High Resolution Measurements of the Superconducting Energy Gaps of Assymetrical all-MgB₂Thin Film Josephson Junctions

We have previously analyzed substructure within the two superconducting energy gaps of Magnesium diboride (MgB ) using tunneling spectroscopy [1,2]. The samples consisted of 1-gap(Pb/Sn)/2-gap(MgB₂) heterojunctions. Here, we report similar measurements of all-MgB₂ Josephson junctions consisting of two c-axis MgB films with very small contribution from the larger σ gap. Due to differences in growth conditions, the two MgB electrodes have different T's and gap values. We analyze data using a modified tunneling model where each electrode is represented as a weighted sum of two BCS densities of states. Our results show (1) a transition from SIS to NIS behavior with temperature and (2) the presence of multiple quasiparticle peaks due to the sums and differences in pairwise combinations of π and σ gap values within each electrode.

¹“Energy Gap Substructures in Conductance Measurements of MgB₂-based Josephson Junctions” , S. Carabello, et al., Supercond. Sci. Technol. 28 (2015) 055015.
² "Momentum-dependent multiple gaps of MgB₂ probed by electron tunneling spectroscopy on MgB₂/native oxide/Pb junctions”, K. Chen, et. al, Nat. Commun. 3 : 619 doi: 10.1038/ncomms1626 (2012).