Chiara Tarantini

Bi ₂ Sr ₂ CaCu ₂ O _x (Bi-2212) is an interesting high temperature superconductor (HTS) for high field applications due to its unique ability to develop high critical current density ( J c ) in polycrystalline round wires. The performance of all HTSs is highly sensitive to the doping states, particularly near the critical doping where the pseudo-gap opens and J c , the upper critical field H _{c 2} and other superconducting properties, but not T c , are maximized. However, due to Bi-2212’s complex wire microstructure, the impact of doping on the conductor performance has been poorly understood. Here we systematically investigate the superconducting behaviour across a wide doping range, separating for the first time the individual effects induced by oxygen-doping on the grain and the grain boundary (GB) properties. Our results can be interpreted as grains and GBs being affected by separate pseudo-gap openings with the grain irreversibility field H _Irr being controlled solely by the grain pseudo-gap, whereas the GB H _Irr is influenced by both. Moreover, the J c performance at 4.2 K is strongly related to the grain vortex pinning properties, but with some J _c limitation arising from the GBs. These findings demonstrate the impact of a fundamental physical phenomenon, the pseudo-gap, on the overall conductor performance, and they highlight the complexity of optimizing this material. These results also suggest that targeted adjustments to Bi-2212 processing may enable significant improvements in J c . Moreover, since the pseudo-gap is not unique to Bi-2212, similar limiting effects of different grain and GB boundary doping almost certainly operate in other HTSs, which also could gain from alternative processing strategies making them more suitable for magnets for high-field research, fusion or medical applications, and for high-power cable technologies.

RE Ba ₂ Cu ₃ O _7-x ( RE BCO, RE : rare earth) coated conductors suffer from great critical current I _c differences between different manufacturers, I _c variations within individual manufacturers, and often significant lengthwise fluctuations. The understanding of such variations is complicated by the lack of a direct correlation between I _c and the critical current density J _c . In fact, although J _c is the fundamental property determined by the local vortex pinning landscape, I _c is often limited by variable current blocking mechanisms. An important practical complexity is commercial practices of performing J _c and I _c evaluations based on I _c at 77 K and self-field (sf), where connectivity variation dominates over vortex pinning variations. However, at higher fields and lower temperatures, vortex pinning becomes more complex and highly variable, making predictions of I _c and J _c at arbitrary temperature, magnetic field, and field orientation quite uncertain. To address some aspects of this problem, we conducted detailed spool-to-spool performance characterization on recently manufactured RE BCO coated conductors. Despite J _c (77 K, sf) varies by only ~11%, J _c (77 K, 1 T) of its minimum and maximum (for H // ab -plane) show variations of ~21% and ~32%, respectively. This emphasizes the shortcoming in using J _c (77 K, sf) as parameter for evaluating even the low field performance. An even more remarkable spool-to-spool J _c variation of ~68% was observed at 20 K and 15 T for H // ab -plane. To identify the origin of such lack of reproducibility we performed microstructural characterizations, which revealed, within the REBCO layer, large variation in the density of copper oxide (CuO _x ) particles ranging from 0.1 to 2 μm in size. We believe that they play a decisive role in reducing the effective cross-section of the RE BCO layer by not simply blocking current themselves, but also by nucleating off-axis RE BCO grains, whose misoriented grain boundaries adversely impact RE BCO grain-to-grain connectivity. The RE BCO growth associated with high density CuO _x particles also leads to the more disordered spatial arrangement of degrade the orientation of BaHfO ₃ precipitate arrays, which, self-aligning along the ab -planes, generate strong pinning enhancing J _c ( H // ab ) at all temperatures. In this way, we established that variations of both connectivity and vortex pinning are thus directly coupled. Our results also explain why the so-called "lift-factor", typically defined by the ratio between I _c ( T , H ) and I _c (77 K, sf), frequently turns out to be unreliable.

In recent years there have been growing concerns about the proper evaluation of physical properties of superconductors, in particular for quantities extracted from magnetic characterizations. Errors can and often do occur due to the following issues: i) several measurement instruments still use Gaussian & cgs-emu units instead of the preferable International System (SI) units; ii) there are decades of valuable publications where, however, equations were expressed in Gaussian & cgs-emu or other unit systems or where constants were normalized to unity, which requires proper understanding and unit conversion in order to correctly evaluate the measured physical quantities; iii) the conversion between unit systems sometimes appears challenging and may not be properly performed. In this paper we will describe how to properly convert physical quantities relevant for the evaluation of magnetic and other properties focusing on the still most used unit systems, SI and Gaussian & cgs-emu. We will provide examples of how to properly verify and understand the physical formulae. We will include examples for the correct method to determine the critical current density J _c of a superconductor from the measurement of its magnetic hysteresis loop through the Bean model, and the correct conversion to SI of the equations for H _c2 ( T ) according to the Werthamer-Helfand-Hohenberg (WHH) formulation. The goals of this paper are to make the readers aware of the unit conversion issue, to provide useful hands-on tools for proper conversion and to strongly encourage future exclusive use of the SI units and formulae.

