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

Dissipation by surface states in superconducting RF cavities

Recent experiments on superconducting cavities have found that under large radio-frequency (RF) electromagnetic fields the quality factor can improve with increasing field amplitude, a so-called "anti-Q slope." Linear theories of dissipation break down under these extreme conditions and are unable to explain this behavior. We numerically solve the Bogoliubov-de Gennes equations at the surface of a superconductor in a parallel AC magnetic field, finding that at large fields there are quasiparticle surface states with energies below the bulk value of the superconducting gap. As the field oscillates, such states emerge and disappear with every cycle. We consider the dissipation resulting from inelastic quasiparticle-phonon scattering into these states and investigate the ability of this mechanism to explain features of the experimental observations, including the field dependence of the quality factor. We find that this mechanism is likely not the dominant source of dissipation and does not produce an anti-Q slope by itself; however, we demonstrate in a modified two-fluid model how these bound states can play a role in producing an anti-Q slope

Surface oxides, carbides, and impurities on RF superconducting Nb and Nb3Sn: A comprehensive analysis

Surface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium-tin (Nb3Sn) in situ under different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb3Sn contains thick metastable oxides, while electrochemically synthesized Nb3Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb3Sn surface structures for high-performance superconducting RF and quantum application

Smooth, homogeneous, high-purity Nb3Sn superconducting RF resonant cavity by seed-free electrochemical synthesis

Workbench-size particle accelerators, enabled by Nb3Sn-based superconducting radio-frequency (SRF) cavities, hold the potential of driving scientific discovery by offering a widely accessible and affordable source of high-energy electrons and X-rays. Thin-film Nb3Sn RF superconductors with high quality factors, high operation temperatures, and high-field potentials are critical for these devices. However, surface roughness, non-stoichiometry, and impurities in Nb3Sn deposited by conventional Sn-vapor diffusion prevent them from reaching their theoretical capabilities. Here we demonstrate a seed-free electrochemical synthesis that pushes the limit of chemical and physical properties in Nb3Sn. Utilization of electrochemical Sn pre-deposits reduces the roughness of converted Nb3Sn by five times compared to typical vapor-diffused Nb3Sn. Quantitative mappings using chemical and atomic probes confirm improved stoichiometry and minimized impurity concentrations in electrochemically synthesized Nb3Sn. We have successfully applied this Nb3Sn to the large-scale 1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at multiple operation temperatures, notably lower than vapor-diffused cavities. Our smooth, homogeneous, high-purity Nb3Sn provides the route toward high efficiency and high fields for SRF applications under helium-free cryogenic operations

On the Incommensurate Phase of Pure and Doped Spin-Peierls System CuGeO_3

Phases and phase transitions in pure and doped spin-Peierls system CuGeO_3 are considered on the basis of a Landau-theory. In particular we discuss the critical behaviour, the soliton width and the low temperature specific heat of the incommensurate phase. We show, that dilution leads always to the destruction of long range order in this phase, which is replaced by an algebraic decay of correlations if the disorder is weak.Comment: 4 pages revtex, no figure

A quantitative analysis of complexity of human pathogen-specific CD4 T cell responses in healthy M. tuberculosis infected South Africans

Author Summary: Human pathogen-specific immune responses are tremendously complex and the techniques to study them ever expanding. There is an urgent need for a quantitative analysis and better understanding of pathogen-specific immune responses. Mycobacterium tuberculosis (Mtb) is one of the leading causes of mortality due to an infectious agent worldwide. Here, we were able to quantify the Mtb-specific response in healthy individuals with Mtb infection from South Africa. The response is highly diverse and 66 epitopes are required to capture 80% of the total reactivity. Our study also show that the majority of the identified epitopes are restricted by multiple HLA alleles. Thus, technical advances are required to capture and characterize the complete pathogen-specific response. This study demonstrates further that the approach combining identified epitopes into "megapools" allows capturing a large fraction of the total reactivity. This suggests that this technique is generally applicable to the characterization of immunity to other complex pathogens. Together, our data provide for the first time a quantitative analysis of the complex pathogen-specific T cell response and provide a new understanding of human infections in a natural infection setting

Impact of distinct poxvirus infections on the specificities and functionalities of CD4+ T cell responses.

