Suppressed paramagnetism in amorphous Ta2O5−x oxides and its link to superconducting-qubit performance

Abstract

Amorphous-oxide layers in thin-film capacitors are linked to reduced transmon-qubit T1 coherence times. Ta-based capacitors outperform Nb-based ones, suggesting that amorphous Ta2O5−x is less lossy than Nb2O5−x. 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 that 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. We also show that hyperfine couplings between Nb nuclei and local magnetic moments in Nb2O5−x can form “two-level systems” (TLSs) or “two-level fluctuators” with energy splittings of 100–1000 MHz or higher. This reveals a TLS mechanism in amorphous Nb2O5−x oxide layers that is likely inactive in Ta2O5−x. Our work provides a fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits that can be used to guide materials selection and processing.

Source-reported materials — not catalogue approval

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