Abstract The nanorods are effective in improving the critical current density in YBa 2 Cu 3 O 7 films, but their pinning strength and morphology depend on the fabrication process. To understand the critical current density mechanism, the vortex excitation mode as well as static pinning configuration should be discussed. In this study, we prepared YBa 2 Cu 3 O 7 + BaHfO 3 /YBa 2 Cu 3 O 7 composite multilayers (CMLs). This is a segmented nanorod system where the length of nanorods can be controlled. Two types of CMLs were fabricated to extract the influence of the pinning strength of the nanorods, where the nanorod pinning strength is relatively represented by J c (77 K,1T) of a YBa 2 Cu 3 O 7 + BaHfO 3 single layer film: 0.73 MA cm −2 for strong-pinning nanorod and 0.08–0.27 MA cm −2 for weak-pinning nanorod. In the multilayer with strong-pinning nanorods, the critical current density decreased with increasing the pure layer ratio, indicating that the vortex excitation from the nanorods was accelerated by the pure layer. In the multilayer with weak-pinning nanorods, dependence of the critical current density on the pure layer ratio exhibited the peak due to the competition of excitation modes. Surprisingly, when we compare the results in different types of CMLs, the critical current density for high pure layer ratio is almost the same despite different nanorod pinning strength, where the nanorod edges rather than the nanorod core is a dominant pinning center. Design of the excitation mode as well as static configuration is very effective in achieving high critical current density in YBa 2 Cu 3 O 7 films.

Abstract Searching for new superconductors, especially unconventional superconductors, has been studied extensively for decades but remains one of the major outstanding challenges in condensed matter physics. Medium/high-entropy alloys (MEAs-HEAs) are new fertile soils of unconventional superconductors and generate widespread interest and questions on the existence of superconductivity in highly disordered materials. Here, we report on the effect of Ni-doped on the crystal structure and superconductivity properties of strongly coupled TiHfNbTa MEA. XRD results indicate that the maximum solid solution of (Ti1/4Hf1/4Nb1/4Ta1/4)1-xNix is about 7.7%. Resistivity, magnetic susceptibility, and specific heat measurements demonstrated that (Ti1/4Hf1/4Nb1/4Ta1/4)1-xNix HEAs are all bulk type-II superconductors and follow the trend of the increase of Tc with the increase of Ni-doped contents. The specific heat jump (∆Cel/γ"T" _"c" ^" mid" ) of all (Ti1/4Hf1/4Nb1/4Ta1/4)1-xNix are much larger than the BCS value of 1.43, suggesting all these HEAs are strongly coupled superconductors. Additionally, large Kadawaki-Woods ratio (KWR) values suggest that there is a strong electron correlation effect in this system. The (Ti1/4Hf1/4Nb1/4Ta1/4)1-xNix HEA system is a new ideal material platform for the study of strong correlation behavior and strongly coupled superconductivity, which provides an insight into the physics of high-temperature superconductors or other unconventional superconductors.

Abstract This manuscript presents the design, fabrication, and operation of high temperature superconducting conductor on round core (CORC®) wires utilizing variable critical current REBCO tapes that contain occasional dropouts of critical current (I c). Performance evaluations conducted at temperatures ranging from 25 K to 77 K demonstrated comparable superconducting characteristics between CORC® wires with and without intrinsic variations in I c within the REBCO tapes. This is a clear indication that the high level of current sharing between tapes in CORC® wires allows current to bypass local defects. A 70 mm bore, 4-layer CORC® solenoid in which 25 m of CORC® wire was wound into 81.5 turns, generated a peak magnetic field of 4.6 T at an I c of 4460 A at 25 K. The I c of the solenoid corresponds closely with the sum of the expected average tape I c, not its minimum I c driven by the tape dropouts. Experimental results confirmed stable dissipative operation at 4021 A (87% I c) continuous current with minimal power losses, highlighting the feasibility of manufacturing longer-length CORC® conductors from tapes with significant local dropouts for superconductor applications including fusion, accelerators, power transmission, and rotating machinery.

Abstract Reel-to-reel deposition is a key technique for continuous production of long REBa2Cu3O7-δ (REBCO) tapes, while the transverse uniformity of the tape has not been extensively investigated. This work investigated the transverse variation in both superconducting performance and structural characteristics of long EuBCO tapes fabricated via reel-to-reel multi-plume multi-turn pulsed laser deposition. The J c exhibits pronounced transverse non-uniformity, with the lowest J c at the left edge reaching only ~73% of that at the center part. Structural anaylsis reveals that a strong correlation between this non-uniformity and microstructural variations across the tape width. Loose structures were observed on both tape edges, whereas droplet-shaped particles were exclusively found on the left side. The EuBCO layer in loose structure regions exhibited pronounced arched morphology, which impedes the supercurrent transport. This arched structure is attributed to volumetric expansion induced by the partial decomposition of the buffer and EuBCO layers. These decomposition phenomena are most likely associated with the elevated localized overheating near the tape edges, which hypothetically originates from the torsional stress during helical travelling process of the tape in the reel-to-reel deposition process.

Abstract Solenoids constructed by assembling ring-shaped bulk high-temperature superconductors (HTSs) have been demonstrated to be capable of trapping a sufficiently strong and uniform magnetic field for nuclear magnetic resonance and magnetic resonance imaging. These systems, however, were magnetised using field cooling, requiring a large and expensive secondary superconducting magnet to generate the external field. Pulsed-field magnetisation (PFM), on the other hand, is a much more economical and practical method to magnetise bulk HTSs, as it can be carried out using a normal conductor (such as copper) to generate a brief pulsed magnetic field. The trapped field following PFM is found to be significantly lower than that achieved with field-cooling. This is due to the large quantity of heat generated within the bulk during the rapid change in the applied field. While this heating effect has been exploited to improve the pulse magnetisation efficiency of disc-shaped bulks, it has proven detrimental when magnetising ring-shaped bulks. Here, we present a refinement of the PFM procedure of ring-shaped bulks that can assist with increasing the trapped field. By inserting bulk HTS discs into the bore of a ring and pulse magnetising the discs before the rings, and subsequently removing the discs, we show that a higher field can be trapped in the ring.

