Fabrication of multiband MgB2 tunnel junctions for transport measurements

The coexistence of multiple superconducting condensates in a material gives rise to intriguing phenomena, such as the possible presence of number-phase fluctuations. In this work, tunnel junctions were fabricated, aiming for the detection of such phenomena. Planar junctions with normal conducting and superconducting counter-electrodes have been realized. It was found from spectroscopic investigations that the MgO barrier used for the planar junctions is not ideal. To optimize for the two-band behaviour of MgB2, ab-plane ramp-type MgB2 tunnel junctions were fabricated. The use of Al2O3 barriers substantially improved the homogeneity of the junctions, as was concluded from the nearly ideal magnetic field dependence of the critical current and excellent scaling of the junction properties with the junction widths. The current–voltage characteristics show clear Shapiro steps under microwave irradiation. However, the spectrum showed a degradation of the MgB2 in the ramp region and the signature of the σ-gap was absent. For tunnel junction spectroscopy in the ab-direction and the detection of new, two-band related modes, the fabrication process still has to be further optimized.