Break-junction tunneling spectra of MgB2: Influence of boron quality

Tunneling spectra of a superconductor-insulator-superconductor (SIS) junction formed by the break-junction technique have been carried out for two different boron quality MgB2 samples. One is a polycrystalline MgB2 pellet (RRR ≈ 4) and the other is a high purity MgB2 wire (RRR ≈ 25) of approximately 150 μm in diameter. Both samples exhibit a multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fitting at 4.2 K, the pellet gap parameters are ΔS = 2.2 ± 0.3 meV, ΔM = 6.0 ± 1.5 meV and ΔL = 10 meV, whereas the only clear and very reproducible gap in the wires is the small gap at ΔS = 2.5 ± 0.1 meV. The wires show almost constant values for the fitting parameters of the two-gap model, in contrast with the pellets that show a large variation.