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.
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.