Symposium NM02—Superconducting Materials and Applications

In composite superconductors, the local critical current can vary along the length due to variable vortex pinning interactions, as well as variations in filament connectivity due to filament shape variation and, especially in HTS conductors, variations in grain-to-grain connectivity. This critical current (I_c) distribution can be extracted from a d²V/dI² treatment of the V-I curves, as originally proposed by Baixeras and Fournet [1] and used by Warnes [2] to study filament-sausaging degradation of critical current density (J_c) in Nb-Ti. High J_c is now available in Bi-2212 round wires and there is intense interest in farther optimization of these wires. Brown [3] recently studied wires with almost 6 times variation in J_c made over almost a decade and found that their normalized J_c(H) characteristics were almost identical, from which we concluded that their vortex pinning was identical and that J_c was determined by their grossly varying connectivity. In Nb-Ti there is concern about filament connectivity only when filaments are overdrawn and they start to sausage, unlike Bi-2212 where cross-sectional non-uniformity is built into the filament shape due to the large shape anisotropy of Bi-2212 grains and many poor grain-to-grain connections further degrade filament connectivity. Attainable J_c values in Nb-Ti reach ~ 0.2 of the depairing current density (J_d), while reaching only ~ 0.01 J_d in Bi-2212, suggesting significant room for raising J_c if better connectivity can be obtained (Bi-2212 does however have higher J_c than Nb-Ti now). Our measurements show that J_c in Bi-2212 depends significantly on both powder source and the heat treatment used to densify and grain-align the 2212. We find that the standard deviation to mean ratio s/m of various Bi-2212 wires range from ~ 0.1 - 0.2, while our benchmark Nb-Ti conductor is about 0.05 at ~ 0.5H_irr. We find it interesting that (i) there are clear trends that J_c rises as s/m declines and (ii) that the s/m values are only about twice as high as our fully optimized Nb-Ti wire, which unlike Bi-2212 is isotropic and has one single value of H_irr. We conclude that this technique is valuable for comparison of various Bi-2212 conductors and for comparison to more traditional Nb-Ti and Nb₃Sn conductors.

References
[1] J. Baixeras and G. Fournet, “Vortex displacement losses in a non-ideal type II superconductor,” Journal of Physics and Chemistry of Solids, vol. 28, no. 8, pp. 1541–1547, Aug. 1967.
[2] W. H. Warnes and D. C. Larbalestier, “Critical current distributions in superconducting composites,” Cryogenics, vol. 26, no. 12, pp. 643–653, Dec. 1986.
[3] M. D. Brown et al., “Prediction of the J_c(B) behavior of Bi-2212 wires at high field,” IEEE Transactions on Applied Superconductivity, vol. 29, no. 5, pp. 1–4, Aug. 2019.

Materials losses from native oxides on superconducting metal surfaces, interfaces between materials, and bulk dielectrics are the dominant source of decoherence in superconducting quantum processors. Superconducting microwave resonators can act as a convenient qubit proxy for assessing loss performance and characterizing loss mechanisms such as two-level system loss, non-equilibrium quasiparticles, and magnetic flux vortices. This talk will provide an overview of accurate resonator loss measurement, summarizing techniques that have been evolving for over two decades, and will conclude with recommendations for future measurements in this field.

Advances in precursor powder and processing technology has increased the potential of Bi₂Sr₂CaCu₂O_x (Bi-2212) as a capable material for application in high field high temperature superconducting (HTS) magnets. The record high critical current densities (J_c [4.2K, 30T] = 4670A/mm²) was achieved by improved precursor powder, in particular the recent Engi-Mat powder. Further understanding and optimization of the processing stage will provide another avenue for further increases in the J_c of Bi-2212 round wires (RW). The use of Over Pressure Heat Treatment (OPHT) has greatly simplified the processing of Bi-2212 RW with less time and resources spent in the processing stage. Notwithstanding these advantages of OPHT, the exact influence of the key OPHT parameters, most especially the rate of cooling (R_F) and the time in the melt (t_melt) on the micro- and nano- structure of fully heat treated Bi-2212 RW is still not fully understood. In this study, we systematically vary R_F and t_melt for 2 wire diameters (1mm and 1.2mm) and investigate how this affects the microstructure, grain texture, and overall J_c of the recent Bi-2212 RWs made with the new Engi-Mat powder. We found that the 1mm diameter wire showed minimal variation in J_c (about 5%) and microstructure, while at 1.2mm, the wires showed significant variation microstructure and J_c (about 20%). Overall, our results showed the possibility of achieving minimal variation in J_c with a wide OPHT processing window. This study is particularly important for scaling up Bi-2212 processing from short sample OPHT to large coil processing while maximizing the J_c of Bi-2212 RW. Further details will be given in the presentation.

Bulk superconductors (SC) have recently found a mighty application in magnetic shielding [1], with MgB₂ bulks as a very promising option [2]. For this purpose, a combination of modelling procedure and growth technique able to manufacture properly shaped products with high and homogeneous critical current density, can be a successful approach guiding the whole optimization process.
This work focuses on the shielding properties of tube- and cup-shaped MgB₂ bulks with small height/outer radius aspect ratio. Moreover, the effect of adding a ferromagnetic (FM) sheet around the superconductor is addressed. All the SC samples were produced by processing the starting commercial MgB₂ powders added with hexagonal BN into high-density bulks via spark plasma sintering, obtaining fully machinable samples [3]. Starting from the experimental analysis, a 3D modelling evaluation was then carried out.
In more details, first, the properties of both SC and hybrid SC/FM shields were measured in both axial- (AF) and transverse-field (TF) configurations using cryogenic Hall probes [4,5]. In a second step, the experimental results were reproduced by finite element calculations with COMSOL Multiphysics® [4], exploiting a 3D magnetic vector-potential (A) formulation [6]. The good agreement obtained by comparing the computed induction fields with those measured experimentally allowed us to validate this modelling approach for our application.
Finally, this numerical procedure was applied to explore new shield design with similar aspect ratio but optimized performance. In particular, the shielding efficiency of three different cup-shaped arrangements were compared both in AF and TF configurations, studying the effect of superimposing ferromagnetic vessels of different sizes around the superconductor.

Acknowledgements
Slovak team acknowledges the support of Grants APVV-15-0257 and APVV-16-0418.

[1] K. Hogan et al., Supercond. Sci. Technol. 31, 015001 (2018)
[2] D. Barna et al., IEEE Trans. Appl. Supercond., 29, 4101310 (2019)
[3] G. Aldica M. Burdusel, V. Cioca, P. Badica Patent No RO130252-A2, DPAN 2015-383635
[4] L. Gozzelino et al., Supercond. Sci. Technol., 32, 034004 (2019)
[5] L. Gozzelino et al., Supercond. Sci. Technol., 33, 044018 (2020)
[6] M. Solovyov and F. Gömöry, Supercond. Sci. Technol., 32, 115001 (2019)

The toroidal field (TF) magnets of magnetic confinement fusion (MCF) power plants need to operate at high current density for extended periods to be cost effective. This requires these TF magnets to use superconducting current carriers. To aid in their design, this work aims to better understand how ReBCO coated conductors will perform when subjected to the exotic particle environment present in electricity producing MCF devices. This has been achieved using a combination of irradiation damage simulation and ion bombardment experiments.

Simulation based investigations have been used to determine the predominant changes to ReBCO during neutron irradiation in several settings (fusion and fission). Ion bombardment experiments included 2 irradiation campaigns performed with room temperature samples. These used 2MeV He+ and 3.1MeV O2+ ions to mimic the effects of neutron irradiation. The results using each ion species, were compared with those of published neutron irradiation experiments. The 3^rd ion bombardment campaign involved the in-situ irradiation and superconducting property measurement of ReBCO samples whilst at their operating temperature. This required a new, dedicated experiment to be designed and built, in collaboration with Surrey University’s Ion Beam Centre.

A summary of the findings of the irradiation damage simulations, results from the 3 irradiation campaigns and a summary of the in-situ irradiation and testing experiment will be presented, along with proposed next steps.

