Theoretical investigation of magnetic losses in amorphous Ta₂O₅.

Experimental investigations have shown improved qubit coherence times (T₁) when Ta superconducting coplanar waveguide (CPW) transmission lines are used for qubit readout instead of Nb transmission lines. The authors of these studies have suggested that this behavior is likely a result of a reduced number of loss channels in the amorphous Ta₂O₅ oxide layer as compared to the amorphous Nb₂O₅ oxide layer. A recent work has shown that d-channel magnetic defects in the amorphous Nb₂O₅ oxide layer of Nb CPW transmission lines are a potentially significant source of qubit decoherence. Owing to the increased spatial extent of Ta 5d orbitals compared to Nb 4d orbitals, we hypothesize that amorphous Ta₂O₅ will host fewer localized moments than amorphous Nb₂O₅. We confirm this hypothesis using spin-polarized density functional theory (DFT) calculations on magnetic moment formation in amorphous Ta₂O₅ and discuss the impact on magnetic losses.