Tunneling spectra of a superconductor–insulator–superconductor (SIS) junction formed by the break-junction technique were carried out for 2 different B-quality samples. One was a polycrystalline MgB2 pellet (RRR ≈ 4) and the other was a high-purity MgB2 wire (RRR ≈ 25) of approximately 150μm in diameter. Both samples exhibited a multiple-gap feature, which could be expressed by the correlated 2-gap model. From the conductance fitting at 4.2K, the pellet gap parameters were ΔS = 2.2meV, ΔM = 6.0meV and ΔL = 10meV, whereas the only clear and very reproducible gap in the wires was the small gap at ΔS = 2.5meV. The wires exhibited almost constant values for the fitting parameters of the 2-gap model, in contrast with the pellets that showed a large variation.

Break-Junction Tunneling Spectra of MgB2 - Influence of Boron Quality. R.A.Ribeiro, T.Ekino, T.Takasaki, T.Takabatake, J.Akimitsu: Physica C, 2005, 426-431[1], 450-3