Upper critical fields and superconducting transition temperatures of some zirconium-base amorphous transition-metal alloys

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Abstract
Superconducting upper critical fields Hc2(T), transition temperatures Tc and normal-state electrical resistivities ρn have been measured in the amorphous transition-metal alloy series Zr1−xCox, Zr1−xNix, (Zr1−xTix)0.78Ni0.22, and (Zr1−xNbx)0.78Ni0.22. Structural integrity of these melt-spun alloys is indicated by x-ray, density, bend-ductility, normal-state electrical resistivity, superconducting transition width, and mixed-state flux-pinning measurements. The specimens display Tc=2.1−3.8 K, ρn=159−190 μΩ cm, and |(dHc2dT)Tc|=28−36 kG/K. These imply electron mean free paths l≈2−6 Å, zero-temperature Ginzburg-Landau coherence distances ξG0≈50−70 Å, penetration depths λG0≈(7−10)×103 Å, and extremely high dirtiness parameters ξ0l≈300−1300. All alloys display Hc2(T) curves with negative curvature and (with two exceptions) fair agreement with the standard dirty-limit theory of Werthamer, Helfand, Hohenberg, and Maki (WHHM) for physically reasonable values of spin-orbit-coupling induced, electron-spin-flip scattering time τso. This is in contrast to the anomalously elevated Hc2(T) behavior which is nearly linear in T that is observed by some, and the unphysically low-τso fits to WHHM theory obtained by others, for various amorphous alloys. Current ideas that such anomalies may be due to alloy inhomogeneity are supported by present results on two specimens for which relatively low-