Iron based superconductors (FBS) are interesting materials that have high intragranular J_c of ~6x10⁵ Acm^-2 as thin film single crystal at 4.2 K and 15 T, high irreversibility field of > 90 T, T_c of >35 K, and very low critical field anisotropy. An increase of intergranular J_c is needed to make these materials suitable for high field magnet applications. Intergrain connectivity issues must be overcome for achieving high J_c. Extrinsic factors, such as impurity phases and grain boundary porosity, are the present cause of poor intergrain connectivity in FBS polycrystalline bulks and wires. Combining high-pressure heat treatment and clean synthesis protocol, we studied how the milling energy density and heat treatment (HT) temperature affect the superconducting properties of Ba_0.6K_0.4Fe₂As₂ (K-Ba-122). By changing RPM and milling time of planetary ball mill, we precisely evaluated milling energy density E_BM, ranging from 65 MJ/kg to 1 GJ/kg in different bulks. A novel two-stage heat treatment was used for each bulk. In 1st stage, different HT temperature and ambient pressure was applied in different bulks. We fixed the 2nd stage HT temperature and pressure consistent (600 °C and 193 MPa). We obtained after the 1st HT a T_c of ~38 K, which is close to the highest T_c reported for optimally doped K-Ba-122 single crystals. However, in all cases, T_c drops by 3-4 K after the high pressure 2nd HT. Optimization of 1st stage HT for any E_BM was achieved at 750^oC, where magnetization J_c after the complete two-stage HT was highest compared with other 1st HT temperatures. Maximum J_c of 1.1×10⁵ Acm^-2 at 1T is achieved by synthesizing bulk at 100 MJ/kg and 750 ^oC 1st HT, followed by the high-pressure 600 ^oC 2nd HT. In this specific case, T_c after 1st HT was 37.6 K and after 2nd HT 34.6 K. The milling energy of 100 MJ/kg at 1st milling appeared to contribute reducing the formation of current-blocking FeAs phase. In this presentation, we will discuss about the correlation between milling energy, heat treatment temperature, connectivity and improved superconducting properties in polycrystalline K doped Ba-122.

This study presents new results on investigation of the levitation force between a permanent magnet and pre-magnetized GdBCO superconducting tape stacks containing from 20 to 100 tapes of 12 mm × 12 mm. The stacks were cooled with liquid nitrogen in the field of a superconducting magnet with a magnetic induction of 3 T and then they were placed in a cryostat of the levitation force measuring system. Stacks of 100 tapes were cooled in different applied fields with magnetic induction in the range of 0.1-1 T. The field captured by the tapes was measured with a Hall sensor 5 minutes after magnetization. Experimental data on levitation force value and the effect of lateral displacement on it for stacks of tapes with different thicknesses and trapped flux have been obtained. Levitation force decay during the lateral displacements above a permanent magnet was observed for different stacks. Based on the obtained results, it was concluded that pre-magnetized stacks show greater repulsive forces than non-magnetic ones, and the difference increases with the number of tapes in the stack. Capture of the magnetic flux not only provides a greater levitation force, but also leads to a greater attenuation of the levitation force at lateral displacements which increases with increasing magnetization. The captured flux depends on the external field as follows: it increases with an increase in the applied field up to 0.4 T, after which it reaches saturation. The observed physical processes were simulated, and obtained results are in good agreement with the experimental data.
This work was supported by a grant from the Russian Science Foundation (Project 17-19-01527).

We report on the dissipation caused by step current pulse in NbTi superconducting filament on fuzed silica. The non-equibroum state is induced by a current exceeding the the critical current Ic. A voltage appears after a certain delya time td, and close to Tc the dissipation is governed by the phase slip center phenomena. It bears similarity to single photon detection using a superconducting nanowire biased with a current skower than the critical current Ic. The film cooling time is deduced from fitting the data with the Time Time-Dependent Ginzburg-Landau (TDGL) theory due to M. Tinkham. We find taht the film cooling time is longer close to Tc than at lower temperature and this attributed the number of phonon population that increased.

Au-Ag nanostructures have been posited as potential room temperature, ambient pressure superconductors in past studies. This presentation will summarize recent progress made towards the growth of films of these materials. Au-Ag nanostructures comprise discrete ~1 nm Ag nanocrystals embedded at specific loading densities into an Au matrix. This motif is nontrivial to produce through standard colloidal techniques that rely upon spherically or axially symmetric growth around a central nucleus. I will present methods developed by us that enable a simplified route for the preparation of these materials. The structural properties of Au-Ag materials produced through this method will be described. This presentation will further provide an overview of their unconventional optical and electrical properties. In particular, we will describe the suppression of plasmon resonances within these materials along with the emergence of a broad non-dissipative resonance over the visible. We will further describe the observation of zero resistance in films of these materials. In certain samples, these states with resistivity below 10^-10 ohm-m are also observed at temperatures as large as 400 K. Other aspects including stability, and sensitivity of these materials towards air exposure, will be discussed.

Recently it was shown that bimetallic nanostructured films composed of gold and silver exhibit certain unconventional electrical and optical properties which are not expected from any conventional metallic films. To account for these observations, we have developed a coarse grained theoretical model to explain the experimental findings. We adopt a jellium model for the gold silver material. A metal-insulator transition mediated by correlation enhancement is described. I will show that the system is associated with two temperature scales with a potential superconducting transition occurring below the electron localization temperature.

Recent studies on engineered Au-Ag nanostructure assemblies have attracted tremendous interest due to their unconventional electrical and magnetic properties. Briefly, these nanostructures have been shown to exhibit an immeasurably low resistive state, typically with resistivity below 10^-10 ohm-m and also a highly diamagnetic state at temperatures approaching and greater than the ambient. Along with these observations, they also exhibit an unusual optical response which is not anticipated from their constituent material. Generally, Au or Ag nanostructures show their definitive plasmon resonances under optical excitation. This corresponds to the collective oscillations of free electrons which leads to an excitation of dipolar modes. Contrary to conventional Au and Ag materials, Au-Ag nanostructures do not show any definitive plasmon resonance at a particular frequency but rather show a broad extinction over the entire UV-VIS optical window even extending up to NIR. We de-convolve the extinction into constituent scattering and absorption components. It is found that extinction entirely comprises of scattering with negligible absorption. Further, these nanostructures do not show any dissipative dynamics in pump-probe spectroscopy. We shed further insights into the unconventional properties of these materials through EELS and structure correlated single particle microscopy. Our findings indicate significant electronic reconfiguration in these nanostructures that gives rise to deviations from the usual dielectric response of Au and Ag.

Coated conductors of YBa₂Cu₃O₇ (CC-YBCO) have emerged as the most attractive opportunity to reach unique performances in an extended range of temperatures and magnetic fields making them attractive for power and magnet applications. Reducing the cost/performance ratio, however, continues to be a key objective. Chemical solution deposition (CSD) is a competitive cost-effective technique which has been used to obtain nanocomposite films and CCs. However, the typical growth rates (0.5-1 nm/s) of the fluorine-based CSD approach remain too low thus limiting its throughput.
We will show in this talk how CSD-based nanocomposite YBCO films can be obtained using metalorganic colloidal solutions including preformed BaMO₃ (M=Zr, Hf) nanoparticles. This approach can be easily combined with Inkjet printing and other scalable deposition techniques for thick film preparation.
Transient Liquid Assisted Growth (TLAG) is a novel growth approach [1] allowing to combine the already well established CSD methodologies with ultrahigh growth rates based on a non-equilibrium liquid-mediated approach (100-1000 nm/s). This novel approach uses fluorine-free metalorganic precursors and is also compatible with the colloidal solution approach to nanocomposite coated conductors. We will show that using properly TLAG requires to generate new knowledge about kinetic phase diagrams that we have reached using fast in-situ XRD analysis (100 nm/frame) under synchrotron radiation. Critical current densities up of 5 MA/cm² at 77K are already realized in thin films and the suitability of using IBAD buffered metallic substrates will be also demonstrated. A modified nanostructure is demonstrated based on epitaxial BaMO₃ nanoparticles thus generating a new opportunity to deeply analyse the key influence of defect structure on vortex pinning of nanocomposite superconductors.

We acknowledge funding from EU-ERC_AdG-2014-669504 ULTRASUPERTAPE and EU-PoC-2020-IMPACT projects, and the Excellence Program Severo Ochoa SEV2015-0496

[1] L. Soler et al, Nature Communications,11, 344 (2020)

We present a simple liquid assisted processing (LAP) method, to be used in-situ during pulsed laser deposition growth to give both rapid growth rates ( >250 nm/min and strong pinning. Achieving these two important features simultaneously has been a serious bottleneck to date. LAP enhances the kinetics of the film growth so that good crystalline perfection can be achieved at up to 60 x faster growth rates than normal, while also enabling artificial pinning centres to be selfassembled into fine nanocolumns. In addition, LAP allows for RE mixing (80% of Y with 20% of Yb, Sm, or Yb+Sm) to create effective point-like disorder within the REBCO lattice and which leads to strongly improved pinning at 30 K and below. Overall, LAP is a simple method which could be adopted by other in-situ physical or vapour deposition methods (i.e PLD, MOCVD, evaporation, etc) to significantly enhance both growth rate and performance.

