Effect of Transition-Metal Impurities on the Critical Temperature of Superconducting Al, Zn, In, and Sn

Abstract

The depression in critical temperature $T_c$ of several dilute solid solutions of transition elements in Al, Zn, In, and Sn was measured as a function of residual resistivity and concentration. In Zn-Mn and Zn-Cr, localized moments are present and $T_c$ decreases linearly with concentration with a slope $-\frac{d{T}_c}{\mathrm{dc}}$ of 315 and 170°K/at.%, respectively. By comparing these numbers with theory, a value of about 1.4 eV was found for the $s-d$ exchange integral. In all other alloys, the critical temperature is still depressed but much less than in Zn-Mn and Zn-Cr, and the depression becomes less rapid with increasing concentration of impurities. However, no saturation is found up to the maximum concentrations reached. The effect here found is different from that taking place in dilute alloys containing nontransition metals in that the depression of $T_c$ is larger, giving rise to suppression of superconductivity at sufficiently high impurity concentration. An analysis of the data for Al-Mn, Al-Cr, Al-Fe, Zn-Fe, Zn-Co, and Zn-Ni shows that in these systems the observed decrease of $T_c$ is satisfactorily accounted for by the smoothing out of the gap anisotropy and by the formation of localized virtual states, whose effect on the superconducting properties has been studied theoretically by Zuckermann. In-Mn and Sn-Mn do not fit the foregoing picture; however, the results obtained rule out the presence of magnetic moments in these two systems in bulk form. The apparent contrast with the results obtained in thin films is discussed. Some results on ternary alloys are also reported. It is shown that no effect on the critical temperature of In-Mn dilute solutions is produced by the addition of Pb.