High-quality Nb3Sn thin films on sapphire prepared by tin vapor diffusion

Stoichiometric Nb3Sn films on sapphire were prepared with a tin vapor diffusion process from sputtered Nb precursors. The superconductive properties of the precursor and the converted films were measured at 1 kHz and 87 GHz. The nanocrystalline Nb films yielded Tc=7.8–9.3 K, ΔTc=0.05–0.30 K, Jc (4.2 K)=0.8–2.8 MA/cm2, and ρ(Tc)=1–35 μΩ cm, indicating a high sensitivity to granularity and impurities. In contrast, the 0.5–3.0 μm thick and large-grained Nb3Sn films showed reproducibly Tc=18.0 K, ΔTc=0.1 K, Jc (4.2 K)=5–6.5 MA/cm2, and ρ(Tc)=7.7–9.1 μΩ cm. A reduced energy gap Δ/kTc=1.8–2.2 and a penetration depth λ0 (T=0 K)=65–80 nm were deduced from the surface impedance measurements. The residual surface resistance dropped below the sensitivity limit of 0.3 mΩ.