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1. Direct current magnetic Hall probe technique for measurement of field penetration in thin film superconductors for superconducting radio frequency resonators

   https://doi.org/10.1063/5.0083309  

   Senevirathne, I. H.; Gurevich, A.; Delayen, J. R. (May 2022, Review of Scientific Instruments)

   Superconducting Radio Frequency (SRF) cavities used in particle accelerators are typically formed from or coated with superconducting materials. Currently, high purity niobium is the material of choice for SRF cavities that have been optimized to operate near their theoretical field limits. This brings about the need for significant R & D efforts to develop next generation superconducting materials that could outperform Nb and keep up with the demands of new accelerator facilities. To achieve high quality factors and accelerating gradients, the cavity material should be able to remain in the superconducting Meissner state under a high RF magnetic field without penetration of quantized magnetic vortices through the cavity wall. Therefore, the magnetic field at which vortices penetrate a superconductor is one of the key parameters of merit of SRF cavities. Techniques to measure the onset of magnetic field penetration on thin film samples need to be developed to mitigate the issues with the conventional magnetometry measurements that are strongly influenced by the film orientation and shape and edge effects. In this work, we report the development of an experimental setup to measure the field of full flux penetration through films and multi-layered superconductors. Our system combines a small superconducting solenoid that can generate a magnetic field of up to 500 mT at the sample surface and three Hall probes to detect the full flux penetration through the superconductor. This setup can be used to study alternative materials that could potentially outperform niobium, as well as superconductor–insulator–superconductor (SIS) multilayer coatings on niobium. 

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2. MAGNETIC FIELD PENETRATION TECHNIQUE TO STUDY FIELD SHIELDING OF MULTILAYERED SUPERCONDUCTORS

   https://doi.org/10.18429/JACoW-SRF2021-SUPTEV001  

   I. H. Senevirathne; A. Gurevich; J. R. Delayen; A-M Valente-Feliciano (July 2021, SRF)

   The SIS structure which consists of alternative thin lay- ers of superconductors and insulators on a bulk niobium has been proposed to shield niobium cavity surface from high magnetic field and hence increase the accelerating gradient. The study of the behavior of multilayer supercon- ductors in an external magnetic field is essential to opti- mize their SRF performance. In this work we report the de- velopment of a simple and efficient technique to measure penetration of magnetic field into bulk, thin film and mul- tilayer superconductors. Experimental setup contains a small superconducting solenoid which can produce a par- allel surface magnetic field up to 0.5 T and Hall probes to detect penetrated magnetic field across the superconduct- ing sample. This system was calibrated and used to study the effect of niobium sample thickness on the field of full magnetic flux penetration. We determined the optimum thickness of the niobium substrate to fabricate the multi- layer structure for the measurements in our setup. This technique was used to measure penetration fields of Nb 3 Sn thin films and Nb 3 Sn/Al 2 O 3 multilayers deposited on Al 2 O 3 wafers. The system was optimized to mitigate thermo- magnetic flux jumps at low temperatures. 

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3. Impact of submicron ${\mathrm{Nb}}_3\mathrm{Sn}$ stoichiometric surface defects on high-field superconducting radiofrequency cavity performance

   https://doi.org/10.1103/PhysRevResearch.6.043133  

   Willson, Sarah A; Harbick, Aiden V; Shpani, Liana; Do, Van; Lew-Kiedrowska, Helena; Liepe, Matthias U; Transtrum, Mark K; Sibener, S J (November 2024, Physical Review Research)

