Understanding mechanism of performance improvement in nitrogen-doped niobium superconducting radio frequency cavity

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