Nanometer-scale surface chemistry limits the performance of superconducting radio-frequency cavities and quantum circuits. We present an ab initio framework connecting density-functional theory interfacial energetics with strong-coupling Eliashberg theory for capped $\mathrm{Nb}$ and $\mathrm{Ta}$ surfaces. This approach identifies $\mathrm{Au}$ and $\mathrm{Au}$-based alloys ($\mathrm{Au}\mathrm{Pd}$, $\mathrm{Au}\mathrm{Pt}$) as effective passivation layers. Our model further predicts that combining a noble-metal capping layer with an appropriate wetting/adhesion layer yields far more robust adhesion than a capping layer alone under realistic conditions, enabling thinner caps and thus addressing a central challenge in superconducting surface passivation.

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• Quantum computation
• Elemental superconductors
• Surfaces
• Ultrathin films
• Density functional theory
• Eliashberg theory