   ${\mathrm{Nb}}_3\mathrm{Sn}$film coatings have the potential to drastically improve the accelerating performance of Nb superconducting radiofrequency (SRF) cavities in next-generation linear particle accelerators. Unfortunately, persistent ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric material defects formed during fabrication limit the cryogenic operating temperature and accelerating gradient by nucleating magnetic vortices that lead to premature cavity quenching. The SRF community currently lacks a predictive model that can explain the impact of chemical and morphological properties of ${\mathrm{Nb}}_3\mathrm{Sn}$defects on vortex nucleation and maximum accelerating gradients. Both experimental and theoretical studies of the material and superconducting properties of the first 100 nm of ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces are complicated by significant variations in the volume distribution and topography of stoichiometric defects. This work contains a coordinated experimental study with supporting simulations to identify how the observed chemical composition and morphology of certain Sn-rich and Sn-deficient surface defects can impact the SRF performance. ${\mathrm{Nb}}_3\mathrm{Sn}$films were prepared with varying degrees of stoichiometric defects, and the film surface morphologies were characterized. Both Sn-rich and Sn-deficient regions were identified in these samples. For Sn-rich defects, we focus on elemental Sn islands that are partially embedded into the ${\mathrm{Nb}}_3\mathrm{Sn}$film. Using finite element simulations of the time-dependent Ginzburg-Landau equations, we estimate vortex nucleation field thresholds at Sn islands of varying size, geometry, and embedment. We find that these islands can lead to significant SRF performance degradation that could not have been predicted from the ensemble stoichiometry alone. For Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces, we experimentally identify a periodic nanoscale surface corrugation that likely forms because of extensive Sn loss from the surface. Simulation results show that the surface corrugations contribute to the already substantial drop in the vortex nucleation field of Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces. This work provides a systematic approach for future studies to further detail the relationship between experimental ${\mathrm{Nb}}_3\mathrm{Sn}$growth conditions, stoichiometric defects, geometry, and vortex nucleation. These findings have technical implications that will help guide improvements to ${\mathrm{Nb}}_3\mathrm{Sn}$fabrication procedures. Our outlined experiment-informed theoretical methods can assist future studies in making additional key insights about ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric defects that will help build the next generation of SRF cavities and support related superconducting materials development efforts. Published by the American Physical Society2024 

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4. The demonstration of Majorana zero modes in scalable gold nanowires

   https://doi.org/10.1117/12.2565976  

   Wei, Peng; Manna, Sujit; Xie, Yingming; Law, Kam Tuen; Lee, Patrick; Moodera, Jagadeesh (August 2020, Spintronics XIII)

   Drouhin, Henri-Jean M.; Wegrowe, Jean-Eric; Razeghi, Manijeh (Ed.)

   Majorana zero modes (MZMs) are expected to emerge in material heterostructures combining superconductivity, ferromagnetism, and spin-orbit coupling (SOC). Particularly, inducing superconductivity and magnetic exchange interactions in well-defined Shockley surface states (SS) of high quality ultrathin Au(111) layers, which intrinsically have strong SOC, has been predicted as an excellent platform for MBS. In this talk, our success in creating such heterostructure in epitaxially grown Au(111) heterostructures will be presented. Signatures of superconductivity induced in the two-dimensional SS of Au(111) thin film are observed by means of electron tunneling spectroscopy. The behavior of such superconducting state under a planar Zeeman field will be shown. Evidence of a pair of MZMs in a fabricated Au(111) nanowire system will be demonstrated. 

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5. Promoting subsurface Sn incorporation at Nb(100) oxide surface sites leading to homogeneous Nb3Sn film growth for superconducting radiofrequency applications

   https://doi.org/10.1116/6.0003892  

   Willson, Sarah A; Farber, Rachael G; Sibener, S J (December 2024, Journal of Vacuum Science & Technology A)

   For next-generation superconducting radiofrequency (SRF) cavities, the interior walls of existing Nb SRF cavities are coated with a thin Nb3Sn film to improve the superconducting properties for more efficient, powerful accelerators. The superconducting properties of these Nb3Sn coatings are limited due to inhomogeneous growth resulting from poor nucleation during the Sn vapor diffusion procedure. To develop a predictive growth model for Nb3Sn grown via Sn vapor diffusion, we aim to understand the interplay between the underlying Nb oxide morphology, Sn coverage, and Nb substrate heating conditions on Sn wettability, intermediate surface phases, and eventual Nb3Sn nucleation. In this work, Nb-Sn intermetallic species are grown on a single crystal Nb(100) in an ultrahigh vacuum chamber equipped with in situ surface characterization techniques including scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Sn adsorbate behavior on oxidized Nb was examined by depositing Sn with submonolayer precision on a Nb substrate held at varying deposition temperatures (Tdep). Experimental data of annealed intermetallic adlayers provide evidence of how Nb substrate oxidization and Tdep impact Nb-Sn intermetallic coordination. The presented experimental data contextualize how vapor and substrate conditions, such as the Sn flux and Nb surface oxidation, drive homogeneous Nb3Sn film growth during the Sn vapor diffusion procedure on Nb SRF cavity surfaces. This work, as well as concurrent growth studies of Nb3Sn formation that focus on the initial Sn nucleation events on Nb surfaces, will contribute to the future experimental realization of optimal, homogeneous Nb3Sn SRF films. 

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