Abstract Reproducible low-loss microwave interconnects are essential for scaling modular superconducting quantum processors and networks. Wire bonds provide a simple and flexible route to galvanic connections in such systems, but their microwave loss and reproducibility at millikelvin temperatures remain insufficiently quantified. Here we demonstrate reproducible low-loss aluminum wire-bond interconnects in both superconducting cable resonators and planar tantalum resonators. In the cable devices, a wire-bond connection is placed at the midpoint of a high-quality resonator, where the different current distributions of odd and even modes separate connection-induced loss from intrinsic cable dissipation within the same device. Across devices fabricated with different bond numbers and ultrasonic powers, a stable bonding window yields no resolvable connection resistance above the practical resolution of order 0.1 mΩ. Measurements of planar resonators on two chips independently show no systematic degradation of the internal quality factor from wire-bond ground terminations within the device-to-device variation. These results establish aluminum wire bonding as a reproducible low-loss interconnect for superconducting microwave circuits, supporting the scalable assembly of modular superconducting quantum processors and networks.
Abstract Distributed fiber-optic sensing is a promising approach for quench detection and protection of high-temperature superconducting (HTS) systems, offering immunity to electromagnetic interference and scalability to large cable and magnet systems. In this work, we demonstrate the response of continuous fiber Bragg grating (cFBG) sensors integrated on the outer layer of a two-layer spiral-wound REBCO cable. The cable consists of three 2-mm-wide coated conductors per layer wound around a 3-mm brass core, with the optical fiber wound helically on the outer surface at a 20-mm pitch. We present results from two quench scenarios: heater-induced and over-current induced quench. In the heater experiment, an optical response is observed before any measurable voltage is generated, demonstrating the potential of temperature-based detection to provide early warning ahead of voltage-based methods. In the over-current experiment, a measurable optical response is observed within tenths of a second of the onset of rapid voltage increase. These results demonstrate the feasibility of cFBG sensing in spiral-wound HTS cables and support continued development of fiber-optic monitoring systems for HTS applications.
Abstract REBCO coated conductors are of significant interest for high-field superconducting applications owing to their exceptional critical current retention under high magnetic fields. However, depending on the fabrication route, microstructural inhomogeneities such as porosity, Cu x O precipitates, and a-axis oriented grains can emerge at various length scales, disrupting current transport and limiting the critical current density. This study investigates and characterizes such defects in commercial REBCO conductors of varying specifications. Top-view SEM images of the REBCO layer were acquired following chemical etching of Cu and Ag layers to expose the microstructure for analysis. Conventional image analysis and segmentation techniques prove insufficient for reliably quantifying these defects, while manual identification remains prohibitively labor-intensive. To overcome these limitations, a machine learning approach is explored to enable rapid, automated, and accurate defect detection. Specifically, an open-source computer vision model, Mask R-CNN, is fine-tuned on domain-specific SEM image data. The fine-tuned model achieved a validation mean average precision of 45.23% and enabled quantitative defect analysis across 63 tapes. Partial correlation analysis, controlling for confounding between defect types, revealed independent associations with critical current density that varied in strength and sign across defect types and measurement conditions. These findings motivate further targeted characterization to establish the microstructural origins of these relationships and inform conductor optimization.
Abstract Fast and reliable quench detection remains a major challenge in the development of high-temperature superconducting (HTS) magnets with large stored energies in the megajoule range and above. Several temperature-based detection methods are investigated at EPFL Swiss Plasma Center, including twisted-pair superconducting wires (SQD), shielded thermocouple chains (TCC) and fiber-optics sensing (FOS). All three methods require strong thermal coupling to the main winding while remaining electrically insulated, thereby eliminating the need for high-voltage penetrations typically associated with voltage taps in fusion magnets. Their integration into the winding pack and their performance during quench events are studied on multiple samples of increasing size and stored energy, made of laminated stacked-tape soldered (LASSO) ReBCO conductors. These include a single racetrack coil tested in SULTAN using a 20 m-long 12-tape LASSO conductor and a double pancake coil tested in JORDI using a 50 m-long 15-tape LASSO conductor. Potential applications of the LASSO conductor and its industrial availability are also discussed.
