The voltage-dependent tunneling conductance of trapezoidal potential barriers has been calculated using two extreme models of (1) the WKB approximation and (2) perfectly sharp boundaries between the metal electrode and the insulator. We show that for both models the conductance-voltage plot is roughly parabolic at low voltages (;S 0.4 V). The minimum conductance is not at zero bias unless the barrier is symmetrical and identical Fermi energies are chosen for the two metal electrodes. The inclusion of image forces does not radically alter the shape of the conductance-voltage dependence. Using reasonable barrier shapes, the asymmetry of the calculated conductance about V =0 is not as large as we frequently observe experimentally. We point out that this extreme asymmetry appears to be associated with the presence of organic impurities in the oxide layer of the junction.

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.

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A new family of rare earth-rhodium-tin intermetallic compounds, with the representative formula (RE)Rh~Sn~, has been synthesized in single crystal form. The compounds containing the heavier rare earths are superconducting and those with the lighter rare earths are generally magnetic. The compound ErRh Sn 36 exhibits reentrant superconductivity with T~= 0.97 K and Tm = 0.57 K as determined from ac magnetic susceptibility measurements. The synthesis and X-ray characterization of the series are described and the results of electrical resistivity, upper critical magnetic field, magnetic susceptibility, specific heat and neutron scattering measurements on the Er compound are given.