Probing the Superconducting Gaps of Carbon Substituted MgB2 Films using SIS Tunnel Junctions

We report scanning tunneling spectroscopy of superconducting-insulating-superconducting (SIS) tunnel junctions consisting of a polycrystalline MgB$_{2}$ micro-wire as the probe tip and a series of C-substituted thin films as samples. The MgB$_{2}$ micro-wire is affixed to a 250$\mu $m Pt-Ir wire and approached to each C-substituted film using an Oxford CryoSXM in STM mode. We are then able to scan over the film surface and probe different areas of the film locally using this SIS configuration. The micro-wire tip, for which we have previously characterized the gaps, is used as a reference for the C-substituted films and to enhance energy resolution by pushing the spectral features to higher energies. The films were grown by hybrid physical-chemical vapor deposition and extensive magnetic and structural characterization of identical films have been reported previously. We investigate the evolution of $\Delta _{\pi }$ and $\Delta _{\sigma }$ with carbon substitution and correlate these results with T$_{C}$ and H$_{C2}$ results.