High Residual Resistivity Ratio Research Articles

Characterization of vanadium diffusion barriers in Nb-Sn composite wires

Ductile vanadium has been explored for use as a diffusion barrier material in niobium-tin composite wires. Vanadium is found to co-process well, standing over 99.995% area reduction without anneal. Resistance ratios greater than 300 to 1 (0°c to 20K) have been measured in reacted composite wire with noncopper current densities greater than 1700 A/mm2at 10 Tesla, 4.2K. Vanadium forms an intermetallic compound on the tin side which should not be superconducting above 4.2K. The composite residual resistance ratio (RRR) is seen to be dependent on reaction conditions being higher as the reaction temperature is dropped. To determine the source of this effect a copper-vanadium composite was fabricated. Features similar to the superconducting wire were observed. There is a reasonable overlap of reaction conditions suitable for both the superconductor and the copper making high RRR possible for optimized conductors.

Electronic relaxation rates of copper, aluminum, and potassium determined from the temperature dependence of the anomalous surface impedance

Changes in frequency of an extremely stable isothermal $\mathrm{LC}$ oscillator are associated with the temperature dependence of the surface impedance of a thermally isolated metallic sample placed within the inductor. Even in the anomalous skin-effect regime, where the electronic mean free path is much greater than the rf skin depth, there is a small yet measurable temperature dependence to the surface impedance. The formulas for the surface impedance (valid for the anomalous through classical skin-effect regimes) by Reuter and Sondheimer are used to extract, from the changes in frequency of the oscillator, information related to electronic relaxation rates. The electronic relaxation rates determined in this way are compared to relaxation rates obtained from published dc electrical resistivity data. Cu, Al, and K have yielded consistent temperature dependencies with respect to the dc measurements in certain temperature ranges. A comparison of dc electrical conductivity and surface-impedance measurements is also made through the use of the theoretical work of Manz, Black, Pashaev, and Mills, who find, for the temperature-dependent surface impedance in the extreme anomalous limit, a new relaxation rate different from that in the relaxation-time approximation. This new relaxation rate in certain limits is predicted to be proportional to that characterizing the dc electrical resistivity. The results obtained from copper are the best verification of this prediction. Data from aluminum are about a factor of 5 greater than the model-dependent prediction of Manz et al. Difficulty in achieving a high residual resistance ratio in potassium prevents a comparison through the theory of Manz et al. However, a ${T}^{2}$ dependence observed with potassium at low temperatures is about 150 times greater than that observed in recently reported dc electrical resistivity data. A further theoretical investigation by Black and Mills examined the contribution of electron-electron normal scattering processes to surface impedance in the extreme anomalous limit. Their findings suggest that these electron-electron $N$ processes may fully contribute to the surface-impedance relaxation rate. Our copper and potassium data qualitatively support this suggestion.

Oxygen annealing of silver for obtaining low electrical resistivity: technique and interpretation

A single crystal Ag rod has been annealed in stages in a deliberate but low oxygen partial pressure. The change in the residual resistivity ratio (RRR) after each anneal is reported. During the course of the anneals the RRR is increased from 140 to 9500. The details of the annealing procedure are given, and the dynamics of internal oxidation of impurities in Ag are discussed. It is shown that for achieving high RRR, high annealing temperatures and low partial pressure of oxygen should be employed. In addition, the necessity of including size effects in the interpretation of electron transport phenomena in incompletely oxygen annealed samples is demonstrated.