We present an analysis of the 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 cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 meV and superconducting critical temperature (Tc) up to 16.3 K. Scanning Electron microscopy (STEM) performed on cross section of the sample's surface region shows a 2 microns thick Nb3Sn surface layer. The elemental composition map exhibits a Nb over Sn ratio of 3 and reveals the presence of buried sub-stoichiometric regions that have a ratio f 5. 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 Tc regions could play an important role in the dissipation mechanism occurring during RF tests of Nb3Sn-coated cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.
@article{arxiv.1503.03410,
title = {Analysis of Nb3Sn surface layers for superconducting RF cavity applications},
author = {Chaoyue Becker and Samuel Posen and Nickolas Groll and Russell Cook and Christian M. Schlepuetz and Daniel Leslie Hall and Matthias Liepe and Michael Pellin and John Zasadzsinski and Thomas Proslier},
journal= {arXiv preprint arXiv:1503.03410},
year = {2015}
}