G.H. Wostenholm

The resistivity of the β-W compounds

Abstract The very marked negative curvature in the resistance characteristics of superconducting β-W type compounds, such as Nb 3 Sn, has traditionally been associated with the existence of a very sharp peak in the electronic density of states at the Fermi level, but this explanation has been challenged recently by several authors. In this work the Bloch-Mott-Wilson model for the resistivity of transition metals is modified to account for the negative curvature and the more limited variation of resistance observed for less ideal samples. It is assumed that conduction by d-electrons is generally significant. The rate of scattering of d-electrons by phonons is shown to be reduced when the mean free path of the d-electrons is very short, thus limiting the intrinsic resistivity when the total resistance is rather high. Plausible agreement is demonstrated between the corresponding expression for the resistivity and the published data.

Superconductivity of molybdenum and tungsten carbides

Abstract The superconducting critical temperatures of a total of 18 samples of α-Mo2C, β-Mo2C, η-MoC1−x and α-W2C have been determined resistively. The critical temperatures lie within the ranges 6°–7.3°, 5.1°–7.2°, 7.4°–8.9° and 2.95°–3.05°K, respectively. The critical temperature for β-Mo2C is much higher than found previously, and similar to that for α-Mo2C.

The thermal and electrical conductivities of niobium-65% titanium alloys

The thermal and electrical conductivities of samples of as-rolled and annealed 35% Nb-65% Ti alloy have been measured between 4 and 20K and between the superconducting critical and 270K respectively. Analysis shows that the thermal conductivity is almost entirely due to phonon carriers, with the magnetic primarily limited by electron-phonon scattering by an unusually small number of electrons. The electrical resistivity data are analyzed to show that s-d interband scattering occurs most frequently. The enhancement of the electrical conductivity due to the existence of short-range superconducting order above the bulk critical temperature is discussed.

Superconductivity and the structure of W2C

The superconducting critical temperatures of samples of α, β and γ-W2C and the low temperature orthorhombic phase recently described by Telegus et al. have been determined. The critical temperatures lie within the ranges 3.05–3.35, 3.1–3.85, 2.85–3.05 and 2.4–4.05 K, respectively. The critical temperatures of samples of the orthorhombic β and Telegus et al. forms are generally higher than found previously for W2C.

The anomalous low temperature lattice thermal conductivity of Nb3Sn and Nb3Al

Abstract The thermal conductivity was measured at temperatures in the range approximately 2–27 K for arc-melted samples of the β-W-type compounds Nb 3 Sn and Nb 3 Al. The samples had relatively high electrical resistivities, and the electronic carrier contribution to the thermal conductivity was correspondingly small, allowing the lattice component of the thermal conductivity to be estimated fairly unambiguously. Both samples exhibited atypical variations in the lattice thermal conductivity, with distinct anomalies occurring at temperatures of approximately 14 K and 10 K for Nb 3 Sn and Nb 3 Al respectively. This behaviour apparently correlates with the anomalous heat capacity and intrinsic resistivity variations observed previously for these materials.

The anomalous low temperature lattice thermal conductivity of VP3Sn and related compounds

Abstract The thermal conductivity has been measured below 20 K for the β-W type compounds V3Sn, V2,4 Ti0.6Sn, V3Sn0.5Al0.5 and hexagonal Ti3Sn. The thermal conductivity varies normally at temperatures below an anomaly lying at temperatures in the range 5.5 to 10 K, which may be related to the Batterman-Barrett martensitic transformation. At higher temperatures, the lattice thermal conductivity component varies approximately linearly with temperature, which correlates with the anomalous lattice heat capacities and intrinsic resistivities reported previously for β-W type compounds.

The resistivity of V3Sn and related compounds

Abstract The resistivities of the superconducting β-W-type compounds V3Sn, V3Sn0.5A10.5 and V2.4Ti0.6Sn have been measured between Tc and 300 K. The resistivities exhibit a saturation effect which is particularly marked for the ternary compounds. The resistivity varies approximately as Tr from 20 to 50 K, where r lies in the range 1.05 – 1.64, in contrast with the T2 variation reported for Nb3Sn. A semi-empirical expression for the resistivity is shown to be obeyed closely at all temperatures. The magnitude of the resistivity of hexagonal Ti3Sn is shown to be similar to V3Sn, although a T2.85 variation is obeyed from 20 to 50 K in this case.

The lattice thermal conductivity of superconducting vanadium — titanium alloys

Abstract The thermal conductivities have been measured for concentrated vanadium-titanium alloys in the superconducting and normal phases below 15 k. The thermal carriers are mainly phonons, and the thermal conductivity is limited primarily by scattering by electrons and statistical concentration fluctuations.

The electrical resistivity of niobium and niobium-zirconium alloys

The electrical resistivity of samples of niobium and 75% Nb 25% Zr alloy have been measured between the superconducting critical temperatures and room temperature. The temperature dependent component of the resistivity has been compared with phenological equations, and it is shown that phonon induced transitions between s and d bands are largely responsible for the observed resistivity. The effect of finite fluctuations of the superconducting order parameter is discussed.

The low-temperature thermal conductivity of niobium-zirconium alloys

The thermal conductivities of two samples of nominal 75% Nb-25% Zr alloy have been measured below 20K. The measured conductivities are substantially higher than found previously for segregated niobium-zirconium alloys, and the phonon carrier component is found to be completely dominant at all temperatures. The effective number of electrons scattering the phonons is unusually small.