Abstract Reducing the megabar pressures required for hydrides while maintaining strong electron–phonon coupling remains a major challenge. We address this with a Li₃IrH₉-derived dual-substitution strategy, designing quaternary Li2MgXH9 (X = Fe, Ir, Os, Ru) hydrides in which Mg-for-Li chemical precompression is combined with transition-metal substitution to tune the transition-metal d and H-derived electronic states. Harmonic phonon calculations place the lowest dynamically stable pressure at 10 GPa for Li2MgRuH9, 20 GPa for Li2MgFeH9, and 40 GPa for Li2MgIrH9 and Li2MgOsH9. Li2MgRuH9 reaches Tc = 87.1 K at 10 GPa and 115.0 K at 150 GPa, and all four phases remain thermodynamically metastable (ZPE-corrected decomposition enthalpies of 0.521–2.124 eV per formula unit) at their lowest stable pressures. The favorable response of Li2MgRuH9 arises from coupled electronic and lattice effects: low-frequency regions dominated by Ru, Mg, and Li vibrations account for 63.4–83.8% of λ over 10–190 GPa, while H-dominated high-frequency regions contribute 8.5–10.7% of λ yet sustain a high ωlog. At these pressures, index-1 saddle points lie within approximately 0.08 eV of the Fermi level in all four compounds, identifying their low-pressure DOS enhancements as van Hove-like features; in the Ru and Ir systems, pressure drives these saddle points away from the Fermi level and lowers NF. Compression initially raises ωlog; stronger compression then broadens the bands, depletes NF, and weakens the coupling. These results identify Ru-centered Li–Mg hydrides as promising low-pressure superconductors and provide a pressure–composition design principle that separates beneficial phonon hardening from electronic-state depletion in quaternary hydride superconductors.
Abstract The authors acknowledge a recalculation of their critical current density ( J C ) values, confirming an overestimation by a factor of approximately 8 due to a hybrid cgs/SI formulation of the Bean model, with corrected values now on the order of 10 7 A cm −2 at 10 K and low field. This correction, however, does not alter the fundamental conclusions of their study on vortex dynamics in an undoped, uniaxially textured YBCO film. The lower J C values are consistent with the presence of natural pinning centers, which was the focus of their research, and not intended to compete with commercial superconducting wires. Analyses of vortex regimes, crossover fields, power-law exponents, magnetic relaxation, and normalized pinning force curves are invariant to the absolute scaling of J C . This is because these analyses rely on ratios of J C to other parameters (like J 0 ) or normalized quantities (like J C / J 0 or F P /F Pmax ), which remain unchanged despite the scaling factor adjustment. Therefore, the identification of four vortex regimes, the characterization of pinning center sizes and defect densities, and the conclusions regarding the dominant role of natural pinning mechanisms in this specific YBCO system remain valid.
Abstract Compact and mechanically reinforced REBCO insert magnets provide greater flexibility for high-field magnet design and construction. Meanwhile, conduction-cooled REBCO magnets have attracted increasing interest because they can operate below 20 K with lower cooling costs than conventional liquid-helium-cooled systems. An extreme no-insulation conduction-cooled REBCO magnet with a 40 mm cold bore was constructed and preliminarily tested as a prototype for a 40 T high-field magnet project. The prototype was wound using 4 mm-wide, 50 μ m-thick REBCO tapes with 1 μ m-thick copper stabilizer layers on both sides. The ultra-thin REBCO tapes enabled a compact winding structure, while the surface-shunting layer provided additional turn-to-turn current bypass capability. Stycast epoxy was applied to the solder-shunted surfaces of the double-pancake (DP) coils to enhance the thermal conduction between the REBCO coils and the cooling plate. The edge-bonded epoxy also mechanically reinforced the REBCO turns against electromagnetic stresses. The prototype was cooled below 10 K within 14 h and charged to 105 A within 2 h, at which point a localized fault-mode quench occurred in one DP coil. The magnet steadily dissipated the stored energy with the assistance of an external shunt circuit and was subsequently re-operated up to 120 A, generating a center field of 9.3 T after bypassing the damaged coil. Dedicated sudden-discharge tests at 100 A further demonstrated the effectiveness of the external shunt, reducing the peak temperature rise from 33 K to 13 K during power-supply shutdown events.
