Scanning Tunneling and Electron Spectroscopies of Nb Surface Oxides for Qubits and SRF Cavities*

Scanning point contact tunneling (SPCT) spectroscopy has been performed on high purity Nb pieces, including cut outs of superconducting RF cavities as well as on patterned Nb films coated with Al.  Hot spot regions of conventionally processed SRF cavities reveal localized magnetic moments in the Nb oxide as evidenced by a zero bias conductance peak that exhibits Zeeman splitting in a field, and a logarithmic T dependence, consistent with the Appelbaum-Anderson model of spin flip tunneling.  Similar magnetic moment features are observed from the Nb oxide layer at the edges of lithographically patterned Nb films where the Al layer has peeled off.  Electropolished Nb crystals exhibit significant broadening of the BCS density of states attributed to magnetic pairbreaking.  These results are consistent with scanning TEM/EELS studies that show a continuous change of the Nb oxidation state, from the surface Nb₂O₅ to the presence of an amorphous, likely metallic NbO_x layer at the interface with Nb. Combined, these experiments indicate that oxygen vacancies are the probable cause of local moments in the oxide layer.  Magnetic moments and pairbreaking may be playing a role in qubit decoherence.