Enhanced Surface Superconductivity of Niobium by Zirconium Doping

Correlating Electron–Phonon Coupling and In Situ High-Temperature Atomic-Scale Surface Structure at the Metallic Nb(100) Surface by Helium Atom Scattering and Density Functional Theory

“…Additionally, since extensive research on Nb and Nb SRF cavities has allowed them to approach their fundamental limits of operation, the search has begun for new materials and cavity treatments to further reduce R s , maximize the superheating field (H sh ), minimize power loss, and optimize overall performance. 5,7,8,11,12 Due to ≤100 nm penetration depth of RF fields in typical Nb SRF cavities, changes in the surface significantly affect cavity performance. 1,5,6 In fact, surface defects, inhomogeneities, and impurities limit cavity quality factors and operating temperatures, holding back a variety of promising new SRF cavity materials.…”

“…Both Nb’s normal and superconducting state properties have led the material to its ubiquitous use and extensive study in SRF cavities. − Nb’s relatively high ductility allows for facile manufacture of optimal cavity geometries for relatively high quality ( Q ) factors and accelerating gradients. − Additionally, pure Nb’s relatively high thermal conductivity facilitates effective cooling, necessary to enter and maintain Nb’s superconducting state. − Nb’s superconducting state’s relatively high critical temperature ( T C ) and low RF surface resistance ( R s ) contribute significantly to Nb SRF cavities’ high Q factors at attainable operating temperatures. , Since the RF fields only penetrate through the first 40–100 nm of the cavity surface, SRF cavity performance depends critically upon the surface chemistry and surface quality. , Thus, the surface preparation of SRF cavities must be carefully designed and implemented. Furthermore, the superconducting state’s unique behavior at these surfaces results from SRF cavity preparation techniques. ,− However, fundamental studies correlating the atomic-scale surface structure with changes in the superconducting state remain unexplored. How does the surface modify the physical properties driving the formation of the superconducting state and its favorable properties?…”

“…A deeper understanding of atomic-scale surface structure’s effect on the well-known superconducting state of the bulk would reveal the mechanisms determining which SRF cavity preparations aid or hinder the performance of a cavity. Additionally, since extensive research on Nb and Nb SRF cavities has allowed them to approach their fundamental limits of operation, the search has begun for new materials and cavity treatments to further reduce R s , maximize the superheating field ( H sh ), minimize power loss, and optimize overall performance. ,,,, Due to ≤100 nm penetration depth of RF fields in typical Nb SRF cavities, changes in the surface significantly affect cavity performance. ,, In fact, surface defects, inhomogeneities, and impurities limit cavity quality factors and operating temperatures, holding back a variety of promising new SRF cavity materials. ,,,,,, While well-studied, the evolution of surface defects remains a challenging part of the SRF cavity treatment design and implementation. An understanding of the evolution of surface defects and their resulting effects on superconductivity at the surface remains elusive.…”

Distinguishing the Roles of Atomic-Scale Surface Structure and Chemical Composition in Electron Phonon Coupling of the Nb(100) Surface Oxide Reconstruction

“…Extensive study of Nb and Nb SRF cavities has pushed their performance to the fundamental limits of operation. Thus, the SRF community has begun developing new materials and cavity treatments to further reduce R s , maximize the superheating field ( H sh ), minimize power loss, and optimize overall performance. ,,− In fact, defects, inhomogeneities, and impurities at the surfaces limit cavity quality factors and operating temperatures, limiting the implementation of promising new SRF cavity materials. ,,,,,, While well-studied, the formation and evolution of surface defects and compositional inhomogeneities remain a challenging part of SRF cavity treatment design and implementation. Such an understanding of the role of surface structure and chemical composition, as well as their resulting effects on superconductivity at the surface, remains elusive.…”