Skip to main content

The Center for Bright Beams, A National Science Foundation Science and Technology Center

Ab Initio Theory of the Impact from Grain Boundaries and Substitutional Defects on Superconducting Nb3Sn

M. M. Kelley, N. S. Sitaraman, and T. A. Arias

Nb3Sn offers the potential to significantly advance superconducting radio frequency (SRF) technology by improving the efficiency of accelerating cavities, but preserving the material's performance requires special attention to its microstructual properties. While the effects of point defects are well-understood, with superconducting properties that quickly degrade as the material deviates from its ideal stoichiometry, less is known about how performance is impacted by extended defects such as grain boundaries. Grain boundaries are defects that significantly disrupt the crystal structure and provide disorder on length scales comparable to the superconducting coherence length of Nb3Sn. This study provides the first ab Initio investigation to predict the impact of grain boundaries on the superconducting performance of Nb3Sn. In this study we identify an energetically favorable selection of grain boundary structures, examine the impact of grain boundaries on the material's electronic structure, explore the interactions between grain boundaries and point defects, and finally consider how all of these effects impact local superconducting properties.

Local Fermi-level density of states profiles around a clean grain boundary containing the material's ideal stoichiometry and around a grain boundary filled with excess tin defects analagous to observed atomic concentration profiles.
Figure: (top) Local Fermi-level density of states profiles around a clean grain boundary containing the material's ideal stoichiometry and around a grain boundary filled with excess tin defects analagous to observed atomic concentration profiles (bottom left). The depression in the Fermi-level density of states is much wider for the boundary filled with tin defects than for the clean boundary without point defects. Estimates for a local superconducting transition temperature (bottom right) are obtained by averaging the density of states profiles over a sphere with radius corresponding to the superconducting coherence length of Nb3Sn. We find a significant reduction in Tc around a tin-rich grain boundary and only a slight reduction in Tc around a clean grain boundary.
 

Applications and relation to CBB Goals:

The higher superconducting transition temperature of Nb3Sn allows to both increase efficiency and decrease cost of SRF technology, and observations from CBB researchers and affiliates suggest a connection between cavity performance and grain boundary composition. This study explains from first principles why grain boundaries containing tin defects are more detrimental to the superconducting performance than are clean grain boundaries without point defects. CBB researchers employing Ginzburg-Landau simulations can build off this work by utilizing our model on how grain boundary composition affects the local superconducting transition temperature as a function of distance from the boundary plane. These combined efforts will inform CBB experimentalists on how to adjust cavity baking recipes to optimize the material's performance.
 

Link to full publication:

http://arxiv.org/abs/2005.12133