Speaker
Description
Niobium-based superconducting radio-frequency (SRF) cavities have demonstrated exceptionally high quality factors, making them an attractive platform for investigating fundamental loss mechanisms in superconducting quantum systems. To further reduce surface-related dissipation, we investigate a surface-engineering approach based on thermal atomic layer deposition (ALD) of functional oxide thin films designed to suppress the formation of amorphous native niobium oxides, which are widely considered a major source of two-level-system (TLS) losses. Engineered ALD layers, including ZrO₂ and Ta₂O₅, serve as functional barrier and interface layers that tailor the superconductor–dielectric interface, stabilize the niobium surface, and mitigate dielectric losses.
The effectiveness of this approach relies on achieving a sharp, well-controlled interface between the deposited oxide and the underlying niobium. Post-deposition thermal treatments play a critical role in optimizing the interfacial structure, suppressing native oxide evolution, and enhancing cavity performance in both the low-field quantum regime and the high-field accelerator regime. Extensive materials characterization has been performed to correlate thin-film composition, interface evolution, and processing conditions with RF performance, providing insight into the mechanisms governing surface-related dissipation.
To evaluate the effectiveness of these engineered interfaces over the full operating range of SRF cavities, new RF measurements have been performed in the SQMS dilution refrigerator at millikelvin temperatures and down to the single-photon regime, enabling direct investigation of TLS-dominated losses and their saturation behavior. Complementary measurements at the Fermilab Vertical Test Stand (VTS) characterize cavity performance under accelerator-relevant operating conditions, extending the study across a broad range of RF fields. Together, these measurements establish a comprehensive framework for assessing the impact of ALD-engineered functional oxide interfaces from the quantum regime to accelerator operation.
This presentation will describe the ALD processing methodology, interface engineering strategy, materials characterization, and new RF measurements obtained in both the dilution refrigerator and the Vertical Test Stand. Particular emphasis will be placed on the role of functional oxide interfaces and optimized thermal processing in mitigating TLS-related losses, improving the ambient stability of niobium surfaces, and enabling next-generation SRF cavities for quantum and accelerator applications.