Ab Initio amorphous Nb and Ta oxide structures

Amorphous oxide layers in thin film capacitors are linked to reduced transmon qubit T1 coherence times. Ta-based capacitors outperform Nb-based ones, suggesting amorphous Ta2O5−x is less lossy than Nb2O5−x. In the associated manuscript, we investigate the microscopic features of these amorphous oxides using ab initio molecular dynamics and density functional theory, revealing the origins of the superior performance of Ta2O5−x. We establish that oxygen deficiency is less likely to occur in amorphous Ta2O5−x than in Nb2O5−x for 0 ≤ x ≤ 0.25 and for a given oxygen deficiency x, metal Ta-Ta bond formation is enhanced. Such bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous Nb2O5−x. These thermochemical differences quench or highly suppress magnetic moments in amorphous Ta2O5−x and eliminate a potential source of quasiparticles and magnetic flux noise. This dataset compiles the Nb2O5−x and Ta2O5−x structures generated using ab inito molecular dynamics.