The widely variable resistivity of MgB₂samples

Abstract

In this brief review, I consider a rather mundane property of MgB 2 , 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 MgB 2 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, T c 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 MgB 2 samples, then the critical current density must be decreased by the same loss in the effective area. Comparisons of the MgB 2 resistivity with the properties of disordered 'cluster compound' superconductors, of HTS materials and of granular Al (heavily contaminated with oxygen), show similarities between the MgB 2 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 MgB 2 samples, to the increase in resistivity and to a reduction in J c . I speculate that a Josephson junction model of the grain boundaries in MgB 2 might apply to samples with extremely high resistivities. Alternatively, it has been suggested that the two-band nature of MgB 2 can result in an unusual behaviour of its resistivity and T c 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 J c changes as a function of irradiation damage in MgB 2 bulk and films of initially low resistivity.

Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

“…22 We observed that A F of the silicone oil doped sample was larger than that of the malic acid doped sample, as well as the nanocarbon doped sample. As can be seen in Table I, A F values ranged from as low as 0.111 ͑nano-C doped MgB 2 ͒ to as high as 0.249 ͑Si-doped MgB 2 ͒, depending on the dopant.…”

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

“…7 First, we consider the area fraction. This was investigated by state-of-the-art 50-nm-resolution X-ray tomography, as shown in Figure 2.…”

“…The reason why the low-field properties were not improved has been studied, and the lack of enhancement has been mostly attributed to porosity. 7,8 The chemical reaction between magnesium and boron results in about 30% volume reduction, and porosity is known to be difficult to avoid. However, no quantitative analysis has ever been carried out.…”

Fabrication and Transport Critical Current Densities of MgB2/Fe/Cu Multifilament Tapes without Any Intermediate Annealing

“…The active cross-sectional area fraction (A F ) values can be estimated from resistivity measurements by using Rowell's connectivity analysis [30],…”