Measurement of the pair potential in aligned Pb-Cd lamellar eutectic composites

Thermal-conductivity measurements have been used to study the pair potential in multilayer superconductor-normal-metal lamellar composites. Lamellar Pb-Cd eutectic samples were prepared by a high-speed directional solidification technique which generates large single-grain samples with all the lamellas lined up in the same direction. For thin Cd layers where the pair potential $\Delta$ is expected to be uniform throughout the Cd, the pair potential is found to be ∼40% of the value in bulk Pb. This gives direct experimental evidence that the boundary condition requireing continuity of $\frac{\Delta }{\mathrm{NV}}$ applies in the clean regime. Surprisingly, the electron mean free path in the Cd is longer than the width of the lamellas implying that Andreev scattering is suppressed at the Pb-Cd interface.