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, Bshβ, 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
[Nb1βxβTixβN(50 nm)/Nb, Nb1βxβTixβN(80 nm)/Nb, and
Nb1βxβTixβN(160 nm)/Nb] are investigated here by depth-resolved
measurements of their Meissner screening profiles using low-energy muon spin
rotation (LE-ΞΌSR). From fits to a model based on London theory (with
appropriate boundary and continuity conditions), a magnetic penetration depth
for the thin Nb1βxβTixβN layers of Ξ»Nb1βxβTixβNβ= 182.5(31) nm is found, in good agreement with literature values for the bulk
alloy. In contrast, a simple London model without appropriate boundary
conditions overestimates Ξ»Nb1βxβTixβNβ by more than a factor
of two, suggesting that it is inappropriate for quantifying
Ξ»Nb1βxβTixβNβ here. Using the measured
Ξ»Nb1βxβTixβNβ, the maximum vortex-free field,
Bmaxβ, of the superconductor-superconductor (SS) bilayer structure
was estimated to be 610(40) mT. The strong suppression of the surface current
in the Nb1βxβTixβN layer suggests an optimal thickness of βΌ1.4Ξ»Nb1βxβTixβNβ= 261(14) nm.Comment: 13 pages and 8 figure