Energy gap and proximity effect in (formula presented) superconducting wires

Measurements of the penetration depth (formula presented) in the presence of a dc magnetic field were performed in (formula presented) wires. In as-prepared wires (formula presented) shows a strong diamagnetic downturn below (formula presented) A dc magnetic field of 130 Oe completely suppressed the downturn. The data are consistent with proximity coupling to a surface Mg layer left during synthesis. A theory for the proximity effect in the clean limit, together with an assumed distribution of the Mg layer thickness, qualitatively explains the field and temperature dependence of the data. Removal of the Mg by chemical etching results in an exponential temperature dependence for (formula presented) with an energy gap of (formula presented) (formula presented) in close agreement with recent measurements on commercial powders and single crystals. This minimum gap is only (formula presented) of the BCS weak coupling value, implying substantial anisotropy.