Ab Initio Theory of Eliminating Surface Oxides of Superconductors with Noble-Metal Encapsulation

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 Nb and Ta surfaces. This approach identifies Au and Au-based alloys (AuPd, AuPt) 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.