Tunneling Spectroscopy Studies of the Magnetic Surface Oxides of Nb: Impact on Qubit Decoherence

There is now considerable evidence for paramagnetic moments in the surface oxides of Nb originating in oxygen vacancies (Nb⁴⁺) that lead to unpaired electron spins.  It is of strong interest to elucidate how such paramagnetic moments might influence the Nb surface superconductivity as well as provide possible TLS and non-TLS channels relevant for qubit performance. Here we summarize a large set of tunneling spectroscopy studies including point-contact (PCT) measurements utilizing the native Nb oxide as well as vacuum scanning tunneling microscopy/spectroscopy (STM/STS) of backsputtered bare Nb and of capped Nb surfaces. A rich array of spin-flip Kondo physics has been observed including well-defined, sub-gap Shiba states in the superconducting phase of Nb due to nanoscale surface sub-oxides. The gap spacing between Shiba bands varies and in some cases includes quasiparticle states near zero bias that could couple to RF fields and introduce non-TLS losses. We also identify strong zero-bias conductance peaks that can be fit using the Appelbaum theory of spin-flip tunneling. We thus identify two separate types of Kondo effects, those that originate in many-body corrections to the Nb electrode density of states and those that are inter-electrode and a property of the spin-flip tunneling channel. The zero bias spin-flip tunneling peaks co-exist with the superconducting quasiparticle density of states of Nb in the superconducting phase and persist above Tc, exhibiting a logarithmic T dependence as well as Zeeman splitting in a magnetic field. These unique features serve to identify the origins of such zero modes as due to magnetic moments in the insulating regions of the Nb oxide.