  Surface properties are critical to the capabilities of superconducting
microwave devices. The native oxide of niobium-based devices is thought to
consist of a thin normal conducting layer. To improve understanding on the
importance of this layer, an attempt was made to replace it with a more easily
controlled gold film. A niobium sample host microwave cavity was used to
measure the surface resistance in continuous wave operation at 4.0 GHz and 5.2
GHz. Sample conditions studied include temperatures ranging from 1.6 K to 4.2 K
with RF magnetic fields on the sample surface ranging from 1 mT to the maximum
field before the superconducting properties were lost (quench field). The
nominal film thickness of the gold layer was increased from 0.1 nm to 2.0 nm in
five steps to study the impact of the normal layer thickness on surface
resistance on a single niobium substrate. The 0.1 nm film was found to reduce
the surface resistance of the sample and to enhance the quench field. With the
exception of the final step from a 1.5 nm gold film to 2.0 nm, the magnitude of
the surface resistance increased substantially with gold film thickness. The
nature of the surface resistance field-dependence appeared to be roughly
independent from the gold layer thickness. This initial study provides new
perspectives and suggests avenues for optimizing and designing surfaces for
resonant cavities in particle accelerators and quantum information
applications.