UNLABELLED: The factors that determine CD4+ T cell (TCD4+) specificities, functional capacity, and memory persistence in response to complex pathogens remain unclear. We explored these parameters in the C57BL/6 mouse through comparison of two highly related (\u3e92% homology) poxviruses: ectromelia virus (ECTV), a natural mouse pathogen, and vaccinia virus (VACV), a heterologous virus that nevertheless elicits potent immune responses. In addition to elucidating several previously unidentified major histocompatibility complex class II (MHC-II)-restricted epitopes, we observed many qualitative and quantitative differences between the TCD4+ repertoires, including responses not elicited by VACV despite complete sequence conservation. In addition, we observed functional heterogeneity between ECTV- and VACV-specific TCD4+ at both a global and individual epitope level, particularly greater expression of the cytolytic marker CD107a from TCD4+ following ECTV infection. Most striking were differences during the late memory phase where, in contrast to ECTV, VACV infection failed to elicit measurable epitope-specific TCD4+ as determined by intracellular cytokine staining. These findings illustrate the strong influence of epitope-extrinsic factors on TCD4+ responses and memory. IMPORTANCE: Much of our understanding concerning host-pathogen relationships in the context of poxvirus infections stems from studies of VACV in mice. However, VACV is not a natural mouse pathogen, and therefore, the relevance of results obtained using this model may be limited. Here, we explored the MHC class II-restricted TCD4+ repertoire induced by mousepox (ECTV) infection and the functional profile of the responding epitope-specific TCD4+, comparing these results to those induced by VACV infection under matched conditions. Despite a high degree of homology between the two viruses, we observed distinct specificity and functional profiles of TCD4+ responses at both acute and memory time points, with VACV-specific TCD4+ memory being notably compromised. These data offer insight into the impact of epitope-extrinsic factors on the resulting TCD4+ responses

Advancing a Superconducting Sample Host Cavity and its Application for Studying Proximity-Coupled Normal Layers in Strong Microwave Fields

241 pagesThe study of the interaction of a microwave field with a conventional superconducting surface is a rich topic for both application and science. The basic interaction with a small signal is understood and was quantitatively described decades ago. These descriptions rapidly break down in the presence of large amplitude microwave fields. The resulting interaction is difficult to model and depends strongly on the surface features and properties. A variety of behaviors are observed as the field amplitude is increased. The nature of the behavior changes for different materials, surface structures, and frequencies. Theoretical models describing the interaction of a large microwave signal with a superconductor have been proposed, attacking the issue from a variety of perspectives. At this time, no microscopic models exist that are able to even qualitatively explain the variety of behaviors that are observed. Beyond the scientific intrigue of better-describing the microscopic origin of the various observed behaviors of superconductors in these extreme conditions, there exists practical motivation. Particle accelerators employ resonant cavities with superconducting surfaces as a means of transferring energy to the particles. For this application, the goal is minimizing the dissipation of the microwave energy in the superconducting surface while maximizing the applied surface field. To engineer increasingly high performance surfaces, it is required to understand what features/properties are desirable or detrimental for obtaining the application goals. Realizing a large microwave field on a surface is nontrivial. In this work, a driven resonant cavity was used to create the high amplitude fields. This structure was a sample host cavity, designed with an opening such that a flat sample plate could be attached to close the volume. This scheme allows for exposing a detachable flat sample to a large microwave field. It is nontrivial to measure the response of the sample to the microwave field, as it must be decoupled from that of the system as a whole. The method used for this purpose is sensitive to systematic and measurement uncertainty, especially for samples of direct interest for accelerator application. Attempts were made to modify the system to improve its measurement quality and range. The implemented changes led to a significant improvement in performance. Using this sample host cavity, an attempt was made to improve the understanding of a common feature of superconducting surfaces, the native oxide. Specifically, the surface oxide that is present on the best materials known for accelerator application, niobium and niobium-tin. The niobium oxide contains a metallic phase that electrically couples to the superconducting bulk. This coupling, referred to as proximity-coupling, results in the normal conducting oxide layer taking on some superconducting properties. Conversely, the normal conducting layer will influence the properties of the superconductor near the surface. Models and experiments indicate that this metallic oxide may have an important role in the amplitude-dependence of the microwave dissipation in superconducting cavities used for applications. But it is difficult to control the relevant properties of the oxide, and to do so without altering other surface features. This makes it difficult to study the impact of metallic surface oxide phases on the microwave interaction directly. In this study, the choice was made to remove the oxide and replace it with an easier-to-control gold layer. This allowed for a more controlled study of the microwave response of a proximity-coupled system. Using the sample host cavity, high-field RF measurements were performed on these gold-superconductor samples for a range of gold layer thicknesses. A model describing the influence of proximity-coupling on the microwave response was implemented to assist with interpreting the measurements. The data was well-described by this model for small amplitude fields, but the agreement was lost as the field strength increased. It was found that replacing the niobium oxide with a minimal thickness gold layer enhanced the maximum field limitations of the system. This result indicates that the niobium oxide could also be a limiting factor in accelerator applications