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Superconducting critical temperatures Tc and magnetic fields Hc2, lattice parameters a0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 μm or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values of Tc, a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat-treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 μm. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value of Tc (by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreased Tc. Also, small additions of these elements significantly increased Hc2. Specifically, the largest values of Hc2 (∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with a value for the pure Nb3Sn of 23.5 T at 4.2 K. The value of a0 decreased with all of the alloying additions; these variations can be explained qualitatively by several models for the lattice parameter of A15 compounds, but none of them can quantitatively predict the variations. In one system, (Nb,Ti)3Sn, values of the normal-state resistivity just above the transition temperature were measured: adding 3 at. % Ti raises the value to ∼55 μΩ cm from the value of 10–15 μΩ cm for pure Nb3Sn. This increase in the resistivity is thought to be a primary reason for the increased Hc2 for this system as well as the other types of alloyed Nb3Sn which were studied here.

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CraigSaint-James and de Gennes 1 have shown theoretically that a surface superconducting layer with a critical field H c % can exist above the bulk critical field H c when the external field is parallel to the sample surface. They find that H c % = 2.39KH C where K is the Ginzburg-Landau parameter. For type II superconductors (K>1//2) this relation becomes H c % = 1.69# c 2-Experimental evidence 2 " 6 has confirmed the existence of a surface critical field H c % above i/ c 2> * or type II superconductors. For values of K between 1//2 and 1/2.39, surface critical fields are predicted for type I superconductors, although observation of H c % in type I materials has not yet been reported. We have measured the real and imaginary parts (x' and x") oi the ac susceptibility in type I and type II superconductors in both parallel and perpendicular dc magnetic fields. This technique yields much sharper transitions in E c % than have been reported and permits demonstration of the existence of surface effects in superconductors with H c % barely above H c . In a lead alloy containing only 0.1% Bi we find that H c 2 is slightly above H c . This alloy shows a reversible type I magnetization curve. In pure Pb, H c % was not detectable and the paramagnetic ac susceptibility characteristic of the intermediate state 7 in type I superconductors was observed.