Amorphous superconductingZrxCu1−x: Electronic properties, stability, and low-energy excitations

Abstract

Amorphous ${\mathrm{Zr}}_x{\mathrm{Cu}}_{1-x}$ alloys ($0.50\le x\le 0.74$) prepared by melt-spinning are investigated by means of electrical and thermal measurements. The negative temperature coefficient of the electrical resistivity suggests that the criterion $2k_F\approx k_p$ is always fulfilled in this system [$k_F$ is the Fermi wave number, $k_p$ is the position of the first maximum of the structure factor $S\left(k\right)\mathrm{}$]. The high density of states $N_{\gamma }\left(E_F\right)$ as inferred from specific-heat measurements originates from the $d$ electrons of Zr. The bare density of states [determined from $N_{\gamma }\left(E_F\right)$ with the aid of the electron-phonon coupling constant $\lambda$] is 3 times higher than that calculated from the free-electron model and almost independent of the Zr concentration. Phonon-electron scattering as determined by thermal-conductivity measurements shows a similar deviation from free-electron behavior. A decisive test of the Nagel-Tauc condition for high (meta-) stability of the metallic glasses is not possible due to the apparent failure of both free-electron and rigid-band models. All samples investigated are superconducting and belong to the extreme-type-II superconductors. Fluctuation-induced paraconductivity extends up to $1.5T_c$. An analysis of the electron-phonon coupling constant $\lambda$ as determined from $T_c$ indicates that the electron-ion matrix element $〈I_2〉$ varies strongly across the ${\mathrm{Zr}}_x{\mathrm{Cu}}_{1-x}$ series. The size of $\lambda$, the jump of the specific heat at $T_c$, and its drop at low temperatures show that Zr-Cu alloys are weak- to intermediate-coupling superconductors. The relatively high $T_c$ values of the Zr-rich metallic glasses allow the observation of a linear specific-heat term for $T\ll T_c$ in all these samples which is due to localized low-energy excitations. These excitations are also seen—via strong phonon scattering—in the thermal conductivity.