39 research outputs found
Measurements of the amplitude-dependent microwave surface resistance of an Au/Nb bilayer
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.Comment: Submitted to: Superconductor Science and Technolog
Shielding superconductors with thin films
Determining the optimal arrangement of superconducting layers to withstand
large amplitude AC magnetic fields is important for certain applications such
as superconducting radiofrequency cavities. In this paper, we evaluate the
shielding potential of the superconducting film/insulating film/superconductor
(SIS') structure, a configuration that could provide benefits in screening
large AC magnetic fields. After establishing that for high frequency magnetic
fields, flux penetration must be avoided, the superheating field of the
structure is calculated in the London limit both numerically and, for thin
films, analytically. For intermediate film thicknesses and realistic material
parameters we also solve numerically the Ginzburg-Landau equations. It is shown
that a small enhancement of the superheating field is possible, on the order of
a few percent, for the SIS' structure relative to a bulk superconductor of the
film material, if the materials and thicknesses are chosen appropriately.Comment: 7 pages, 5 figure
On superconducting niobium accelerating cavities fired under N2-gas exposure
The dependence of the Q-value on the RF field (Q-slope) is actively studied
in various accelerator laboratories. Although remedies against this dependence
have been found, the physical cause still remains obscure. A rather
straightforward two-fluid model description of the Q-slope in the low and high
field domains is presented with emphasis on the recently experimentally
identified improvement of the Q-value by so-called "N-doping"
Electrochemical Polishing of Chemical Vapor Deposited Niobium Thin Films
Combining chemical vapor deposition (CVD) with electrochemical polish (EP)
operations is a promising route to producing performance-capable
superconducting films for use in the fabrication of cost-effective components
for superconducting radiofrequency (SRF) particle accelerators and
superconducting quantum computers. The post-deposition EP process enables a
critically necessary reduction in surface roughness of niobium thin films to
promote optimal superconducting surface conditions. In this work, surface
morphology, roughness, and crystal orientation of the CVD-grown and EP-polished
niobium films were investigated. The grain growth and polishing mechanisms were
analyzed. The CVD films were found to comprise steps, kinks, and pyramidal
features, resulting in undesirable large peak-to-valley distances. The
electrochemical polish was demonstrated to significantly diminish the height of
pyramids and effectively minimize the overall surface roughness. In contrast to
buffered chemical polishing (BCP), EP results showed a probable dependence on
crystal orientation, suggesting this process was influenced by locally enhanced
current density and thickness variations of oxide dielectrics. These
understandings identify the EP principles tied to CVD-grown Nb films that allow
further refinement of surface profiles for film-based SRF application