Tunneling Spectroscopy Study of Low Baked Bulk Niobium for Superconducting RF Cavity Applications
Point-contact tunneling (PCT) spectroscopy was used to probe the local superconducting properties of cavity-grade niobium following surface treatments representative of superconducting radio-frequency (SRF) cavities. Electropolished (EP) samples were compared with samples baked at 120~$^\circ$C for 48 h and subjected to a two-step bake at 75~$^\circ$C for 2 h followed by 120~$^\circ$C for 48 h. Although the superconducting gap $Δ$ is predominantly distributed around the bulk Nb value (~1.5 meV), regions exhibiting strongly suppressed gaps (minigaps) are observed in 15--29% of junctions on EP samples. Their occurrence decreases to 6% after the 120~$^\circ$C bake and to only 1% after the two-step treatment. Spectra exhibiting minigaps are well described by the Usadel proximity-effect model, consistent with superconducting Nb coupled to normal regions that we attribute to hydride formation. These results provide direct microscopic evidence that low-temperature baking suppresses regions of degraded surface superconductivity in cavity-grade Nb. The pronounced reduction achieved by the two-step bake further supports a link between hydride-related proximity effects and the surface superconducting properties governing SRF cavity performance.