Speaker
Description
Large experiments in fundamental physics, from axion haloscopes [1] to future accelerator technologies like the CERN FCC [2], can benefit enormously from superconductors with low surface impedance in high magnetic fields. In these regimes, vortex motion is the main source of rf dissipation. The achievable performance is thus governed not only by the intrinsic superconducting properties, but also by the material defects which at the same time should provide efficient vortex pinning centers without significantly impacting on the degree of cleanness of the vortex cores, where dissipation due to quasi-particles mainly occurs. Understanding and controlling the interplay between microstructure and overall vortex dissipation is mandatory to optimize the superconducting coatings.
In this work we focus first on Nb3Sn, extending our previous investigations on DC magnetron sputtered films, examining the role of the Sn-rich islands recently observed on those DCMS samples [3] and their impact on the pinning landscape. A comparison is carried out on newly prepared Nb3Sn coatings grown on sapphire substrates, 500 nm thick, where the reduced thickness should strongly suppress the formation of islands and provide a cleaner reference system. We present dual frequency microwave (ν=8 and 27 GHz) measurements in perpendicular fields up to 1.2 T whence the vortex dynamics parameters are extracted. We compare the pinning strength (measured by Labusch parameter kp), the flux-flow dissipation (probed through the flux flow resistivity ρff), and thermal creep, aiming at identifying the effect of mesoscale structural inhomogeneities on the Nb3Sn high frequency dissipation.
We further broaden the investigation to coated-conductors (CCs) REBCO tapes, mechanically peeled to expose the superconducting film from the CC tape by two independent laboratories, ICMAB and ENEA, to assess their potential for axion haloscopes and particle accelerators beam-screen applications. Indeed, REBCO provides very high upper critical field with mature tape technology, but its practical performance in rf resonant structures significantly depends on the preparation stage and on the resulting vortex response after delamination. We find that CCs exhibit very strong (beneficial) vortex pinning, with flux-flow resistivity and quasiparticle states close to single crystals, thus offering an excellent mix for applications. Overall, the study provides a comparative picture of Nb3Sn and REBCO as candidate materials for haloscopes [4] and other high-field applications.
Acknowledgments
Work partially supported by INFN CSN5 project SUPERMAD and FCC collaboration under MoU Addendum FCC-GOV-CC-0218 (KE5084/ATS).
We acknowledge sample preparation and useful discussions with S. Calatroni, G. Celentano, A. Masi, C. Pira, S. Posen, T. Puig, A. Vannozzi.
References
1. D Alesini et al, Phys. Rev. D 99, 101101(R) (2019)
2. S Calatroni, IEEE Trans. Appl. Supercond. 26, 3500204 (2016)
3. D. Fonnesu et al., Sci Rep. 16, 3539 (2026)
4. A. Alimenti et al, Instruments 6, 1 (2022)