Enhancement of Superconductivity in Aluminum Films

Mixtures of Al and ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ were sputtered at room temperature, while mixtures of Al and Ge were sputtered at 77 °K yielding, respectively, a maximum transition temperature of 2.5 and 3.6 °K. The phonon spectrum of an Al film with 3.6 at.% $\mathrm{Ge}(T_c=3.\mathrm{}48 {}_{}{}^{\circ }{}_{}{}^{}\mathrm{K})$ is very similar to that of normal $\mathrm{Al} (T_c=1.\mathrm{}2{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K})$ and only slightly lower in energy. The magnitude at 0 °K and the temperature dependence of the energy gap of the enhanced Al are extremely well fitted by the BCS theory. Density measurements were performed in two different ways on the two mixtures (10 wt% of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ and 10 wt% of Ge) with maximum $T_c$. Density measurements by a direct weighing technique yielded a decrease in density of 5.5%. Electron diffraction experiments showed that the lattice spacing increased from 4.05 (normal aluminum) to 4.19 Å for the most enhanced aluminum, which corresponds to a 10.5% increase in volume. The observed increase in volume is sufficient to explain the increase in $T_c$ without invoking any other mechanism.