Oxygen vacancies in niobium pentoxide as a source of two-level system losses in superconducting niobium

Journal Article · Thu Aug 01 00:00:00 UTC 2024 · Phys.Rev.Applied

DOI:https://doi.org/10.1103/PhysRevApplied.22.024035· OSTI ID:1834188

Bafia, Daniel ^[1]; Murthy, Akshay; Grassellino, Anna ^[1]; Romanenko, Alexander ^[1]

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1. Fermilab

We identify a major source of quantum decoherence in three-dimensional superconducting radio-frequency (SRF) resonators and two-dimensional transmon qubits composed of oxidized niobium: oxygen vacancies in the niobium pentoxide, which drive two-level system (TLS) losses. By probing the effect of sequential in situ vacuum-baking treatments on the rf performance of bulk $\mathrm{Nb}$ SRF resonators and on the oxide structure of a representative $\mathrm{Nb}$ sample using TOF SIMS, we find a nonmonotonic evolution of cavity quality factor $Q_0$ , which correlates with the interplay of ${\mathrm{Nb}}_2{\mathrm{O}}_5$ vacancy generation and oxide-thickness reduction. We localize this effect to the oxide itself and present the insignificant role of diffused interstitial oxygen in the underlying $\mathrm{Nb}$ by regrowing the oxide via wet oxidation, which reveals a mitigation of aggravated TLS losses. We hypothesize that such vacancies in the pentoxide serve as magnetic impurities and are a source of TLS-driven rf loss.

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
• Sponsoring Organization: US Department of Energy; USDOE
• Grant/Contract Number: 89243024CSC000002; AC02-07CH11359
• OSTI ID: 1834188
• Alternate ID(s): OSTI ID: 2474338
• Report Number(s): FERMILAB-PUB-21-678-SQMS-TD; oai:inspirehep.net:1982500; arXiv:2108.13352
• Journal Information: Phys.Rev.Applied, Journal Name: Phys.Rev.Applied Journal Issue: 2 Vol. 22
• Country of Publication: United States
• Language: English

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