Nanoscale lattice and chemical composition measurements of superconducting Nb₃Sn films using scanning transmission electron microscopy

Modern-day superconducting radio-frequency (SRF) cavities for particle accelerators are made from Nb, and after decades of development their RF performance is approaching the theoretical limit for Nb. One of the most attractive materials to replace Nb is Nb₃Sn, because of its higher superheating field and critical temperature. However, due to its intrinsic smaller coherence length, this alloy is more sensitive to degradation of superconducting properties from defects such as tin deficiency and strain. Here, we use scanning transmission electron microscopy (STEM) to examine these defects at nanoscale. By performing the Energy Dispersive X-ray Spectroscopy (EDX) analysis, we identify tin-deficient regions in the film. Samples from two growth methods - vapor diffusion and electrochemical synthesis - are analyzed, and the stoichiometric differences between these methods are found. Next, in order to measure the lattice parameter of the crystal, we performed 4-dimensional STEM maps, where the entire diffraction pattern is collected at each scanning position. Furthermore, by combining the information from the zero-order Laue zone (FOLZ) and higher-order Laue zones (HOLZ) we develop a method to calculate the full 3-dimensional strain tensor. Finally, we determine a correlation between the strain and chemical composition. We highlight the use of our defect analysis to improve the Nb₃Sn SRF cavity development.