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Multiple-Gap Features from Break-Junction Tunneling in the Superconducting MgB2

Tunneling spectroscopy measurements on polycrystalline MgB 2 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 \(\Delta_{\text{L}}\cong 10\) 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.