Abstract The interaction between artificial pinning centers and irradiation-induced defects is critical to the performance of YBa 2 Cu 3 O 7- δ (YBCO) coated conductors in radiation environments. In this work, YBCO coated conductors with 0%–15% BaHfO 3 (BHO) doping levels were irradiated with 14 MeV N ions at fluences of 4.437 × 10 12 , 4.417 × 10 14 and 1.004 × 10 16 ions cm −2 , and the evolution of critical current density ( J c ) and microstructure were investigated. The pristine 5% BHO-doped sample showed the highest J c , indicating that moderate BHO doping provides effective pinning while maintaining the continuity of the YBCO matrix. After irradiation, J c exhibited a strong dependence on fluence, temperature, magnetic field, and BHO content. High-fluence irradiation caused severe degradation in all samples, whereas at lower fluences, the 0% and 15% BHO-doped samples showed improved current-carrying performance under certain conditions, while the 5% and 10% samples degraded. x-ray diffraction and ω -scan analyses revealed irradiation-induced lattice distortion and degradation of the out-of-plane texture. Compared with undoped YBCO, the BHO-doped samples generally exhibited smaller net full width at half maximum increases at the highest fluence, indicating that the evolution of mosaic spread depends on the BHO concentration and suggesting that BHO-related interfaces may influence the accumulation and redistribution of irradiation-induced disorder. High-resolution transmission electron microscopy observations showed lattice bending, local disorder, dislocation-like defects, and slight local amorphization. Molecular dynamics simulations indicated that N-ion irradiation generates vacancies and interstitial atoms. Their accumulation and clustering may contribute to local disorder and localized amorphization. Further SAED analyses showed that some BHO nanoparticles became diffuse and structurally disordered after irradiation, suggesting their participation in defect accommodation and redistribution. These results indicate that irradiation-induced defects interact not only with the superconducting YBCO matrix but also with pre-existing BHO nanoparticles and their interface regions, providing useful guidance for defect engineering of YBCO coated conductors in radiation environments.
Abstract We develop a magneto-optical imaging (MOI) system based on a paramagnetic garnet indicator combined with a polarizing microscope and implement it in a 25 T cryogen-free superconducting magnet, enabling wide-field imaging of superconducting critical states under steady magnetic fields up to 22 T. In the low-field regime, quantitative evaluation of the magnetic field distribution is achieved using a differential imaging method. In higher fields above 10 T, where the low-field analysis is no longer directly applicable due to the field dependence of the indicator response, an alternative approach based on a quadratic approximation of the light intensity is employed. Using these methods, we visualize magnetic flux penetration and trapping across the full millimeter-scale width of commercial REBCO tapes and evaluate the critical current I c up to 22 T, showing good agreement with pulsed-current transport measurements. These results establish magneto-optical imaging as a powerful tool for wide-field characterization of superconducting critical states in high magnetic fields, offering advantages over scanning probe methods such as magnetic force microscopy.
Abstract The vulnerability of REBCO insert magnets to extreme mechanical responses during open-circuit faults has caused severe failures in some high-field systems. This work studies the fault behaviors of a nested no-insulation/metal-insulation (NI/MI) REBCO insert in a 30 T magnet using the rotated anisotropic resistivity method. Experimentally calibrated characteristic turn-to-turn resistances of unshunted and InSn surface-shunted coils (4000 and 100 μ Ω·cm 2 ) are adopted in the simulation. Results of simulations under radially non-uniform turn-to-turn resistance configurations reveal that electromagnetically coupled quench propagation forces the low-resistance layer to protect the high-resistance layer at the cost of excessive current and stress, failing to reconcile flux compensation and mechanical load. Two complementary strategies are then proposed: an axial graded resistance scheme that synchronizes regional discharge time constants to suppress current amplification, and an external magnetic dam that provides passive flux compensation via co-wound superconducting tapes in the over-banding layer. These reduce peak stress by ∼27%, meeting long-term operational safety thresholds. These findings demonstrate that regulating the spatiotemporal distribution of flux change rate through resistance matching is an effective approach to suppress open-circuit fault damage in nested magnets, offering a theoretical basis for quench protection design in high-field nested REBCO magnets.
