Comparison of Nb and Ta Oxides for Potential Magnetic Pairbreaking and Quasiparticle Generation in Superconducting Qubits*

Recent studies of transmon qubits where the Nb capacitor is replaced with Ta have shown increases in T₁ decay time by a factor of 3 to 10. We suggest that this dramatic improvement may be due to differences in the native oxide.  Nb sub-oxides are shown to be magnetic based on scanning point contact tunneling spectroscopy and magnetic susceptibility. There is evidence of magnetic pairbreaking that leads to sub-gap quasiparticle states.  The question of whether Ta exhibits magnetic sub oxides is addressed.  High purity recrystallized foils of Nb and Ta have been prepared by the identical procedure of UHV annealing above 2000C and subsequently exposed to air for periods of time from 30 minutes up to two weeks or longer.  XRD measurements reveal similar grain orientations in each foil.  TEM measurements show that the Ta oxide is roughly 40% thinner than the Nb oxide after long term air exposure. XPS measurements reveal several notable differences in the Ta oxide including the apparent absence of a TaO₂ sub-oxide whereas the Nb foil, as well as other Nb samples, consistently exhibit a corresponding NbO₂ sub-oxide.  Density functional theory calculations of formation energies are presented to aid in the analysis.