Optimization of tailored multilayer superconductors for RF application and protection against premature vortex penetration

Superconducting radiofrequency (SRF) cavities for accelerators is the only superconductivity application that operates in the Meissner state. For decades, bulk niobium, which exhibits the highest 1st critical field among all known superconductors, has been the only material to achieve high accelerating fields in combination with a high quality factor. Specific nanostructures tailored for SRF applications, in the form of superconducting/insulating/superconducting multilayers, were proposed to surpass Nb performances that could improve SRF performance. In this paper, we present the study of a series of NbN/MgO/Nb tri-layers including standard material characterization and specific superconducting characterization (local magnetometry) along with a comparison to the current state-of-the-art theoretical modeling. This study shows that such structures are effective to enhance the penetration field compared to bare niobium, even in the presence of numerous defects.