Superconductivity and the electronic structure of Zr- and Hf-based metallic glasses

The results of a comprehensive study of the superconducting transition temperatures of Zr- and Hf-based metallic glasses are reported. The microscopic origins of superconductivity in these glasses are discussed in terms of recent ultraviolet photoelectron spectroscopy (UPS) measurements and calculations based on the renormalized atom technique. These calculations accurately predict the UPS spectra and the results of low-temperature heat-capacity measurements. The complete description of the electronic structure afforded by these calculations allows, for the first time, a consistent picture of the variation of $T_c$ with $X$ in the glasses ${\mathrm{Zr}}_{1-y}{X}_y$ ($X=3d or 4d$ transition metals). In addition, the dependence of $T_c$ with composition ($y$) can be understood in terms of the $X$ $d$ subband positions relative to $E_F$. The results reported here support our recent contention that the strong depression of $T_c$ observed for $X=\mathrm{F}\mathrm{e},\mathrm{M}\mathrm{n},\mathrm{C}\mathrm{r},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{V}$ glasses is related to the formation of localized magnetic moments and spin fluctuations. An alternate explanation for the low $T_c$ of $X=\mathrm{V}\mathrm{and}\mathrm{Cr}$ glasses based on the idea of an atomic-structure change is also discussed.