？它是否标志着涡旋物质的相变？
• RQ4 渗透深度各向异性γλ与上临界场各向异性γH相比如何？这对涡旋芯结构有何含义？
• RQ5 热涨落和材料无序在多大程度上影响MgB2中涡旋物质的转变？它们如何影响临界电流性能？

主要发现

• MgB2中上临界场各向异性γH(T)随温度升高显著减小，在Tc附近明显偏离各向异性金兹堡-朗道理论的预测。
• 在Tc附近Hc2的角度依赖性中观察到系统性偏离AGLT，表明即使在低场下该理论也已失效。
• 扭矩测量表明，混合态中的有效各向异性为场依赖性，无法用单一各向异性参数解释，暗示σ带与π带的超导性存在差异抑制。
• 在不可逆扭矩测量中，临界电流密度在约0.85Hc2处观察到峰值效应，表明涡旋物质发生了有序-无序转变。
• 中间场中测得的有效各向异性暗示渗透深度各向异性γλ高于当前理论估计值，表明需要更精细的模型。
• 理论计算表明，在低温下γH与γλ之间存在显著差异，且γλ保持远小于γH，凸显了一个关键的未解决定量差异。