Speaker
Description
Deposition of high-purity niobium (Nb) thin films onto copper (Cu) substrates provides a direct path to Deposition of high-purity niobium thin films onto copper substrates bypasses the high material costs and thermal stability limits of bulk niobium for superconducting radio-frequency cavities, yet achievable accelerating gradients remain hindered by premature vortex entry and high-field quality-factor degradation. Although energetic condensation techniques optimize film growth kinetics, conventional average roughness metrics fail to account for field limitations; symmetric triangular surface morphologies maintain an average roughness that is entirely invariant to slope angle, concealing steep nanoscale topographies on surfaces that appear exceptionally smooth under standard profiling. Atomic force microscopy reveals corrugations whose physical dimensions and steep facet slopes directly correspond to the characteristic London penetration depth and superconducting coherence length of clean niobium. Within the London theoretical model, these geometries induce localized magnetic field enhancement at protrusion ridges and significantly degrade the Bean–Livingston surface barrier within adjacent troughs, providing a quantitative origin for the peak magnetic field plateaus observed in state-of-the-art coated structures. Optimizing deposition energetics alongside near-surface impurity engineering offers a promising path to mitigate these geometric vulnerabilities and enable high-gradient cavity operation.