Detection of superconducting inhomogeneities by penetration depth measurements δλ (T, f, Bac)

Small regions of differently strong superconducting interaction have been detected by measuring the penetration depth Δλ(T,f,Bac) and the losses ΔR(T,f,Bac) as deviation from the BCS theory of homogeneous materials. For Nb two deviations are most pronounced: a step-like increase in Δλ(T) and ΔR(T) at K and an enhanced slope λ0= dλ/dy for T > T*. The step Δλ0around 7 K shows the transition of regions to the normal state, which then for T > T* yield an enhanced slope λ0, as explained by proximity effect theory. For Nb3Sn no such deviations from the BCS theory could be detected, showing together with λ0= 170 nm its homogeneity. For thinned Nb3Sn layers the underlaying Nb shows up as step at 9.2 K, which is used to evaluate the mean thickness of the Nb3Sn layer.