Multiple-gap features from break-junction tunneling in the superconducting MgB₂

Tunneling spectroscopy measurements on polycrystalline MgB₂ have been carried out with a break-junction technique. The observed spectra exhibit the multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fittings at 4.2 K, the gap parameters can be classified into Δ_S = 2:2 ± 0:3 meV, Δ_M = 6:0 ± 1:5 meV, and Δ_L = ≅ 10 meV. All these gaps exist up to the same critical temperature T_c. The Δ_M undergoes various ways of temperature dependence, which can be attributed to the proximity-induced gap. We have also observed the conductances showing the features of normal conducting and/or low-T_c phases. Our tunneling data can be explained by the proximity effect combined with the bulk multiple-gap features.