AMSC is exploring the development of advanced wire architectures which can double the I_c of the standard Second Generation (2G) high temperature superconducting (HTS) wire and significantly enhance performance in the presence of magnetic fields. The Second Generation HTS wires are generally based on a composite architecture consisting of a thin YBCO superconducting layer deposited on one side of a thin, flexible metal substrate. This composite HTS/substrate structure is typically surrounded by an electroplated Cu layer or laminated between metal strips to provide mechanical and electrical stability. AMSC has been developing an alternative architecture consisting of 2 distinct HTS layers deposited on each side of a metal substrate. This double HTS layer composite is then surrounded by an electroplated Cu layer or laminated between two metal strips, resulting in a wire package nearly indistinguishable from the standard single HTS layer wire. The novel architecture results in a 2G wire with double the critical current of the standard single layer wire.
The enhanced pinning performance of the wire is achieved by the introduction of a uniform defect structure produced by a reel-to-reel irradiation of the HTS layers by ions such as Au⁵⁺. The ion irradiation produces an extremely uniform distribution of pinning defects that enhances I_c by a factor of up to 3 times. By controlling the irradiation process, the performance can be engineered for the temperatures and fields of the targeted applications.
In this presentation, we will review the performance of the novel 2G wire architecture and describe the structure and properties of the irradiation induced pinning landscapes.
This work is supported by EERE under contract DE-EE0007870.

Advanced metal organic chemical vapor deposition (A-MOCVD) has been developed to grow long-length, thick film REBCO coated conductors in a single pass with superior in-field performance. Multiple long-length 5% Zr doped REBCO coated conductors with film thickness ranging from 1-4µm have been produced in A-MOCVD Reactor. The critical current density of these conductors has been examined by scanning hall probe microscopy. REBCO coated conductors with ~4µm thickness exhibit magnetization critical current density (J_c) over 1.5 MA/cm² at 65 K, 1.5 T and 5.4 MA/cm² at 4.2 K, 13 T. The microstructure, out-of-plane and in-plane texture of REBCO coated conductors will also be presented.

High-throughput screening of materials is a novel development strategy that is envisaged to accelerate discovery, development, and optimization of materials. Chemical solution deposition (CSD) techniques such as drop-on-demand inkjet printing (DoD IJP) are ideal to perform high-throughput studies through the fabrication of complex combinatorial samples with locally-uniform or graded compositions, and high spatial precision that enables parallel investigations on morphological, structural and functional properties.

The current work shows the implementation of such approach to expedite the optimization of REBCO superconducting films fabricated with the newly developed transient-liquid assisted growth chemical solution deposition (TLAG-CSD) method, which has been able to attain growth rates 100 times larger than with other fabrication methods. The preparation of combinatorial samples was done through mixing different rare earth cuprate precursor solutions with DoD IJP to explore the influence of rare-earth and liquid phase compositional changes on the epitaxial growth characteristics. The use of computational methods allowed the successful merging of precursor inks and high sample homogeneity, as confirmed by EDX and high-resolution XRD.

Epitaxial growth studies were then conducted for the fast identification of the best ranges of processing conditions and compositions. We use the advantages of this strategy to rapidly analyze different compositions in one single step by in-situ XRD synchrotron radiation experiments. Finally, we combined this knowledge with machine learning approaches to widen the understanding and rapid optimization of epitaxial growth of high-temperature REBCO superconducting films.

References:
[1] Soler et al. Nat. Comm. 11, 344 (2020).
[2] Rasi et al. J. Phys. Chem. C 124, 15574-15584 (2020).
[3] Queraltó et al. submitted. <div id="gtx-trans" style="position: absolute; left: 5px; top: 334px;"> <div class="gtx-trans-icon"> </div> </div>

The outstanding ability of YBa₂Cu₃O_7−x (YBCO) films to carry high currents at high magnetic fields offers an unparalleled opportunity to be used in large scale superconducting power applications and high field magnets. In an essential need of high performance, high throughput, and low cost manufacturing, chemical solution deposition (CSD) has become a very competitive cost-effective and scalable methodology to produce epitaxial thin films, with enhanced performance at high magnetic fields by the incorporation of secondary phase nanoparticles (NPs) [1]. However, their growth rates are rather small (0.5-1 nm/s). In this regard, we have developed a novel growth approach, entitled, Transient Liquid Assisted Growth (TLAG) [2], which is able to combine CSD methodologies for fluorine-free metalorganic precursors with ultra-fast growth rates (100 nm/s) by facilitating a non-equilibrium liquid-mediated approach and several intermediate reactions. Critical current densities up of 5 MA/cm² at 77K are already realized in TLAG-CSD grown YBCO thin films. In order to further improve the thin film’s crystal quality and superconducting properties, understanding of initial nano-phases in pyrolysis process and all the intermediate reactions during growth, as well as, fine tuning of growth parameters is essential. For this purpose, the microstructure of pyrolyzed YBCO thin films and the intermediate phases of quenched thin films along with grown YBCO thin films, investigated via high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX), will be presented.
Furthermore, the current-carrying capabilities of high temperature superconductors (HTS) are determined by its microstructure and it can be enhanced by the presence of well-controlled nano-defects inside the epitaxial superconducting matrix acting as vortex pinning centers. TLAG-CSD has demonstrated the growth of nanocomposites to increase flux pinning at high magnetic fields by incorporating pre-formed NPs to the metaloganic initial inks. Therefore, using aberration-corrected STEM combined with high angle annular dark field (HADDF) and EELS, we explore the detailed microstructure of grown TLAG-CSD YBCO films and nanocomposites, with a focus on new defects landscape at the atomic level, secondary phases, and strain effects. An additional advantage of TLAG-CSD is that preformed NPs can be incorporated to the metalorganic ink to prepare superconducting nanocomposites with enhanced vortex pinning, which will also be presented. Finally, the recent progress in TLAG-CSD, in terms of microstructure correlation with growth mechanisms and growth rate will be discussed.

In Second-Order Topological Insulators (SOTI), bulk and surfaces are insulating while the edges or hinges conduct current in a quasi-ideal (ballistic) way, being insensitive to disorder. Crystalline bismuth has been shown to belong to this class of materials [1,2,3]. Just like the case of Quantum Spin Hall edges of 2D Topological Insulators, current is expected to be carried without dissipation by counter-propagating ballistic helical states, with a spin orientation locked to the momentum. Such edge or hinge states open many possibilities, from dissipationless charge and spin transport to new avenues for quantum computing. We have investigated Crystalline Bi nanowires based Josephson junctions and found that they exhibit saw-tooth current phase relations robust in high magnetic field which is the signature one-dimensional ballistic edge states. We also demonstrate the topological nature of Andreev states in these junctions when coupled to a microwave resonator in a phase-biased configuration. We find absorption peaks at the Andreev level crossings, whose temperature and frequency dependencies point to protected topological crossings with an accuracy limited by the electronic temperature of our experiment.

References :
[1] A. Murani et al, Nature Communications 8, 15941 (2017).
[2] Frank Schindler et al, Nature Physics 14, 918–924 (2018).
[3] A. Murani et al, Phys. Rev. Lett. 122, 076802 (2019).
Helene Bouchiat’s email is [email protected]

In order to study Majorana and parafermionic quasiparticles a platform is needed that combines superconductivity and topology. A candidate platform is a Josephson junction with standard s-wave superconductors as leads and a topological interlayer in which electron spin is coupled to momentum direction. We will show results for junctions based on topological materials with transport in 3D bulk Dirac semimetals, 2D topological surface states as well as 1D higher order topological hinge states, progressively trying to reduce the number of non-topological additional modes.

For the Dirac semimetal Bi_0.97Sb_0.03 we revealed that the required spin-momentum locking is present for the bulk carriers and that the degeneracy of the Dirac cone can be lifted in parallel electric and magnetic fields [1]. Indeed, Josephson junctions with this material reveal that part of the supercurrent is carried by bound states that are 4pi-periodic, as evidenced from microwave irradiation experiments [2]. The g-factor of the interlayer turns out to be gigantic [3]. We will discuss in what sense these bound states can be considered topological despite their degeneracy in spin.

Going one dimension lower, the spin momentum locking of 2D topological surface states can be used. For the Dirac semimetal Cd₃As₂ we show that part of the supercurrent is carried by the Fermi arc surface states [4]. For the use of the surface states of topological insulators we developed technology that allows for in-situ fabricated interfaces and in-situ defined nanostructures [5]. The use of selective area growth is especially promising for the purpose of reducing the number of modes in the nanodevices by size quantization, in order to maximize the 4pi signal.

