Josephson and Multi-Gap Quasi-Particle Tunneling in Crystalline MgB$_{2}$-based Junctions with an MgO Sputtered Barrier

MgB$_{2}$ is a multi-gap superconductor with a T$_{C}$ of 39K and a hexagonal structure. This simple and stable compound is very attractive for device applications. We have deposited and patterned micron-sized SIS tunnel junctions with highly textured MgB$_{2}$ electrodes grown by MBE co-evaporation with sputter-deposited MgO tunnel barriers, in an entirely in-situ process. This method enabled us to obtain low resistance junctions with very good oxide coverage. We present $I-V$ and \textit{dI/dV} data displaying Josephson pair tunneling as well as the quasi-particle tunneling signature of both the Pi- and Sigma-bands of the MgB$_{2}$. Our experimental gap values agree with theoretical calculations. Although our MgB$_{2}$ films were $c$-axis oriented, growth-related roughness of the bottom MgB$_{2}$ enable $a/b$-axis tunneling and thus explain the observed Sigma-band features. We link our data to a simple model assuming tunneling to occur from both the Pi- and Sigma-bands in parallel, proportionally weighted depending on the interfacial topography.