Abstract To effectively improve the quality of single-domain samarium barium copper oxide (SmBCO) bulk superconductors and accelerate their practical application, understanding and mastering the growth kinetics is of great importance. In this work, within the growth temperature window of 1059–1043 ℃, the effects of slow cooling rate( 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, 1.8, 2.0, 2.2, and 2.6 °C/h, defined as the cooling rate applied within crystal-growth temperature window of the SmBCO bulks) on the crystal growth kinetics, microstructures and superconducting properties of SmBCO bulks fabricated by the TSIG method with a novel Sm₂O₃+011 (Sm₂O₃+1.2BaCuO₂) precursor are systematically investigated. The results show that, in the temperature range of 1059–1043 ℃, the surface single-domain sector area of the samples gradually decreases with increasing cooling rate. When the cooling rate is raised to 2.0 ℃/h, random nucleation occurs and small grains appear near the sample edges. Meanwhile, the growth-affected region along the c-axis of the samples also shrinks gradually. By contrast, the average growth rates along both the a-b plane and c-axis first increase and then decrease, and the cooling rate shows a more significant influence on the c-axis growth rate. Microstructural observations reveal that the number of embedded Sm₂BaCuO₅ (Sm211) particles increases gradually, while their average size decreases with rising cooling rate. Measurements of the critical temperature (Tc) and critical current density (Jc) indicate that the superconducting performance reaches its optimum at a cooling rate of 1.2 ℃/h. As the cooling rate deviates from 1.2 ℃/h (either higher or lower), the superconducting properties of the samples degrade gradually. These findings provide important guidance for further improving the preparation quality and growth efficiency of SmBCO bulk superconductors.
Abstract This article will present a modified method for measuring critical current for superconducting wires and tapes. In this measurement method, the transport current is kept constant, and the magnetic field is increased by a magnet. This method is called the magnetic field sweep method. The research carried out showed that the magnetic field sweep method allows for a better description of the physical phenomena, which occur in superconducting wires made of several superconducting materials, e.g. Nb diffusion barrier and MgB 2 core. The conducted research indicated that the magnetic field sweep method can detect damage to the Nb diffusion barrier, more precisely determine I c in superconducting wires in which an electric field related to transfer length appears (short wires) and identify the magnetic field ranges in which individual pinning centers effectively trap the vortex lattice. All of these are key factors for enabling high critical current density superconducting wires.
Abstract Multiple pairs of bronze pieces were joined along a common seam and then exposed to Nb vapor via sputter deposition during heating at ∼ 715 ∘ C to form a diffusion bond between the pieces. Polishing and alignment of the pieces created smooth surfaces normal to the Nb flux with seams perpendicular to the surface (i.e. parallel to the Nb flux). Conversion of Nb to Nb 3 Sn took place simultaneously with diffusion bonding, resulting in Nb 3 Sn thin films that coated bronze surfaces and spanned seams with uniform thickness. Characterization of superconducting properties via magneto-optical imaging (MOI) suggests that, in several examples, supercurrent flows freely across the seam when cooled to 9 K, while shielding or trapping a low magnetic field. Modification of the process to coat the pieces with Nb before diffusion bonding and Nb 3 Sn formation resulted in varying degrees of seam coverage by the resultant Nb 3 Sn films. The pre-coating method did not produce any examples of comparable quality to those obtained by the hot bronze approach. This work could enable new approaches to joining Nb 3 Sn materials in magnet conductor and RF cavity applications.
