Multiple authors have reported improved transmon qubit coherence times (T₁), exceeding 300 µs, when Ta films are used as the primary metallic element (i.e., forming the capacitive elements of the transmon and the readout resonators) instead of Nb films. The authors of these studies have suggested that this behavior is likely a result of a reduced number of loss channels in the native amorphous Ta₂O_5-x oxide layer of Ta films as compared to the native amorphous Nb₂O_5-x oxide layer of Nb films. We have investigated one such loss channel: weak paramagnetic moments. Using spin-polarized density functional theory calculations we find that amorphous Ta₂O_5-x, like amorphous Nb₂O_5-x, may host weak paramagnetic moments. To complement these findings, we examine the influence of crystal structure, including polyhedral connectivity and bond distortions, and chemical identity on the formation of magnetic moments in oxygen deficient Nb and Ta pentoxides.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract No. DE-AC02-07CH11359.