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Coating Nb with thin superconducting layers (with or without insulating layers, i.e., SS or SIS) with longer penetration depth λ can enhance the accelerating gradient by maintaining the Meissner state above each layer’s superheating field Bsh, due to reduced surface screening currents and interfacial energy barriers. We review previously published studies using radioactive beam-based techniques to investigate SS and SIS for increasing accelerating gradient. Muon spin rotation (µSR) measurements of Nb3Sn(2 µm)/Nb samples revealed interfacial energy barriers through depth profiling of the first-flux-penetration field Bvp, consistent with Nb’s metastable Bsh. Low-energy µSR study at depths <=~ 150 nm in SS Nb1-xTixN/Nb samples confirmed nanoscale current suppression and a bipartite Meissner screening profile, supporting the "counter-current" model and identifying optimal coating thickness for maximizing Bvp. For vortex penetration study in SIS, β-detected nuclear magnetic resonance (βNMR) study optimizes the superconducting and normal-state properties of Nb0.75Ti0.25N in Nb0.75Ti0.25N(91 nm)/AlN(4 nm)/Nb. Resonance measurements in the vortex state showed broadening below Tc ~ 15 K, yielding λ near the intrinsic limit, while spin-lattice relaxation exhibited a metallic Korringa response modified below Tc by a Hebel-Slichter coherence peak.
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