The widely variable resistivity of MgB2 samples

Abstract

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.