In order to avoid any non-topological Andreev modes in the junctions at all, the 1D situation is preferred. For this purpose we focused on higher order topological 1D hinge states that might exist in Dirac semimetals. For Cd₃As₂ we showed that this is indeed the case by analyzing the supercurrent modulation by magnetic field and electric gating [6].

1. Monopole diffusion in Dirac semimetal Bi_0.97Sb_0.03, J. de Boer et al., Phys. Rev. B 99, 085124 (2019).

2. 4pi periodic Andreev bound states in a Dirac semimetal, Chuan Li et al., Nature Mater. 17, 875 (2018).

3. Zeeman-effect-induced 0-π transitions in ballistic Dirac semimetal Josephson junctions, Chuan Li et al., Phys. Rev. Lett. 123, 026802 (2019).

4. Fermi-arc supercurrent oscillations in Dirac semimetal Josephson junctions, Cai-Zhen Li et al., Nature Communications 11, 1150 (2020).

5. Selective area growth and stencil lithography for in situ fabricated quantum devices, P. Schüffelgen et al., Nature Nano. 14, 825 (2019).

6. Reducing Electronic Transport Dimension to Topological Hinge States by Increasing Geometry Size of Dirac Semimetal Josephson Junctions, Cai-Zhen Li et al., Phys. Rev. Lett. 124, 156601 (2020).

The interest in hybrid Topological Insulator (TI) Josephson junctions was boosted after the prediction by Fu and Kane of an unconventional chiral p-wave symmetry of the proximity induced order parameter into the topological surface states. The chiral induced p-wave is a prerequisite for the nucleation of localized Majorana states in a tri-junction geometry, which is instrumental for topological quantum computation. In a multimode hybrid TI Josephson junction with two terminal geometry, Majorana physics manifests as peculiar properties of a part of the Andreev bound states carrying the Josephson current: they give rise to an unconventional 4p periodic current phase relation (CPR) coexisting with a 2p periodic CPR of the conventional Andreev bound states. The relative weight between the 4p and 2p periodic Andreev bound states increases with the transparency of the junction and, in general, by reducing the number of channels. A direct way to achieve a low number of transport channels is to use TIs with reduced dimensionality like very thin and narrow nanoribbons. Indeed, quite recently, various theoretical proposals have shown the advantage to using Josephson junctions with TI nanoribbons, with suppressed bulk conduction, to realize Majorana fermions.
To obtain information about the bound state spectrum we have implement a circuit-QED readout scheme for our Al-Bi2Se3-Al hybrid junctions. Here we embedded a TI Josephson junction-based RF SQUID in a superconducting resonator. The dispersive microwave read out of the resonator/RF-SQUID setup allows us to extract information about the bound state spectrum of the TI junction. In fact, the low amplitude microwave readout ensures a relative low perturbation of the junction (in respect to the critical current) as compared to typical “Shapiro step” measurements, where the large microwave drive can cause undesirable population of higher lying bound states. Moreover, the dispersive readout enables us to characterize decoherence sources eventually affecting the performance of a TI junction based topological quantum bit. Finally, we present preliminary results of two-tone spectroscopy envisaged to extract the phase dispersion of single bound states directly.

Hybrid material systems with a conventional superconductor in proximity to a semiconductor or an unconventional conductor have recently acquired a vast interest due to their potential to hoist exotic phenomena.^[1-3] Current studies on superconductor-topological insulator (TI)-superconductor junctions based on exfoliated film and nanoribbons have shown unconventional current phase relations (CPR) that could be associated with Majorana fermions, which may help realize topologically protected quantum computing.^[4-8] In a 3D TI based junction, the Majorana modes should appear in the surface modes as a doublet of gapless Andreev bound states whose energy varies 4π periodically with the phase across the junction alongside the normal 2π periodic Andreev bound states. But in most of the studies on these systems, there is an unavoidable contribution from the bulk, and experiments based on 3D-TI nanoribbons (TINRs) with a reduced number of transport modes are much needed. We make use of Al-Bi₂Se₃-Al junctions fabricated using TINRs grown by physical vapor deposition. ^[9-11]To extract the current phase relation of our TI-junction, we utilize an asymmetric dc-SQUID measurement technique. The TI-junction to be studied is connected parallel to a reference junction on the same TINR with a higher critical current, typically 10-15 times more, thus forming a SQUID device. This asymmetry in critical current is achieved by varying the TI-junction width, keeping the distance between the electrodes constant. Now, considering a large ratio between the critical currents and a small SQUID loop inductance (i.e., screening parameter much less than one), the phase across the reference junction stays constant on the application of magnetic flux while the phase across the test junction is changing linearly with the applied magnetic flux. From the modulations of the SQUID critical current, we directly determine the CPR of the small TI-junction. We observe clear deviations from standard sinusoidal CPR of typical tunnel junctions in all our devices. This skewed CPR at low temperatures results from highly transparent modes in our TI junctions. For increasing temperature and one obtains the sinusoidal current phase relation consistent with the thermal population of the Andreev bound states.

Reference:
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[2] C. Nayak et al., Rev. Mod. Phys. 80, 1083–1159 (2008)
[3] G. Y. Huang et al., Phys. Rev. B: Condens. Matter Mater. Phys. 95, 155420–6 (2017)
[4] J. Wiedenmann et al., Nat. Comm, 7:10303 (2016)
[5] C. W. J. Beenakker et al., Annu. Rev. Conden. Matter Phys. 4, 113–136 (2013)
[6] L. P. Rokhinson et al., Nat. Phys. 8, 795–799 (2012)
[7] K. L. Calvez et al., Commun. Phys. 2, 4 (2018)
[8] P. Schüffelgen., Nat. Nanotechnol. 14, 825–831 (2019)
[9] G. Kunakova et al., Nanoscale 10, 19595–19602 (2018)
[10] G. Kunakova et al., Appl. Phys. Lett. 115, 172601 (2019)
[11] G. Kunakova et al., J. Appl. Phys. 128, 194304 (2020)

The current phase relationship (CPR), describes the relation between the supercurrent, I_S, through a Josephson junction (JJ) and the gauge invariant phase difference across the junction, Δθ. The exact relation, is determined by the details of the Andreev bound state spectrum that exists in the junction. For conventional SIS junctions, the CPR is sinusoidal, I_S=I_Csin(Δθ), where I_C is the junction critical current. With the advances in materials sciences and the realization of novel link materials, exhibiting non-conventional CPRs, JJ with new functionalities arise having a tremendous potential for applications.

The harmonic content of the CPR in these novel weak link materials can be revealed by experiments relying on detecting the phase-locked response of a current driven Josephson junction when subjected to a high frequency radiation field of frequency, ν_rf [1]. In case the weak link is characterized by a conventional sinusoidal CPR, this resonant response manifests itself as constant voltage plateaus, so called Shapiro steps, in the VI-characteristics at voltages V_n=nΦ₀ν_rf, where n is an integer number and Φ₀ the flux quantum. However, for a weak link having a non-sinusoidal CPR, the response exhibits, in addition to the conventional integer Shapiro steps, steps at fractional values V_n/q=(n/q)Φ₀ν_rf , where the q^th (q ≠ n) fractional step originates from the phase-locked response with the q^th harmonic of the CPR.

Despite the experimental accessibility of these techniques, the detailed interpretation of such experiments is complicated by the non-linear nature of the Josephson response. Moreover, the use of a single junction makes these experiments challenging mainly due to its low response. Josephson junction arrays (JJAs) containing many junctions provide the natural alternative to provide the necessary enhancement of the coherent response. Although JJAs hold a strong promise to study the CPR, the impact of the particular CPR on the high frequency response remains until now unexplored.

We investigate the potential of anisotropic JJAs to study the harmonic content of the CPR in a broad frequency range (down to 50 MHz). Introducing anisotropy results in a giant collective high frequency phase-locked response that reflects the properties of a single junction. We demonstrate experimentally the appearance of giant fractional Shapiro steps in anisotropic JJAs as unambiguous evidence of a skewed current phase relationship. This in contrast to prior observations of giant fractional Shapiro steps in JJAs as resulting from magnetic flux quantization in the two-dimensional array.

