Understanding the Mechanism of the Performance Improvement in Nitrogen-doped Niobium Superconducting Radio Frequency Cavity

TL;DR

This study investigates how nitrogen doping improves niobium superconducting cavities by passivating the surface, reducing oxide thickness, and impeding harmful diffusion processes, thereby enhancing performance.

Contribution

It uncovers the structural and chemical mechanisms behind nitrogen doping's performance enhancement in niobium superconducting cavities.

Findings

Nitrogen doping introduces compressive strain near the Nb/air interface.

Doping impedes oxygen and hydrogen diffusion at the surface.

Surface oxide thickness is reduced by nitrogen passivation.

Abstract

Niobium superconducting radiofrequency cavities enable applications in modern accelerators and quantum computers. However, the surface resistance significantly deteriorates the cavities performance. Nitrogen doping surface treatment can consistently increase cavity performance by reducing surface resistance, but the improvement mechanism is not fully understood. Herein, we employed transmission electron microscopy and spectroscopy to uncover the structural and chemical differences of the Nb-air interface between the non-doped and nitrogen-doped cavities. The results indicate that nitrogen doping passivates the Nb surface by introducing a compressive strain close to the Nb/air interface, which impedes the oxygen diffusion and hydrogen atoms and reduces surface oxide thickness.