Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film

two types of electrons at the Fermi level with widely differing behaviours, one ofthem (σ-bonding) being much more strongly superconducting than the other(π-bonding). This is at odds with usual theories of phonon-mediatedsuperconductivity which assume that all electrons behave in the same manner.Theoretical understanding of the properties of MgB 2 has almost been achieved withtwo energy gaps. It was regarded as behaving more like a metallic than a cupratesuperconductor.On the other side the material appears promising for RF, since it is ratherinsensitive to impurity content; it is highly metallic and has strong linked grainboundaries. Its properties depend greatly on composition and fabrication process.Many properties are anisotropic due to the layered structure. 'Dirty' samples, e.g., withoxides at the crystal boundaries, are different from 'clean' samples. [23]The maximum critical current (Jc) is: 10 5 A/m² at 20 T, 10 6 A/m² at 18 T, 10 7A/m² at 15 T, 10 8 A/m² at 10 T, 10 9 A/m² at 5 T [23] .Up to 2008, the parallel upper critical field (H c2 ) obtained is about 14.8 T, andthe perpendicular upper critical field is about 3.3 T as shown in Fig. 1.10 [23] . In thinfilms the field is up to 74 T, and in fibres up to 55 T [23] .MgB 2 thin film produced by the hybrid physical chemical vapour deposition(HPCVD) has the good performance, which characterized by T c0 = 41.8 K, ρ 0 = 0.26μΩ cm, RRR > 30 [24,25] , and self-field J c = 3.4×10 7 A cm −2 [26] .As far assuperconducting cavities are concerned, still many problems must be solved: thedegradation of superconducting properties of MgB 2 after the immersion in water.Fig.1.10 R BCS of different material17