Suppressed paramagnetism in amorphous $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ oxides and its link to superconducting-qubit performance

Journal Article · 01 June 2025 · Phys.Rev.Applied

DOI:https://doi.org/10.48550/arXiv.2410.13160· OSTI ID:2504567

Pritchard, P. Graham ^[1]; Rondinelli, James M. ^[1]

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1. Northwestern U.

Amorphous-oxide layers in thin-film capacitors are linked to reduced transmon-qubit $T_1$ coherence times. $\mathrm{Ta}$ -based capacitors outperform $\mathrm{Nb}$ -based ones, suggesting that amorphous $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ is less lossy than $\mathrm{Nb}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ . 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 $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ . We establish that oxygen deficiency is less likely to occur in amorphous $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ than in $\mathrm{Nb}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ for $0\le x\le 0.25$ and that for a given oxygen deficiency $x$ , metal $\mathrm{Ta}$ — $\mathrm{Ta}$ bond formation is enhanced. Such bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous $\mathrm{Nb}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ . These thermochemical differences quench or highly suppress magnetic moments in amorphous $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ and eliminate a potential source of quasiparticles and magnetic flux noise. We also show that hyperfine couplings between $\mathrm{Nb}$ nuclei and local magnetic moments in $\mathrm{Nb}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ 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 $\mathrm{Nb}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ oxide layers that is likely inactive in $\mathrm{Ta}$ ${}_2$ $\mathrm{O}$ ${}_{5-x}$ . 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.

• Research Organization: Northwestern U.
• Sponsoring Organization: US Department of Energy
• DOE Contract Number: 89243024CSC000002
• OSTI ID: 2504567
• Report Number(s): FERMILAB-PUB-24-0877-SQMS-V; oai:inspirehep.net:2841057; arXiv:2410.13160
• Journal Information: Phys.Rev.Applied, Journal Name: Phys.Rev.Applied Journal Issue: 6 Vol. 23
• Country of Publication: United States
• Language: English