Suppressed paramagnetism in amorphous <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Ta</mi></math><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi></mi><mn>2</mn></msub></math><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mrow><mi mathvariant="normal">O</mi></mrow></mrow></math><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi></mi><mrow><mn>5</mn><mo>−</mo><mi>x</mi></mrow></msub></math> oxides and its link to superconducting-qubit performance

P. Graham Pritchard; James M. Rondinelli

doi:10.1103/6wyt-fxjg

Suppressed paramagnetism in amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ oxides and its link to superconducting-qubit performance

P. Graham Pritchard ^* and James M. Rondinelli ^†

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA

^*Contact author: patrickpritchard2025@u.northwestern.edu
^†Contact author: jrondinelli@northwestern.edu

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Phys. Rev. Applied 23, 064062 – Published 26 June, 2025

DOI: https://doi.org/10.1103/6wyt-fxjg

Abstract

Amorphous-oxide layers in thin-film capacitors are linked to reduced transmon-qubit $T_{1}$ coherence times. $Ta$ -based capacitors outperform $Nb$ -based ones, suggesting that amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ is less lossy than $Nb$ $_{2}$ $O$ $_{5 - x}$ . 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 $Ta$ $_{2}$ $O$ $_{5 - x}$ . We establish that oxygen deficiency is less likely to occur in amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ than in $Nb$ $_{2}$ $O$ $_{5 - x}$ for $0 \leq x \leq 0.25$ 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 $Nb$ $_{2}$ $O$ $_{5 - x}$ . These thermochemical differences quench or highly suppress magnetic moments in amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ 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 $Nb$ $_{2}$ $O$ $_{5 - x}$ 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 $Nb$ $_{2}$ $O$ $_{5 - x}$ oxide layers that is likely inactive in $Ta$ $_{2}$ $O$ $_{5 - x}$ . 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.

Physical Review Applied

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Suppressed paramagnetism in amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ oxides and its link to superconducting-qubit performance

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

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Suppressed paramagnetism in amorphous $Ta$ $_{2}$ $O$ $_{5 - x}$ oxides and its link to superconducting-qubit performance