Abstract High-temperature superconducting (HTS) resonators are gaining increasing attention for their application in magnetic resonance imaging (MRI) due to their superior performances. However, their diamagnetic behaviour can lead to artifacts in images when cooled in zerofield cooling (ZFC), particularly when the HTS resonator is misaligned with a background magnetic field at an angle. To analyse the electromagnetic behaviour of a typical HTS resonator under the imperfect alignment condition, a numerical model has been developed by employing Maxwell’s equations in conjunction with the nonlinear E-J power law of a HTS film. Firstly, the consistency of the numerical results with the previously reported experimental observation was verified. Subsequently, by comparing the magnetisation behaviours under three different magnetic fields, it was elucidated that the image artifacts stem from a magnetisation geometric effect in the HTS resonator with a transverse geometry, characterised by a large width-to-thickness aspect ratio, and the geometric effect is found to be primarily attributed to a stray field which is beyond the predictions of the Bean model. The study also found that adjusting the width-to-thickness aspect ratio significantly impacts the distribution of penetrated magnetic flux density and screening current density of the HTS resonator, distorting the background magnetic field of a MRI scanner. Furthermore, the pronounced effect of creep behaviour on the resonator’s magnetisation behaviours was explored. Additionally, the developed model was used to predict the magnetisation effect and the consequent static field inhomogeneities generated by a superconducting resonator across various temperatures and magnetic fields, providing insights for designing novel resonators for high-field MRI and broadening their scope of application.

Abstract Hardware Trojans that exploit the unique characteristics of superconducting electronic (SCE) circuits are explored in this paper. Two types of hardware Trojan circuits are proposed: a magnetically-coupled data transmission Trojan embedded in single flux quantum (SFQ) full adder circuit and a pulse-interleaved Trojan implemented within a frequency divider. The working principles of how these Trojan circuits exploit the frequency-sensitive nature of SCE circuits are explained. The proposed Trojan circuits stay hidden during typical low-frequency testing, and only triggered and become active (and potentially dangerous) at higher frequencies. Extensive analysis demonstrate that these Trojans have a high chance of staying stealthy during testing, underscoring the need for more comprehensive security measures that account for the full range of operating frequencies for the protection of both classical superconducting systems and future quantum technologies.

Abstract Accelerating cryomodules with superconducting cavities are key components for particle accelerators. The efficiency, energy gain and operating temperature of cavities drive the operating and capital costs; hence, advances in these areas enable future accelerators. While superconducting cavities are currently made from Nb, Nb3Sn, with a superconducting transition temperature and superheating field approximately twice that of Nb, is poised to substantially improve the efficiency and energy gain. We present results from the first cryomodule with two five-cell 1.5 GHz superconducting radiofrequency Nb cavities coated with Nb3Sn, which attained an accelerating gradient of ⩾ 10 MV m−1 with low cryogenic loss at 4.4 K.

Abstract No-insulation (NI) winding technique enables current redistribution during transient events such as quench through bypass conduction paths. In this study, the controlled area bypass (CAB) method is presented to control the time constants in high-temperature superconducting (HTS) coils. The CAB method applies a solder shunt on the coil surface by confining the solder area with a G10 mask structure. Two insulated coils (INS) and one NI coil, each with 50 turns, underwent CAB processing and were tested. The applied solder coverage ranged from 12.5% to 100% of the total surface area, which increased time constants from 0.66 s to 55.6 s for the NI-CAB coil and from 3.9 s to 24.0 s for the INS-CAB coils. A linear relationship was found between the solder coverage and the time constants. Microscopic analysis confirmed an average solder shunt height of below 200 µm across all samples, with penetration depths of 500 µm for INS-CAB coils and 3.7 mm for the NI-CAB coil. During 50 thermal cycles, the time constants remained stable with fluctuations of about 10%. An analytical circuit model considering the details of the solder shunt geometry was established. The average deviations between the estimations and measurements of the contact resistance are 22% for INS-CAB and 17% for NI-CAB. This study contributes to the further understanding of HTS coils with solder surface shunts and the control of time constants for future HTS coil designs.

Abstract Two-dimensional (2D) and three-dimensional (3D) polycrystalline vanadium films grown on SiO2 substrates have been prepared by magnetron sputtering method. The superconductor-insulator transition (SIT) has been observed by adjusting the film thickness, average crystallite size and magnetic field respectively. The complete superconducting 3D vanadium film deposited at 523 K has an obvious two-step development of superconductivity when a magnetic field is applied. This behavior is attributed to non-uniform coupling due to variations in grain sizes and the intergranular distances across the sample. The metallic state induced by the magnetic field can be explained by a model that incorporates both thermally activated phase slippage and quantum phase slippage. Both the 2D vanadium film with thickness of 12 nm and the 3D vanadium film with average crystallite size of 11.9 nm exhibit a trend towards superconducting transition where the resistance decreases to a nonzero value. The SIT, tuned by adjusting the perpendicular magnetic field, has been observed in the above two films. The scaling behavior near the critical magnetic field B c distinguishes a superconducting phase from an insulating phase, indicating a quantum phase transition induced by a finite-size effect. The upper critical magnetic field H c 2 (0) increases as the grain size decreases, likely due to a reduction in the coherence length resulting from a decrease in the average free path with decreasing size.