Abstract The next generation of high and ultra-high field magnets will increasingly rely on REBCO coated conductors in the form of tapes, tape stacks or cables. However, the intrinsically anisotropic nature of REBCO presents significant operational challenges, particularly regarding the orientation of the applied magnetic field. Consequently, the critical current ( I c ) depends not only on temperature ( T ) and magnetic field amplitude ( B ), but also on the field orientation ( θ ) relative to the tape surface. Accurate characterization of the I c ( B,θ,T ) surface is therefore essential. This study investigates three commercial superconducting tapes from leading manufacturers: Shanghai Superconductor Technology, SuperPower and Faraday Factory Japan/SuperOx. These tapes were fabricated with different rare-earth elements and distinct pinning centers. Measurements were performed at the University of Geneva (UNIGE) and at Tohoku University within the High Field Laboratory for Superconducting Materials (HFLSM). Full-width 4 mm tapes were measured at UNIGE from 4.2 K to 40 K in applied magnetic fields up to 19 T with five distinct orientations. At HFLSM, tapes from the same spools were patterned with microbridges (30 μ m × 1 mm) and measured from 5 K to 55 K in fields up to 24 T, with angular dependence of I c measured over 135°. This extensive dataset was analyzed using the maximum entropy model (MEM) accurately reproducing I c ( B,θ,T ) profiles with a limited set of meaningful parameters. The model successfully captured both single dominant I c peak and more complex angular dependences featuring secondary peaks. The fit components were systematically correlated with the underlaying microstructural features and pinning landscapes for each tape. Subsequently, practical and tape-specific scaling parameters for I c ( B,θ,T ) were derived to describe the unique material formulation and pinning landscape of each tape. The proposed framework provides a physics-based description of the superconducting behavior while explicitly aiming to serve as a practical tool for magnet design and modeling.
Abstract Supersaturation serves as the primary thermodynamic driving force for nucleation, governing the crystallographic orientation and microstructural evolution of superconducting films. In this study, we investigated the impact of the yttrium stoichiometry ( x = 0.8, 1.0, 1.2) of Y x Ba 2 Cu 3 O 7- δ films grown via fluorine-free metal-organic deposition. By tuning the yttrium content, we effectively modulated the transient liquid-phase supersaturation, driving a fundamental transition in the defect architecture and vortex pinning dynamics. The Y-deficient film (Y 0.8 BCO), grown under lower supersaturation, lacks a competing dense 3D defect network. Consequently, its microstructural landscape is dominated by localized planar stacking faults, which provide robust 2D correlated pinning and sustain a broader vortex trapping angle ( θ T ). Conversely, high supersaturation in the Y-rich film (Y 1.2 BCO) generates a dense 3D network of isotropic defects. These fine-scale disorders dimensionally match a reduced in-plane coherence length ( ξ ab ), acting as efficient core pinning centers that enhance the self-field critical current density ( J c ) to 2.0 MA cm −2 at 77 K. However, a crucial high-field trade-off emerges: under magnetic fields (>2.7 T), excessive lattice disorder depresses local superconducting properties, rendering the pristine stoichiometric YBCO film more robust. Ultimately, balancing the transition between 2D planar and 3D isotropic pinning regimes via stoichiometry-driven supersaturation engineering provides an effective strategy for optimizing the in-field performance of REBCO coated conductors.
Abstract Currently most optimizations of the superconducting properties of REBa 2 Cu 3 O 7-δ (REBCO, RE123) are done based on process parameters. Defect-density based methods offer a more direct way for such optimization due to the fact that the superconducting properties of a REBCO conductor emerge from its defect landscape. In this study, an X-Ray Diffraction (XRD) method will be presented that helps to determine the stacking fault density (SFD) of REBa 2 Cu 4 O 8 (RE124) stacking faults reasonably well. It is based on the fact that RE124 stacking faults introduce peak shifts and broadening to (00L) reflections of RE123. Both quantities vary with the Miller index L, which enables the identification and quantification of these stacking faults. The method is tested using Sm123 thin film samples produced via Chemical Solution Deposition (CSD). A Cu excess is used to facilitate the growth of extended Sm124 stacking faults through a high-temperature oxygenation. The resulting samples show exactly the behaviour predicted by the presented theory. The SFDs determined via XRD are then cross-checked by Transmission Electron Microscopy (TEM) micrographs. It is found that the SFD from peak shift describes the majority of the samples to a sufficient degree of accuracy. Therefore, the presented method is applicable to quantify extended Re124 stacking faults in RE123 thin films.
