Abstract
We show that the growth, composition, and superconductive properties of ultrathin $\textnormal{Pb}_{1-x}\textnormal{Ga}_x$ alloy films depend strongly on the atomic structure of the substrate interface. Whereas alloy films on the Si(111)-($7\times7$) surface grow in multilayers, reminiscent of the quantum growth phenomenon observed in pure Pb layers, alloy films on the Si(111)-$(\sqrt3 \times \sqrt3)R30^\circ$-Pb and Si(111)-$(\sqrt3 \times \sqrt3)R30^\circ$-Ga interfaces gradually switch to classical layer-by-layer growth. The ($7\times7$) interface is unique in that it enables formation of atomically smooth alloy films containing up to 6\% of Ga, which is far beyond the solid solubility limit. In contrast, the ($\sqrt3 \times \sqrt3$) interfaces promote phase separation. The contrasting influences of these interfaces are also reflected by their superconductive properties, most notably by the differences in the critical current density, exceeding more than one order of magnitude.