$\textit{Ab initio}$ Theory of Eliminating Surface Oxides of Superconductors with Noble-Metal Encapsulation

Abstract

Nanometer-scale surface chemistry limits the performance of SRF cavities and quantum circuits. We present an ab initio framework connecting DFT 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 (WAL) yields far more robust adhesion than a capping layer alone under realistic conditions, enabling thinner caps, and thereby addressing a central challenge in superconducting surface passivation.

Figures (7)

• Figure 1: Illustration of Au capping layer, wetting/adhesion underlayer (WAL), and Nb/Ta substrate.
• Figure 2: Interface map for candidate caps on (a) Nb(110) and (b) Ta(110). Axes: interface energy $\gamma_{\mathrm{int}}$ vs surface-energy contrast $\Delta\gamma=\gamma_{\mathrm{cap}}-\gamma_{\text{sub}(110)}$. Dashed line: $S=0$ wetting boundary; shaded gradient: wetting (green) to islanding (red).
• Figure 3: Normalized transition temperature for Nb$|$N (a) and Ta$|$N (b) bilayers versus $d_N/d_S$. Horizontal black dashed line: $T_c^{\text{bilayer}}/T_c^{\text{substrate}}\ge 0.8$. Experimental data are shown as solid dots yagoubov2001developmentchattaraj2025preventingbanerjee2003proximitypotenza2007layerchang2025eliminating
• Figure 4: Calculated $T_c$ of Nb and Ta bulk as a function of $\mu^*$. The dashed lines represent $\mu^*=0.13$ and $\mu^*=0.14$ used in this letter for Ta and Nb respectively.
• Figure 5: Computed Eliashberg spectral function for metals in our bilayer studies.