[1] S. Shapiro, Phys. Rev. Lett. 11, 80 (1963)
[2] R. Panghotra, B. Raes, Clécio C. de Souza Silva, I. Cools, W. Keijers, J.E. Scheerder, V.V. Moshchalkov, and J. Van de Vondel, Communications Physics 3, 53 (2020)

Raising T_c and setting new T_c records by discovering new compounds, modifying known compounds, and/or developing novel mechanisms to meet new challenges in science and technology have lured many scientists and engineers to superconductivity ever since its discovery. Great progress has been made in the last three decades in both the stable compounds, such as cuprates and iron-pnictides and -chalcogenides, and the unstable compounds of hydrogen-rich molecular solids. Both have T_cs within practical ranges: air-conditioning for the former and cryogen-free ambient for the latter. While the temperature barrier has been brought down, the pressure becomes a formidable obstacle to practical applications. Should this obstacle be overcome by retaining these high pressure (HP)-induced high-T_c phases at ambient, their technological impact would be profound and limited only by the imagination. We have thus explored the possibility of retaining these HP phases at ambient via pressure-quenching following appropriate thermodynamic paths like those for diamond from graphite and black phosphorus from red phosphorus. Model material systems of Sb, Bi, FeSe, and Cu-doped FeSe have been chosen for the study. We have successfully retained at ambient many superconducting and non-superconducting phases induced under HP up to 64 GPa in these materials. The thermal stability of the pressure-quenched phases has also been resistively examined. The experimental details are briefly described below:

I. Non-superconducting Sb and Bi Single Crystals
1. Determined resistively the phase diagrams, R-P and Tc-P, of the superconductors investigated and compared them with existing ones when available.
2. Identified the HP-induced phases with their different T_cs to retain at ambient.
3. Pressure-quenched these HP-induced phases by rapid removal of the pressure at 77 K.
4. Cooled the pressure-quenched phases from 77 K down to 1.2 K to determine whether each superconducting phase has been quenched in. If so, warmed up the sample to room temperature and cooled it down again to 1.2 K. The superconducting transition will disappear if the superconducting phase is associated with pressure-quenching and if the thermal stability of this phase ceases to exist at 300 K.
5. By repeating such R-T measurements on cooling to 1.2 K after warming to various temperatures above 77 K, the thermal stability of each pressure-induced superconducting phase quenched at 77 K is determined.
6. An anomaly in the R-T curve is also detected in the temperature region when the superconducting transition disappears.
7. Indeed, some of the HP-induced superconducting phases have been successfully pressure-quenched at ambient at 77 K with different thermal stability ranges.

II. Superconducting FeSe Single Crystals
Following the same steps taken for Sb and Bi, we have shown that the HP-induced superconducting phases, as well as the non-superconducting phases, can be pressure-quenched at ambient with a thermal stability up to 300 K.

III. Non-superconducting Cu-Doped FeSe Single Crystals
Following the same protocol for the above compounds, we have unambiguously pressure-quenched in at ambient the HP-induced superconducting and non-superconducting phases.

We have thus demonstrated that the superconducting and non-superconducting phases in Sb, Bi, FeSe, and Cu-doped FeSe single crystals induced under HP up to 64 GPa can be pressure-quenched in at 77 K and ambient pressure with different thermal stabilities up to 300 K. The observations hold great promise for practical applications of the high-temperature superconducting cuprate HBCCO with a T_c of 164 K at ~ 31 GPa and of C-S-H with a T_c of 287 K at 267 GPa.

The work in Houston is supported by US Air Force Office of Scientific Research Grants FA9550-15-1-0236 and FA9550-20-1-0068, the T. L. L. Temple Foundation, the John J. and Rebecca Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston.

One-dimensional c-axis aligned BaZrO₃ (BZO) nanorods are regarded as strong one-dimensional artificial pinning centers (1D-APCs) in YaBa₂Cu₃O_7-x (YBCO) films. However, a microstructure analysis has revealed a highly defective, oxygen-deficient YBCO column of 2-3 nm in thickness around the BZO 1D-APCs due to the large lattice mismatch of ~7.7% between the BZO (3a=1.26 nm) and YBCO (c=1.17 nm). A hypothesis of reduced pinning potential height due to this defective YBCO column has been confirmed recently on much lower pinning efficiency of the BZO 1D-APCs as compared to BaHfO₃ (BHO) 1D-APCs that form a coherent interface with YBCO. Herein, we report a dynamic Ca/Cu replacement approach on tensile strained YBCO lattice immediately after the BZO 1D-APCs formation to induce c-axis elongation of the YBCO lattice near the BZO 1D-APC/YBCO interface to prevent the interfacial defect formation by reducing the BZO/YBCO lattice mismatch. An YBCO elongated c-axis up to 1.24 nm via Ca/Cu replacement and a defect-free, coherent BZO 1D-APC/YBCO interface are confirmed in transmission electron microscopy and elemental distribution analyses. Excitingly, up to five-fold enhancement of J_c (H) at magnetic field H=9.0 T//c-axis and 65-77 K was obtained in the ML samples as compared to their BZO/YBCO single-layer counterpart’s. This has led to a record high pinning force density F_p together with significantly enhanced H_max at which F_p reaches its maximum value F_p,max for the BZO 1D-APCs at H//c-axis. At 65 K, F_p,max ~157 GN/m³ and H_max ~ 8.0 T in the 6 vol.% BZO/YBCO ML samples represent a significant enhancement over F_p,max ~36.1 GN/m³ and H_max ~ 5.0 T on the 6 vol.% BZO/YBCO SL counterparts. This result not only illustrates the critical importance of the BZO 1D-APC/YBCO interface in the pinning efficiency, but also provides a facile scheme to repair this interface for higher pinning efficiency of the BZO 1D-APCs.

Acknowledgements
This research was supported in part by NSF contracts Nos: NSF-DMR-1909292 and NSF-DMR-1508494, the AFRL Aerospace Systems Directorate, the Air Force Office of Scientific Research (AFOSR), and the U.S. National Science Foundation (DMR-1565822 and DMR-2016453) for TEM characterization.

CaKFe₄As₄ is a newly discovered iron pnictide superconductor which shows optimal critical temperature T_C of 38 K at the stoichiometric compound. Electron doping with Ni or Co atoms induces a decrease in T_C and the onset of a particular hedgehog antiferromagnetic order, without sign of structural instability or nematicity. The influence of this new form of magnetic order on the superconducting state has been till now poorly investigated. Here, we present quasiparticle interference study on CaK(Fe_0.95Ni_0.05)₄As₄ using scanning tunneling microscope at dilution fridge temperature[1]. Unlike in the undoped system[2], we reveal in the Ni doped system a four-fold highly anisotropic superconducting state, in association with the hedgehog antiferromagnetic order present in this system.

[1] J.Llorens et als., arXiv:2009.11246 (2020)
[2] A.Fente et als., Phys. Rev. B 97, 134501 (2018)

Iron-based superconductors (IBSC) represent a group of superconducting materials that attracted significant attention in recent years both for academic research and practical applications due to the exotic coupling of magnetism and superconductivity and to their large critical current densities at high magnetic fields. All iron-based superconductors are based on a common structural element, namely a layer composed by Fe atoms tetrahedrally coordinated by pnictogen (P,As) or chalcogen (S,Se,Te) atoms. Several elements and molecules can be intercalated among these planes, giving rise to the different families of the IBSCs. Among these, the so called “1144” compounds are obtained when alternate layers of alkaline and alkaline-earth metals are intercalated among Fe-As planes. These group of IBSCs is characterized by critical fields and critical currents among the highest for what concerns IBSCs single crystals, hence, being very attractive in perspective of high field applications.
In previous works, we have demonstrated the appeal of a mechanochemically assisted synthesis route as opposed to the common high temperature (e.g. T ~ 900 °C) process, allowing a finer control of the experimental variables. In particular, we exploit High Energy Ball Milling (HEBM), a low cost and easily scalable mechano-chemical powder processing methodology, to ensure the intimate mixing of the elemental powders, and therefore a high degree of homogeneity and reactivity in the starting reactants. Coupling the HEBM with intermediate temperature (e.g. T ~ 700 °C) thermal treatments represents an effective strategy to drop the issues related to volatile compounds (e.g. As, K) and ensure at the same time the required homogeneity.
In this work we focus on the properties of Ca/K-1144 polycrystalline materials obtained by the mechanochemically assisted synthesis route. The materials have been characterized in their structure and morphology by means of X-ray diffraction and scanning electron microscopy techniques. Superconducting properties are evaluated by means of electrical resistance and magnetization measurement carried out in different magnetic field conditions up to 18 T. High field measurements allow to highlight granularity phenomena – often observed in these class of compounds – most likely due to weak links at grain or grain aggregates boundaries and to the random orientation of the crystallites that characterizes these bulk samples.

The possible coexistence between magnetism and superconductivity has fascinated scientists ever since the latter’s discovery over 100 years ago. New insights into the interaction between these two forms of order may lead to the realisation of exciting new applications including superconducting spintronics, skyrmionics and fluxonic devices for the next generation of computing hardware. RbEuFe₄As₄ is a recently-discovered spin-singlet iron-based superconductor with a superconducting transition temperature of T_c = 37K and a magnetic transition temperature of T_m, = 15K, below which both superconductivity and magnetism are present. This large coexistence temperature window makes RbEuFe₄As₄ an ideal material in which to study the interactions between the two forms of order in detail. Magnetic force microscopy and optical conductivity measurements on RbEuFe₄As₄ have suggested a weak suppression of superconductivity in the vicinity of T_m [1], while angle-resolved photoemission spectroscopy measurements indicate a more complete isolation of the two sublattices [2]. Here we use quantitative scanning Hall probe magnetic imaging of superconducting vortices in RbEuFe₄As₄ to probe changes in key superconducting parameters near the magnetic ordering temperature. Model fits to the vortex profiles as a function of temperature reveal a significant increase in the penetration depth near T_m, followed by a gradual reduction at lower temperatures, revealing that the magnetic ordering leads to a significant reduction in the superfluid density [3]. We corroborate these results with a recently-developed model describing the suppression of superconductivity by correlated magnetic fluctuations [4]. The qualitative agreement between our data and the model suggests that the coupling between the Eu moments and Cooper pairs is weak enough that superconductivity is never destroyed, yet still strong enough to substantially weaken superconductivity. Our results have important implications for understanding coexistence phenomena in other materials systems, which could pave the way to exploiting such materials in future hybrid applications.
[1] V. Stolyarov, A. Casano, M. Belyanchikov, A. Astrakhantseva, S.Y. Grebenchuk, D. Baranov, I. Golovchanskiy, I. Voloshenko, E. Zhukova, B. Gorshunov, A. V. Muratov, V. V. Dremov, L. Y. Vinnikov, D. Roditchev, Y. Liu, G.-H. Cao, M. Dressel, and E. Uykur. Unique interplay between superconducting and ferromagnetic orders in EuRbFe4As4, Phys. Rev. B 98, 140506 (2018).
[2] T. Kim, K. Pervakov, D. Evtushinsky, S. Jung, G. Poelchen, K. Kummer, V. Vlasenko, V. Pudalov, D. Roditchev, V. Stolyarov, D. V. Vyaikh, V. Borisov, R. Valenti, A. Ernt, S. V. Eremeev, and E. V. Chulkov. When superconductivity does not fear magnetism: Insight into electronic structure of RbEuFe₄As₄, arXiv preprint arXiv:2008.00736 (2020).
[3] D. Collomb, S. J. Bending, A. E. Koshelev, M. P. Smylie, L. Farrar, J-K. Bao, D. Y. Chung, M. G. Kanatzidis, W-K. Kwok, and U. Welp. Observing the suppression of superconductivity in RbEuFe₄As₄ by correlated magnetic fluctuations, arXiv preprint arXiv:2010.09901 (2020).
[4] A. E. Koshelev, Suppression of superconducting parameters by correlated quasi-two-dimensional magnetic fluctuations, Phys. Rev. B 102, 054505 (2020).

Nowadays, superconductors are commonly utilized in several applications such as energy generators and storage due to their unique capability of transferring electricity without energy losses. In some applications, their nanoscale patterning enhances their performance and gives rise to new physical phenomena.
Innovative schemes have taken advantage of the third dimension (3D) for the development of advanced electronic components. The fabrication of complex 3D nano-architectures opens fascinating novel routes in the fields of material science, physics and nanotechnology. Thus, 3D nano-superconductors could be implemented in the next generation of energy efficient electronic devices. Nevertheless, their fabrication and characterization are still challenging and only a few works addressing the growth of real 3D nanosuperconductors have been reported so far ^1–4.
In this contribution, we introduce a direct-write nanolithography method to fabricate at-will advanced 3D nano-superconductors. This specific technique called focused ion beam induced deposition (FIBID) is based on chemical vapour deposition process assisted by a charged particle beam focused to a few nanometers.
Particularly, we have prepared 3D superconducting W-C hollow nanowires by decomposing tungsten hexacarbonyl molecules with a highly-focused He⁺ ion beam, with outer diameters down to 32 nm and inner ones down to 6 nm ⁵. In addition, by modifying the ion beam current, hollow nanowires with controllable inner and outer diameters have been achieved ⁶. The growth of the vertical W-C nanowire occurs around the ion beam spot, mainly due to the interaction of secondary electrons with the adsorbed precursor molecules, whereas a cavity at the center of the nanowire is created due to the He⁺ beam milling effect on the growing material. As shown by transmission electron microscopy, nanowires microstructure displays grains of large size fitting with face-centered cubic WC_1-x phase. In addition, we have grown nanohelices with at-will geometries, with dimensions down to 100 nm in diameter, and aspect ratio up to 65. These nanotubes and nanohelices become superconducting at 7 K and show large critical magnetic field and critical current density. Particularly, these nanohelices display superconductivity up to 15 T depending on the direction of the field with respect to the nanohelix axis. This suggest that their helical 3D geometry and their orientation in a magnetic field play a significant role in the superconducting phase transition, which can be qualitatively explained using an approach for the properties of thin-film superconductors. Moreover, fingerprints of vortex and phase-slip patterns are experimentally identified and supported by numerical simulations based on the time-dependent Ginzburg-Landau equation ⁷.
The fabrication of such advanced 3D nanomaterials with outstanding properties makes this technique at the cutting edge of nanofabrication methods based on focused beams of charged particles.

References
(1) Li, W. et al., J. Nanosci. Nanotechnol. 2010, 10 (11), 7436–7438.
(2) Romans, E. J. et al., Appl. Phys. Lett. 2010, 97 (22), 222506.
(3) Li, W. et al., Nanotechnology 2012, 23 (10), 105301.
(4) Porrati, F. et al., ACS Nano 2019, acsnano.9b00059.
(5) Córdoba, R. et al. Nano Lett. 2018, 18 (2), 1379–1386.
(6) Córdoba, R. et al., Beilstein J. Nanotechnol. 2020, 11 (1), 1198–1206.
(7) Córdoba, R. et al., Nano Lett. 2019, 19 (12), 8597–8604.

Acknowledgement:
“This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 NFFA-Europe.”

Topological defects such as vortices and phase slips in a superconductor system manifest spatial patterns and dynamics that are closely associated with the geometric design in curved micro- and nanostructures of superconductors [1]. This study is motivated by the recent progress in fabrication of complex 3D nanoarchitectures (e.g., open nanotubes and nanohelices) by using the advanced 3D roll-up self-organization and nanowriting techniques based on focused ion beams. To simulate the superconducting properties of complex nanoarchitectures, a numerical platform has been developed based on a set consisting of the time-dependent Ginzburg-Landau equation coupled with the Maxwell equations [2].

The topological transitions between vortex-chain and phase-slip transport regimes unveiled in curved superconductor nanostructures as a function of the applied magnetic field under a strong transport current [3] open up a possibility to efficiently tailor the superconducting properties of nanostructured materials by inducing a nontrivial topology of superconducting screening currents. We report on a topological transition between superconducting vortices and phase slips under a strong transport current in an open superconductor nanotube with a submicron-scale inhomogeneity of the normal-to-the-surface component of the applied magnetic field. When the magnetic field is orthogonal to the axis of the nanotube, which carries the transport current in the azimuthal direction, the phase-slip regime is characterized by the vortex/antivortex lifetime ∼ 10⁻¹⁴ s versus the vortex lifetime ∼ 10⁻¹¹ s for vortex chains in the half-tubes, and the induced voltage shows a pulse as a function of the magnetic field. This non-monotonous behavior is attributed to the occurrence of a phase-slip area at such magnetic fields, when the quasi-stationary pattern of vortices changes from single to double chains in each half-turn, followed by reentrance of the superconducting state with a chain of moving vortices when the magnetic field further increases. A three-fold voltage peak occurs in an ultrathin open Nb tube of radius 400 nm at the magnetic field about 10 mT. The topological transition between the vortex-chain and phase-slip regimes determines the magnetic-field–voltage and current–voltage characteristics of curved SC nanomembranes to pursue high-performance applications in advanced electronics (e.g., as novel superconductor switching-based detectors) and quantum computing.

References
[1] Fomin, V. M. Self-rolled Micro- and Nanoarchitectures: Topological and Geometrical Effects, De Gruyter, Berlin-Boston, 2021, 148 p.
[2] Smirnova, E. I.; Rezaev, R. O.; Fomin, V. M. Low-Temperature Physics 2020, 46, 325-330.
[3] Rezaev, R. O.; Smirnova, E. I.; Schmidt, O. G.; Fomin, V. M. Communications Physics 2020, 3, 144, 1-8.

Acknowledgement
This work has been supported by DFG projects #FO 956/5-1, #FO 956/6-1, COST Action #CA16218 (NANOCOHYBRI), MEPhI Academic Excellence Project # 02.a03.21.000, RFBR and Tomsk Region under the project #19-41-700004.

In this work a vapor-solid reaction route was developed for MgB2 conductors with a multifilamentary structure of subelements with a Mg infiltration-based subelement design. The individual subelements consisted of Mg rods surrounded by B powder, all encased in a Nb chemical barrier. This design, used previously for liquid infiltration routes, was used for a lower temperature vapor-solid reaction route, with significant improvements in layer Jc, conductor Ic, and conductor n-value. This result is explained in terms of a two process model, including percolation through the strand, and local reactive diffusion to form the MgB2 phase. The model show that much deeper infiltration and a more complete reaction is possible with a vapor-solid route. Whole conductor Je values of 9 x 104 A/cm2 are obtained at 4 T, 4.2 K, (Ic = 500 A in our 0.83 mm OD, 18 filament conductor). Index values above 30 were obtained at 6 T. These conductors had co-additions of 2 mol% C and 2 wt% Dy2O3. A variety of C doping fractions and Dy2O3 additions are compared, and the best properties are found in samples with co-additions. It is seen that Dy2O3 suppresses Tc more slowly than C doping and thus is a useful co-additive at elevated temperatures. Co-additions are in every case superior to single additive designs, although the optimal amount of Dy2O3 additions depends on the intended temperature of use. This result is explained in terms of the relative fraction of inter- and intra-band scattering with C doping and Dy2O3 additions.

Transportation is a leading source of green-house gas emissions, with aircraft contributing to 10% of the CO₂ emissions at 900 million metric tons of CO₂/yr. Employing electric aircraft drivetrains would greatly diminish CO₂ emissions. However, implementation of all-electric, hybrid, and turboelectric propulsion systems does present many difficult challenges in regards to power capability, specific power, and specific energy. Superconducting power transmission and cryogenic machines have unique properties that can help overcome these challenges. Superconducting power transmission has no ohmic losses, lower transmission voltage, and utilizes smaller, lightweight conductors. Cryogenic machines utilizing superconducting wire possess very high efficiency and supply a large current density. This presentation discusses cryogenic/superconducting considerations for electric aircraft drivetrains and provides an analysis of mass and heat loss scaling laws for various components of the electric drivetrain: such as metal conductors, busbars, current leads, metal/superconducting T-joints, HTS cables, and cryoflex tubing.

In the push to develop high power electric aircraft, superconducting technology promises to significantly reduce mass and volume of motors and generators. However, challenges related to AC Loss and thermal management are a significant factor in preventing the proliferation of aerospace superconducting technologies. Increasing the resistance of the metal matrix stabilization has only gone so far in reducing coupling currents for higher frequency applications. In this research, Multiphysics simulations were used to investigate using a very high thermal conductivity electrical insulator replacing the main slightly resistive metal matrix of an MgB₂ composite wire. The insulator separates the MgB₂ filaments entirely, only allowing transient current sharing to occur with the high purity (typically Niobium) diffusion barrier. The results of these simulations and a process to make this composite in a realistic manner using a molten salt impregnation process are then presented.

Optimizing the critical current density Jc of REBa2Cu3O7-x (REBCO, RE = rare earth) coated conductors at a given temperature T and magnetic field B is particularly important in applications such as high magnetic field coils. To that end, it is necessary to know more about how the introduced artificial pinning centers (APCs) immobile the motion of quantized vortices. In order to solve this problem, we fabricated coated conductor samples in which BaMO3 (BMO : M=Hf, Zr, Sn) APCs were introduced into REBCO thin films and investigated in detail the effect on Jc under 4.2 – 20 K and B//c, 0-25 T magnetic fields. The thin film preparation conditions were adjusted to change the size and density of the BMO nanorods, and to control the concentration of random pinning centers such as oxygen vacancies. Information on nanostructures including BMO-APCs was obtained from REBCO samples by TEM observation, and vortex pinning simulation was performed by large-scale time-dependent Ginzburg-Landau (TDGL) equations for high kappa limit, with nanostructure information as input parameters. Thereafter, the predicted Jc and the measured Jc were compared. According to the experimental results, the maximum value of the global pinning force Fp (= Jc x B) of the sample varied from 0.7 to 1.7 TN/m3 depending on the concentration of BMO. When the concentration of BMO was high, the Fp value reached a peak of 1.7 TN/m3 at 8-10 T and formed a plateau. The plateau continued to the high magnetic field region of 25 T, the upper limit of the present experiment. When the concentration of BMO was low, the maximum value of Fp decreased to about half and gradually increased toward the high magnetic field side. On the other hand, according to TDGL simulation, when the concentration of BMO was high, Fp reached about 1.7 TN/m3 in the vicinity of 8-10 T (strong pinning regime), and then the Fp value was saturated to form a similar plateau (collective pinning, plastic pinning regime). In addition, when the BHO concentration was halved, Fp decreased to about half, and the experimental results were successfully reproduced almost accurately. Large-scale TDGL simulations were expected to have splendid Jc prediction power, but now it is clear that the experimental Jc values of REBCO coated conductors with APCs can be simulated almost accurately. Additional random pin effects such as oxygen vacancies can be also incorporated into the TDGL simulation. The optimal structure of APC that maximizes Jc can be predicted under given T and B conditions by these methods, which can help improve the performance of coated conductors with real complex microstructures.

For the social implementation of superconducting cables, the key point is how long the liquid nitrogen used to cool the superconducting cables can be circulated maintaining a predetermined temperature. In order to obtain the properties required to get a future prospect for longer length cable system, we participate a national project called Ishikari project that include the construction of two dc power lines, “Line 1” with 500m-length (5kA, 100 MVA) and “Line 2” with 1000m-length (2.5 kA, 50 MVA) and the circulation and current test using the system. The project had been started from 2013 and successfully ended on 2016 [1,2]. From the analysis of various data we have got the prospect that we can circulate the LN2 for more than 20 km. [3]. However, there are issues to be considered for the realization of 10-20 km system, such as improvement of thermomechanical characteristics, simplification of cable connection, weight reduction, and further improvement of cooling efficiency. We will discuss about these technical issues and show some of the basic research concerning them.
[1] S. Yamaguchi, et al., IEEE Trans. Appl. Supercond., 25, # 5402504 (2015).
[2] N. Chikumoto, et al., IEEE Trans. Appl. Supercond., 26, # 5402104 (2016).
[3]H. Watanabe, et al., IEEE Trans. Appl. Supercond., 27, # 5400205 (2017).

Iron-based superconductors exhibit high upper critical fields and low electromagnetic anisotropy, making them particularly attractive for high-field applications. IEE-CAS continues to address iron-based wire development and fabricating methods to improve characteristics and performance of the wires and tapes. The highest J_c value has reached 0.15 MA/cm² at 4.2 K and 10 T with a combination of densification and texturing processes in mono-core silver sheathed Ba_1-xK_xFe₂As₂ (Ba-122) tapes. By employing high strength Cu/Ag and stainless steel/Ag as sheath materials, larger than 0.1 MA/cm² at 4.2 K in an external magnetic field of 10 T was recently achieved for Ba-122 tapes by the scalable flat-rolling process. Furthermore, transport J_c of 100 m long tapes was further enhanced, higher than 30000 A/cm² (4.2 K, 10 T). We will show a summary of the recently achieved properties and give an outlook on the next development steps on our program roadmap.

We had recently proposed the existence of room temperature, ambient pressure superconductivity in Au-Ag nanostructures. This presentation will provide an overview of the structural, electronic, magnetic and optical properties of these materials. It is shown that below a certain critical temperature that is dependent on sample structure and composition, these materials show transitions to a diamagnetic, zero resistance state. We further show that along with emergence of unusual magnetic and electrical properties, these materials also exhibit unconventional optics. In particular, we observe the suppression of usual metallic plasmon resonances and the appearance of a broad dissipation less resonance. A theoretical framework for correlating and explaining some of these material properties is described.

