On properties of superconducting Nb3Sn used as coatings in RF cavities

Nb3Sn layers of several μm thickness on bulk niobium have been prepared by heating the niobium samples in a saturated tin vapor at 1050°c. The layers have been analyzed by scanning electron microscopy and electron microprobe analysis; furthermore, measurements of penetration depth λ(T), pinning, critical field Bc2(T), rf surface resistance R(T) and rf peak field have been carried out. As a result λ(T=0) ≃ 170 nm, Ginzburg-Landau parameter , London penetration depth λL= 40 nm, coherence length ξF= 40 nm and mean free path ℓ = 2 nm have been found by using the values 2Δ/kTc= 4.2 and Bc(0) = 0.54T. These material parameters - in connection with weak flux pinning - indicate quite homogeneous Nb3Sn layers. With the cavity measurements a surface resistance Rres(3 GHz) = 1.7 . 10^-7Ohm could be achieved; the highest observed electric surface field so far was MV/m. The apparently good quality of the Nb3Sn oxide interface is indicated by the BCS like λ(T)-dependence and the comparably weak electron loading of Nb3Sn cavities.