Analysis of Nb₃Sn surface layers for superconducting radio frequency cavity applications

We present an analysis of Nb₃Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb₃Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ± 0.4 meV and superconducting critical temperatures (T_c) up to 16.3 K. Scanning transmission electron microscopy performed on cross sections of the sample's surface region shows an ∼2 μm thick Nb₃Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb₃Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low T_c regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb₃Sn-coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators