Determining the Energy Gaps of Assymetrical All-MgB2 Thin Film Josephson Junctions

We have previously measured at very high resolution the momentum-dependent substructure within the two superconducting energy gaps (σ and π) of Magnesium diboride (MgB2 ) using tunneling spectroscopy of MgB2 heterojunctions [1,2]. In this presentation, we report tunneling spectroscopy measurements of all-MgB₂ thin film Josephson junctions. These were fabricated using two c-axis MgB2 films with very small contribution from the larger σ gap. The two MgB electrodes on either side of the junction show different critical temperatures and energy gap values. We attribute this to differences in growth conditions. Using a modified tunneling model where each electrode is represented as a weighted sum of two BCS densities of states, we analyze the differential conductance results we obtained. Our results show (1) a temperature-resolved transition from SIS to NIS behavior 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).