Energy gap and proximity effect in ${\mathrm{MgB}}_2$ superconducting wires

Measurements of the penetration depth $\lambda \mathrm{}(T,H)\mathrm{}$ in the presence of a dc magnetic field were performed in ${\mathrm{MgB}}_2$ wires. In as-prepared wires $\lambda (T,H<\mathrm{}130\mathrm{}\hspace{0.5em}\mathrm{Oe})$ shows a strong diamagnetic downturn below $\approx 10\hspace{0.5em}\mathrm{K}.$ 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 $\lambda \mathrm{}\left(T\right)\mathrm{}$ with an energy gap of $2\Delta {\mathrm{}\left(0\right)/T}_c\approx 1.54$ $\left[\Delta \mathrm{}\right(0\left)\mathrm{}\approx 2.61\hspace{0.5em}\mathrm{meV}\right],$ in close agreement with recent measurements on commercial powders and single crystals. This minimum gap is only $44\%$ of the BCS weak coupling value, implying substantial anisotropy.