Effect of metal encapsulation on bulk superconducting properties of niobium thin films used in qubits
Phys. Rev. Applied 26, 044041 – Published 14 October, 2026
DOI: https://doi.org/10.1103/sbx1-g6vb
Abstract
While the Josephson junction is central to qubit operation, overall quantum coherence requires thorough understanding of the entire metallic environment—leads, capacitive pads, and readout resonators. The established contributors to decoherence are surface oxides and hydroxides, as well as absorbed species, such as hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with nonoxidizing metals is a successful strategy to mitigate these issues. To achieve guided optimization, it is significant to understand the microscopic behavior of capped films. Although most attention was paid to the surface and interfaces, bulk properties were not systematically studied. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium gold and compare them to bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature and the lowest upper critical field, London penetration depth, and critical current. These observations agree well with the microscopic theory of anisotropic normal and superconducting states of niobium with varying amounts of scattering. We conclude that surface encapsulation significantly affects bulk properties, and gold-coated films are the cleanest. Since these changes reside in the bulk of the niobium rather than in the overlayer, they may in turn influence the processes that limit quantum coherence. Considering such a strong variation of properties, we believe that the question of whether the surface or bulk is the dominant contributor to quantum decoherence is wide open. However, they likely both contribute via different mechanisms, which makes it very hard to pinpoint where the bottleneck is.
