Energy gap and proximity effect in MgB₂ superconducting wires

Measurements of the penetration depth λ(T,H) in the presence of a dc magnetic field were performed in MgB₂ wires. In as-prepared wires λ(T,H < 130 Oe) shows a strong diamagnetic downturn below ≈ 10 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 λ(T) with an energy gap of 2Δ(0)/T_c ≈ 1.54 [Δ(0) ≈ 2.61 meV], 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.