Speaker
Description
Coating Nb with thin layers of one or more superconductors with longer penetration depths, $\lambda$, has been proposed to achieve accelerating gradients, $E_{\mathrm{acc}}$, beyond Nb's fundamental limit. Such heterostructures can sustain the Meissner state above each layer's superheating field, $B_{\mathrm{sh}}$, due to the strong suppression of the screening currents in the surface layers by a ''counter-current'' in the substrate and the presence of interfacial energy barrier between material junctions. We infer the presence of interfacial energy barrier by measuring the first-flux-penetration field, $\mu_{0} H_{\mathrm{vp}}$ in superconductor-superconductor (SS) $\mathrm{Nb_3Sn}$(2 µm)/Nb samples using muon spin rotation ($\mu$SR). Using thin Ag foils as energy moderators for the implanted muon spin probes, we profiled $\mu_{0} H_{\mathrm{vp}}$ at sub-surface depths between 10 µm and 100 µm, finding that $\mu_{0} H_{\mathrm{vp}}$ is depth-independent with a value of $234.5 \pm 3.5$ mT, consistent with Nb's metastable $B_{\mathrm{sh}}$ and a surface energy barrier preventing flux penetration. Similarly, evidence for current suppression in SS $\mathrm{Nb_{1-x}Ti_{x}N/Nb}$ samples was observed from nanoscale (depths $\lesssim$ 150 nm) measurements of their Meissner screening profiles in applied fields $\leq 25$ mT using the low energy $\mu$SR. The observed bipartite form of the screening profiles quantitatively confirm the Meissner response predicted by the ''counter-current'' model, which we use to identify the optimal $\mathrm{Nb_{1-x}Ti_{x}N/Nb}$ coating thickness for maximizing the $\mu_{0} H_{\mathrm{vp}}$. Our results of strong suppression of the Meissner currents in the surface layer suggest that multilayered structures with several superconducting and insulating layers are necessary to reach the highest $E_{\mathrm{acc}}$.
