Quasiparticle spectroscopy in technologically relevant niobium using London penetration depth measurements: experiment and theory

Journal Article · 06 November 2024 · Materials for Quantum Technology

DOI: https://doi.org/10.1088/2633-4356/ad8c0c · OSTI ID:2476442

Ghimire, Sunil ; Joshi, Kamal R. ; Datta, Amlan ; Goerdt, Aidan ; Tanatar, Makariy A. ; Schlagel, Deborah ; Kramer, Matthew J. ; Marshall, Jayss ; Kopas, Cameron J. ; Mutus, Joshua Y. ; Romanenko, Alexander ; Grassellino, Anna ; Prozorov, Ruslan

Abstract The London penetration depth, $\lambda \left(T\right)$ , was measured in various forms of niobium, including foils, thin films, single crystals, and samples from superconducting radio-frequency (SRF) cavities. We observed a significant difference in $\lambda \left(T\right)$ at low temperatures, $T<T_{\mathrm{c}}/3$ , due to low-energy quasiparticles. In particular, an unusual downturn of $\lambda \left(T\right)$ on cooling in the SRF cavity samples required to take into account deep in-gap bound states. Theoretical modeling using the generalized Dynes density of states shows that such in-gap states lead to a downturn or a peak in $\lambda \left(T\right)$ upon cooling. Combined, experimental and theoretical findings provide a method for detecting two-level systems or states related to magnetic impurities in the bulk of niobium. This result is particularly relevant for the quantum informatics sciences technologies used in qubits and circuit quantum electrodynamics architecture based on SRF cavities.

• Research Organization: Ames Laboratory (AMES), Ames, IA (United States); Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States); Iowa State U.; Rigetti Computing
• Sponsoring Organization: US Department of Energy; USDOE
• Grant/Contract Number: 89243024CSC000002; AC02-07CH11359
• OSTI ID: 2476442
• Report Number(s): FERMILAB-PUB-23-337-SQMS-TD
• Journal Information: Materials for Quantum Technology, Journal Name: Materials for Quantum Technology Journal Issue: 4 Vol. 4; ISSN 2633-4356
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: United Kingdom
• Language: English