K-doped BaAs₂Fe₂ (K-Ba122) superconductor is a promising material for applications. However, it has been found challenging to achieve high critical current density (J_c ) in untextured bulk sample. In this paper we investigated bulk samples prepared by varying the milling energy density, which affects the grain and grain boundary microstructures, and we investigated their magnetic performance to better understand what causes their different J_c . We found that in our samples, which all have small grain size, T_c does not appear directly correlated to J_c . Moreover, AC susceptibility reveals in at least one case obvious signs of multiscale supercurrents, not caused by granularity but that directly influence the overall J_c performance. Considering the microstructural features and the magnetization response we ascribed the J_c differences to lack of connectivity on a larger scale due to nano-cracks at some grain boundaries, which subdivided the samples into macroscopic regions and inevitably limited the overall performance. We discuss possible routes to overcome those extrinsic current-blocking defects.

It is still largely elusive whether K-doped BaFe₂As₂ (K-Ba122) is granular due to the intrinsic blocking effects at the grain boundaries (GBs). We investigated the remnant magnetization characteristics of a K-Ba122 polycrystalline bulk. Remnant magnetization is effective to evaluate the contribution of magnetization from multi-scale current loops particularly if electromagnetic granularity is present due to weakly coupled GBs. The derivative of remnant magnetization of the K-Ba122 sample showed only a single peak, which was markedly dependent on the specimen size and shifted toward lower field as the specimen size decreases, strongly indicating that the current loop is intergrain. However, the high angle annular dark field scanning transmission electron microscope (HAADF-STEM) analysis revealed that this sample still has nano-cracks at GBs, degrading the continuous network of strong intergrain connectivity. Such extrinsically degraded network of intergrain connectivity can also be seen as a size-dependence of remnant magnetization J_c, which indicated the presence of strongly localized regions with much higher J_c than the bulk J_c. Our study suggested that the extrinsic GB nano-cracks are still the major cause of connectivity degradation, and the intrinsic current suppression might not be practically detectable at clean, fully connected GBs.

Nb${}_{\text{3}}$Sn is the original high-field superconductor but understanding its optimization in wire forms, which are always inhomogeneous, is compromised by a lack of basic understanding of its true property variations when it is alloyed with H_c${}_{\text{2}}$ enhancing additions (e.g., Hf, Ta, Ti, Zr). Since improving the high-field performance of Nb${}_{\text{3}}$Sn is important for many future applications in high energy physics, fusion, and medicine, we are studyingbulk binary and alloyed samples fabricated by ball milling itsconstituent powders and reacting them in a Hot Isostatic Press at pressures up to 2000 bar and temperatures up to 1800 °C. Remarkably, we found that these bulk samples, reacted to produce the A15 phase at temperatures between 1200 and 1800 °C, show minority traces of H_c${}_{\text{2}}$ (0 K), evaluated with a 99% criterion, that reach almost 40 T, more than 27% higher than the best value previously reported for Nb${}_{\text{3}}$Sn. These results are both scientifically interesting but potentially of great practical importance if they can be exploited for improving the high-field properties of Nb${}_{\text{3}}$Sn wires. One caution is that these samples do not have added Cu as wires do and that the high H_c${}_{{2}}$ reactions were all done at well above 1000 °C.

High critical current density Jc is generally the most important property of any superconductor used for applications. The critical current Ic is most frequently measured by transport 4 probe V-I techniques and the measured Ic is then divided by the cross-sectional area to give the critical current density Jc. This method is normally error-free. Occasionally however, Ic and Jc are computed from measurements of the magnetic moment and two recent papers (Goyal et al. 2024 Nat. Commun. 15, 6523 and Polichetti et al. 2024 arXiv:2410.09197) illustrate some of the perils of reporting properties of REBa2Cu3O7-δ (REBCO) films grown on a coated conductor substrate using only magnetic evaluations. Correct data analysis is made difficult by the persistent use of cgs units in commercial instrument outputs and by misapplication of handbook Bean model formulae, which in this case led to an order of magnitude Jc overestimate, which of course renders the original world record claim invalid. The laudable attempt of the authors to explain their original mistake has unfortunately its own errors. Here we explain explicitly how errors are still present in the second paper and provide correct expressions for the conversion of magnetic moments to Jc values using Bean model expressions. An important conclusion is that record Jc values ought always to be validated by real transport measurements and by cross-reference to similar samples and their reported Jc values.

The realization of single crystal and bicrystal films of superconducting materials is of great interest because they allow the investigation of the intragrain performance as well as the understanding of potential limitations in the grain boundary transparency. For many years, the realization of a high-quality (Ba,K)Fe2As2 film has been challenging. Here, the realization of (Ba,K)Fe2As2 epitaxial thin films on single crystal SrTiO3(001) and [001]-tilt-type SrTiO3 bicrystal substrates with high superconducting properties is demonstrated. The epitaxial growth of (Ba,K)Fe2As2 was enabled by implementing an undoped BaFe2As2 buffer layer between the SrTiO3 substrate and (Ba,K)Fe2As2 film. The film exhibits a high Tc of 38.0 K and an extremely high Jc of 14.3 MA/cm2 at 4.2 K. Artificial grain boundaries of (Ba,K)Fe2As2 were also successfully achieved on bicrystals with misorientation angles up to 36.8° by the same preparation methods. The artificial grain boundaries exhibited an identical Tc of 38.0 K and an excellent transfer of the grain orientation from the bicrystal substrates with high crystallinity comparable to that of the high-quality Ba(Fe,Co)2As2 films. This enables the investigation of the intrinsic (Ba,K)Fe2As2 grain boundary nature, which will clarify its potential for superconducting applications, like Josephson junctions, wires, and magnets.