Abstract Improving the quality factor Q 0 and the accelerating gradient E acc of superconducting radio frequency cavities, which are an essential component of modern particle accelerators, remains a challenge. While medium temperature heat treatment (240 ∘ C to 350 ∘ C) generally improves the quality factor Q 0 at moderate fields, it often limits the accelerating gradient due to quenches at around 20 MV/m to 30 MV/m . We demonstrate here that this limitation can be overcome by applying a newly developed combination of mid- and low temperature heat treatment to cavities. After such a treatment, quality factors of up to 4 ⋅ 10 10 and accelerating gradients of up to 40 MV/m can be achieved, which are well suited for cavities for a possible European XFEL upgrade but also other projects requiring a higher accelerating gradient like the International Linear Collider Technology Network and the linear collider facilities, etc. Furthermore, advanced investigations of the surface resistance and calculated diffusion lengths of our medium temperature heat treatment data revealed correlations consistent with those previously reported in the literature.
Abstract Focused ion beam (FIB) processing is widely used to define nanoscale weak links and Josephson junctions in YBa 2 Cu 3 O 7− x (YBCO). However, the specific thickness window required for junction behavior and its microscopic origin remain unclear, particularly the distinction between Ga-ion doping and lattice distortion effects. We performed transport measurements on FIB-patterned YBCO nanobridges and investigated the recovery of Tc through post-annealing. To identify the underlying electronic modification, we employed density-functional theory (DFT) to calculate the hopping parameters and bandwidth changes in distorted YBCO crystal structures. Our measurements show that FIB-induced Tc suppression is partially reversible through annealing, indicating that lattice distortion is the dominant factor in weak-link formation rather than irreversible Ga incorporation. DFT calculations reveal that while distortion significantly reduces the bandwidth and secondary hopping, the primary Cu–Oelectronic coupling channel remains functional. These results support an S–S’–S-like weak-link model, where the superconducting order parameter is locally suppressed while sufficient electronic coupling is preserved. This study provides a microscopic framework for understanding and optimizing the formation of Josephson junctions in FIB-patterned high-Tc superconductors.
Abstract Two-dimensional superconductors provide a versatile platform to explore fundamental physics and applications where the properties of the two-dimensional samples are characterized by the transport of multiple or single layers. However, spatial inhomogeneities introduced during sample preparation can critically alter transport properties, potentially masking intrinsic physics and producing extrinsic signatures that resemble unconventional phenomena. In this work, we systematically investigated the transport signatures arising from different origins of inhomogeneity in superconducting van der Waals NbSe 2 samples. Two distinct categories of spatial variation were examined: growth-induced inhomogeneity (introduced via non-stoichiometric crystal growth) and fabrication-induced inhomogeneity (achieved by modulating layer thickness and step morphology during mechanical exfoliation). Both types of inhomogeneities resulted in anomalous resistance peaks, with growth-induced inhomogeneity producing more pronounced and configuration-dependent transport signatures. The transport response showed strong geometrical dependence. The observed behavior was consistently described within a framework in which spatially separated regions of the device undergo superconducting transitions at different temperatures, reflecting a distribution of local T c values in the samples. The fitting results further reveal that an inhomogeneous superconducting landscape can generate even more complex anomalous transport signatures, including double-peak structures and sign reversals in resistance. In conclusion, our results demonstrate that transport anomalies should be carefully analyzed and inhomogeneity in sample and measurement geometry should be ruled out before attributing them to intrinsic physical mechanisms.
Abstract Distributed windings offer significant electromagnetic advantages for fully superconducting rotating machines, especially higher power density. Non-planar REBCO coils are required for the realization of it but remain challenging to design and fabricate due to the limited strain tolerance of the REBCO layer. In this work, we extend a previously proposed single-turn segmented non-planar coil design to a 20-turn configuration, deriving the geometric parameters of each turn analytically to account for the finite tape thickness. Based on the parameterized coil geometry, we develop a robotic winding procedure using a discrete representation of the tool trajectory and orientation to enable controlled and repeatable tape placement. We validate the approach by manufacturing a no-insulation REBCO test coil and characterizing its electromagnetic performance at 77 K. A critical current of 70 A is measured, in good agreement with the modified load-line estimate of 74.5 A, and the measured central magnetic flux density deviates by less than 1 % from the Biot–Savart calculation.
