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

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 Zr_1-xCoₓ, Zr_1-xNiₓ, (Zr_1-xTiₓ)0.78Ni0.22, and (Zr_1-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 {μ}{Ω} cm, and |(dHc2dT)_Tc|=28-36 kG/K. These imply electron mean free paths l≈2-6 {}, zero-temperature Ginzburg-Landau coherence distances ξG0≈50-70 {}, penetration depths λG0≈(7-10)×10³ {}, 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-τₛₒ fits of Hc2(T) to WHHM theory are coupled with superconductive evidence for inhomogeneity: relatively broad transitions at Tc and Hc2 current-density-dependent transitions at Hc2 and (in one specimen) a J-dependent, high-H (>Hc2), resistive "beak effect." In the Zr_1-xCoₓ and Zr_1-xNiₓ series, Tc decreases linearly with x (and with unfilled-shell average electron-to-atom ratio 〈e/a〉 in the range 5.05≤〈e/a〉≤6.40 in fair agreement with previous results for these systems and contrary to the Tc vs 〈e/a〉 behavior of both amorphous and crystalline transition-metal alloys formed between near neighbors in the Periodic Table. Upper-critical-field and normal-state electrical resistivity measurements suggest that the molar electronic specific-heat coefficient γₘ decreases with x in parallel with Tc in the Zr_1-xCoₓ and Zr_1-xNiₓ series. In the equal-〈e/a〉 (Zr_1-xTiₓ)0.78Ni0.22 system, Tc decreases with x; in the (Zr_1-xNbₓ)0.78Ni0.22 system, Tc first increases and then decreases with x (hence with 〈e/a〉). These diverse 〈e/a〉 dependencies of Tc appear consistent with the ultraviolet-photoemission-spectroscopy indicated split-band model of such amorphous transition-metal alloys and the associated idea that the alloying dependence of Tc cannot be described by general Tc vs 〈e/a〉 rules.