The widely variable resistivity of MgB2 samples
The widely variable resistivity of MgB2 samples
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DOI：
10.1088/0953-2048/16/6/201
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发表时间：
2003
影响因子：
3.6
通讯作者：
John M Rowell
中科院分区：
文献类型：
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作者：
John M Rowell
在这篇简短的评论中，我考虑了MgB2的一个相当普通的性质，即它的电阻率作为温度的函数。事实证明，从单晶到薄膜、线和多晶块体的各种样品的电阻率数据的比较是令人惊讶的。大多数的MgB2样品表现出比在相对少量的单晶、体样品和薄膜中看到的低值高得多的结晶度。在许多情况下，电阻率比单晶值增加几个数量级。即使在这些高值下，电阻率通常仍然具有金属温度依赖性，并且令人惊讶的是，Tc通常处于或接近39 K的体值。电阻率的增加归因于样品的有效载流横截面积的减少。如果横截面积的这种损失是导致MgB2样品电阻率增加的主要因素，那么临界电流密度必须降低有效面积中的相同损失。无序的“簇化合物”超导体的性质，高温超导材料和颗粒状铝（严重污染的氧）的MgB2电阻率的比较，显示MgB2的行为和高温超导和铝的研究结果之间的相似性。我讨论了各种影响，可能有助于减少在MgB2样品中的有效样品面积，电阻率的增加和减少Jc。我推测MgB2晶界的约瑟夫森结模型可能适用于具有极高介电常数的样品。或者，有人提出，MgB 2的双带性质可能导致其电阻率和Tc的异常行为，因为材料从“干净”到“脏”的限制。我的结论是，在各种各样的散装和薄膜样品的电阻率和运输临界电流密度的测量将是非常有用的，因为将作为辐射损伤的函数在MgB2散装和最初的低电阻率的薄膜的电阻率和Jc变化的研究。
In this brief review, I consider a rather mundane property of MgB2, namely its resistivity as a function of temperature. It turns out that a comparison of resistivity data for a wide variety of samples, from single crystals to films, wires and polycrystalline bulk, is surprisingly informative. The majority of samples of MgB2 exhibit resistivities that are much higher than the low values that are seen in a relatively small number of single crystals, bulk samples and films. In many cases, the resistivity is increased over the single crystal values by orders of magnitude. Even at these high values, there is often still a metallic temperature dependence of the resistivity, and surprisingly, Tc is often at or near the bulk value of 39 K. The resistivity increase has been ascribed to a reduction in the effective current-carrying cross-sectional area of the sample. If this loss of cross-sectional area is the dominant factor contributing to the increase in resistivity of MgB2 samples, then the critical current density must be decreased by the same loss in the effective area. Comparisons of the MgB2 resistivity with the properties of disordered ‘cluster compound’ superconductors, of HTS materials and of granular Al (heavily contaminated with oxygen), show similarities between the MgB2 behaviour and the results of the HTS and Al studies. I discuss various effects that might contribute to the reduction in effective sample area in MgB2 samples, to the increase in resistivity and to a reduction in Jc. I speculate that a Josephson junction model of the grain boundaries in MgB2 might apply to samples with extremely high resistivities. Alternatively, it has been suggested that the two-band nature of MgB2 can result in an unusual behaviour of its resistivity and Tc as the material changes from the ‘clean’ to ‘dirty’ limits. I conclude that measurements of both the resistivity and the transport critical current density in a wide variety of bulk and film samples would be very useful, as would a study of the resistivity and Jc changes as a function of irradiation damage in MgB2 bulk and films of initially low resistivity.