Title: Novel Surface Probes of Nb for the Development of SRF Cavities and Qubits
Speaker: John Zasadzinski
We discuss the development over the past ten years of novel surface probes of Nb that have provided unique insights into the performance of SRF cavities as well as 2-d and 3-d qubits. Such probes include Raman microscopy/spectroscopy as well as scanning point contact tunneling (PCT) spectroscopy used in conjunction with more standard tools of TEM and XPS. The Raman spectra have revealed micron sized patches on Nb that develop after cavity processing that contain deep carbonaceous impurities including amorphous carbon and chain-type hydrocarbons as well as coherent NbC precipitates observed directly with atomically resolved TEM. Scanning PCT has provided insight into the improvement in performance of N-doped cavities, revealing a homogenous superconducting gap parameter with a low value of the Dynes broadening parameter G as well as a thicker insulating oxide with improved barrier height. PCT studies of hot spot regions in conventionally processed SRF cavities show an inhomogeneous gap parameter with relatively larger G and clear evidence of magnetic moments in the Nb oxide that give rise to zero bias conductance peaks originating in spin flip tunneling. The magnetic oxides are likely due to oxygen vacancies and we suggest that the microscopic origin of the Dynes G lies in a conducting magnetic oxide that causes pairbreaking at the Nb-oxide interface. Recent XPS comparison of the oxides of Nb and Ta offer insights into the improvement of transmon qubits using Ta capacitors.