Effects of Dislocations on the Superconducting Transition Temperature of Aluminum and Aluminum Alloys

The effect of dislocations on the superconducting transition temperature $T_c$, and residual resistivity ratio $\rho$ of pure aluminum and aluminum containing dilute concentrations of chemical impurities has been examined. Dislocations were produced in the pure metal by cold-working, quenching, deformation in uniaxial tension, and fatigue. Each of these methods of deformation produces a known kind of dislocation and dislocation arrangement. In every case in which an increase in $\rho$ was observed, there was a corresponding decrease in transition temperature $\Delta T_c$, from the pure-metal value. Annealing of the cold-worked metal in the proper temperature range produced a decreasing $\rho$ and a decreasing $\Delta T_c$. For every method of deformation and at every stage of annealing, the correlation between electron scattering and transition temperature ($\Delta T_c$ versus $\rho$) was the same as with low-concentration electropositive impurities. For the deformation of impure aluminum, it was found that dislocations plus zinc (electropositive) impurities showed a correlation between $\rho$ and $\Delta T_c$ which was the same as if the total scattering were due to zinc impurities. For dislocations plus germanium or silicon (electronegative) impurities, $\Delta T_c$ is greater than one would obtain using the measured $\rho$ and the curves for germanium or silicon alone. The latter results are discussed on the basis of the work of Markowitz and Kadanoff, and it is shown that the portion of $\Delta T_c$ which can be associated with changes in the anisotropy of the energy gap correlates with changes in electron mean free path in the manner expected.