Fabrication and characterization of MgB$_{2}$/ thermal oxide barrier /Pb Josephson tunnel junctions

Cross-bridge Josephson tunnel junctions were fabricated, using MgB$_{2}$ films grown by hybrid physical-chemical vapor deposition (HPCVD) and barriers made by thermal oxidation at different temperatures. The junctions showed clear Josephson tunneling characteristics with high supercurrents, high $I_{c}R_{N}$ products ($I_{c}R_{N}$ products $\sim $1.8 mV at 4.2 K), and small subgap current leakage. The external DC magnetic field dependence was also measured and showed clear Fraunhofer pattern. The properties of the thermal oxide barrier depend sensitively on the oxidation temperature. The potential height and barrier thickness of 0.7 eV and 1.8 nm, respectively, were inferred from conductance measurements at high bias voltage. Two superconducting gaps of 2.0 meV and 7.5 meV for MgB$_{2}$ were observed from these sandwich-type tunnel junctions. These results suggest the potential of using MgB$_{2}$ thermal oxide layers as a barrier for practical Josephson tunnel junction fabrication. This work is supported by ONR and NSF.