Evidence for current suppression in superconductor-superconductor bilayers

Abstract

Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, $B_{\mathrm{sh}}$, which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [$\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$(50 nm)/Nb, $\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$(80 nm)/Nb, and $\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$(160 nm)/Nb] are investigated here by depth-resolved measurements of their Meissner screening profiles using low-energy muon spin rotation (LE-$\mu$SR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin $\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$ layers of ${\lambda }_{\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}}=$ 182.5(31) nm is found, in good agreement with literature values for the bulk alloy. Using the measured ${\lambda }_{\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}}$, the maximum vortex-free field, $B_{\max }$, of the superconductor-superconductor (SS) bilayer structure was estimated to be 610(40) mT. The strong suppression of the surface current in the $\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$ layer suggests an optimal thickness of $\sim 1.4{\lambda }_{\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}}=$ 261(14) nm.

arXiv comment13 pages and 8 figures

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Cited by in corpus (6)

• Comment on "Strong Meissner screening change in superconducting radio frequency cavities due to mild baking" [Appl. Phys. Lett. 104, 072601 (2014)]
• Measurements of the first-flux-penetration field in surface-treated and coated Nb: Distinguishing between surface pinning and an interface energy barrier
• Niobium's intrinsic coherence length and penetration depth revisited using low-energy muon spin spectroscopy and secondary-ion mass spectrometry
• Search for inhomogeneous Meissner screening in Nb induced by low-temperature surface treatments
• Implantation studies of low-energy positive muons in niobium thin films
• Superconducting properties of thin film $\mathrm{N}{\mathrm{b}}_{1-\mathrm{x}}\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\mathrm{N}$ studied via the NMR of implanted ${}^8$Li