Low Temperature Superconductors (LTS) have today more widespread application than ever, with a large market pulled by Magnetic Resonance Imaging (MRI), Nuclear Magnetic Resonance (NMR) spectroscopy and large-scale science projects. In particular, Nb₃Sn is the prime candidate to develop accelerator magnets for the post-LHC collider, where unprecedented collision energies in the region of 100 TeV are envisaged. This goal has led to a baseline configuration requiring dipoles generating 16 T in a 100 km tunnel that translates into a requirement for critical current densities substantially beyond state-of-the-art for commercial Nb₃Sn wires and eventually close to the ultimate limit of the material. On the other hand, High Temperature Superconductors (HTS) were intended at the beginning as a vehicle for a new class of applications in the electric utility network. However, the game changing opportunity now coming from HTS and, in particular, from REBCO tapes results from the possibility to operate at high current densities in very high magnetic fields and at high cryogenic temperature. These properties raised the interest of both high-energy physics and fusion communities. High fields beyond the reach of LTS, in the range of 20 T, would enable a further increase of the collision energy in the next generation hadron colliders and are making possible the conception of compact fusion devices, which are crucial for the fusion’s commercial realization. Moreover, the low sensitivity of the critical current to temperature, apart from the obvious simplification of the refrigeration system, would bring an improved stability of the magnet system, which is an essential requirement both in a particle collider and in a dynamic fusion environment. In this talk, I will address the current state and the directions in which the technology needs are driving the property evolution of Nb₃Sn and REBCO with a particular focus to the recent developments at UNIGE.

Research supported by the Swiss National Science Foundation (Grant No. 200021_184940), by the European Organization for Nuclear Research (CERN), Memorandum of Understanding for the FCC Study, Addendum FCC-GOV-CC-0112 (KE3646/ATS) and Addendum FCC-GOV-CC-0175 (KE 4663/ATS) and by the Accelerator Research and Innovation for European Science and Society (ARIES) project. ARIES has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871.

Fusion power offers the prospect of an almost inexhaustible source of energy for future generations. The design and R&D of future fusion reactor concepts is expected to benefit largely from the experience gained in the design, construction and operation of ITER. However, harnessing fusion energy and deploying reliable magnetic confinement fusion power plants, requires to overcome the design challenges and to address the remaining readiness gaps.
Many realizations of the DEMOnstration Fusion Power Plant (DEMO) are proposed in the world, featuring different concepts and approaches. In this framework Europe is starting the Conceptual Design Phase for building a superconducting tokamak and starting operations around the middle of the century. The aim is demonstrating the production of 500 MWs of net electricity, the feasibility of operation with a closed tritium fuel cycle, and maintenance systems capable of achieving adequate plant availability.
Superconducting magnets have the crucial role to produce high magnetic fields used for initiating, confining, shaping and controlling the fusion fuel in the form of plasma. The main challenges related to magnets regard the design layout (pancakes vs. layers), the superconducting materials (Low-Temperature vs. High-Temperature Superconductors) and technology (React&Wind vs. Wind&React manufacturing approach) and the possible use of radial plates.
An overview of the state-of-the-art and the main perspectives of the superconducting magnet technology considered for DEMO will be illustrated.

The study of pinning landscape is crucial to implement any coated conductor technology based on superconducting materials. In particular, the race to obtain a more isotropic material among High Temperature Superconductors has been pursued by the introduction of effective pinning centers in HTS. The advance of Iron-Based Superconductors can be boosted not only by the fact that Fe-chalcogenides are some of the most isotropic superconductors but also overcoming some conventional beliefs. The Fe(Se,Te) thin films, indeed, are very weak anisotropic superconductors with low values of both the anisotropy factors γ_j and γ_H, in the critical currents and critical fields, respectively. Nevertheless, a common feature with HTS is their layering crystallographic structure. Therefore a puzzling issue arises from the angular dependence of pinning in Fe(Se,Te) compared with YBCO films. We measure the irreversibility field H_irr(θ) and the activation pinning energy U_p(θ) as a function of the angle between the c-axis of the films and the applied magnetic fields up to 16 T. H_irr(θ) experimental data reveal similarities and differences between YBCO and Fe(Se,Te) films. Owing to their common material layered structure they both show an anisotropic 2D behavior of the intrinsic pinning contribution, whereas the extrinsic pinning component unveils the 3D isotropic pinning of Fe(Se,Te) against the 2D pinning anisotropy of YBCO. A stronger endorsement comes from U_p(θ) data that show a steep peak around θ=0° (H paralell to the ab-planes) for both YBCO and Fe(Se,Te) films, which is the fingerprint of their flux pinning ability into the layering structure. By continuously rotating θ towards 90° degrees, only YBCO shows a smooth spread peak around θ=90° (H paralell to the c-axis), whereas no peak is observed for Fe(Se,Te). All these features promote Fe-chalcogenide superconducting material as the right candidate for coated conductor technology.

The replacement of Y³⁺ in YBa₂Cu₃O_7-δ (YBCO) by rare earth elements (REs) such as Gd³⁺ to form alternative REBCO compounds has been extensively studied and demonstrated to be very effective for the preparation of materials with enhanced superconducting properties. Chemical Solution Deposition (CSD) of GdBCO is widely adopted for the production of films, but film growth is more sensitive to process parameters with respect to YBCO synthesis. With the aim of overcoming this limitation and improving superconducting properties, mixed Y_1-xGd_xBa₂Cu₃O_7-δ compounds have been successfully studied. To our knowledge,the introduction of Gd in the form of Gd₂O₃ or GdBCO, added in excess with respect to YBCO, has been reported only by two papers (on bulk or film grown by sol-gel) and demonstrated to have a positive influence on superconducting properties. In this work, we propose a similar study applied to YBCO film deposited by CSD. The aim is to obtain the advantages related to the presence of Gd using the robust and reproducible process adopted for YBCO.In particular, when Gd is added in excess with respect to Y, it can form oxides as Gd₂O₃ or it can substitute Y to form GdBCO, whereas Y can form other oxides such as Y₂O₃. In both cases, many different defects might originate and act as artificial pinning centers, contributing to the overall improvement of superconducting properties.
CSD of Gd-doped YBCO (YBCO-Gd) film was carried out following the metal organic decomposition approach and in-situ route. Three dopant concentrations, i.e. 5, 10 and 20mol%, were studied. Film morphology and crystalline structure were deeply investigated by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction and photoelectron spectroscopy (XRD and XPS). In general, a well c-axis oriented grain structure was observed through XRD. However, YBCO-Gd5% and 10% samples show, with respect to pure YBCO, improved film coalescence and an increase of stacking faults number, as recognized by TEM. XPS allowed verifying the Gd distribution in the film and which phase Gd formed. Superconducting properties, assessed through electrical and magnetic measurements, were evaluated at different temperatures, magnetic field directions and intensities. Higher zero-field critical current densities were measured in the temperature range from 10 K to 77 K with 5% and 10% Gd concentrations (i.e. 28, 27 and 13 MA/cm² respectively for YBCO-Gd5%, YBCO-Gd10% and YBCO at 10K). This improvement by a factor two remains up to 12T and from 10 K to 65 K.
Preliminary data on YBCO-Gd20% evidenced the need for further process optimization aimed at reducing the porosity and the a-axis content well evident in the studied films. Nevertheless, the in-field superconducting properties were better than pure YBCO and comparable to YBCO Gd5% and YBCO Gd10%, although a different pinning strength acting in YBCO Gd20% samples could be recognized.
The obtained results confirm the efficacy of Gd addition for the enhancement of transport properties and the existence in YBCO-Gd films of a pinning mechanism particularly significant at low temperatures and different with respect to pure YBCO. The possible origin of such outcomes will be proposed and discussed.

Acknowledgments
V.P. work has been carried out within the framework of the XXXIII Doctoral program in Chemical Sciences,Department of Chemical Science and Technologies,University of Rome Tor Vergata.
This work has been carried out within the framework of the EUROfusion Consortium and has received partial funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement N°633053.The views and opinions expressed herein do not necessarily reflect those of the European Commission.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement N°823717 – ESTEEM3(Nano-engineered YBCO Superconducting Tapes for High Field Applications,NESTApp).

Anisotropy is fundamental in layered superconductors YBa₂Cu₃O_7-δ (YBCO) and Fe-based compound like FeSe_xTe_1-x(FeSeTe). The sources of anisotropy are many. The unavoidable anisotropic electron effective mass (whence the anisotropic coherence length) gives rise to what we call here intrinsic anisotropy γ. The layered structure and extended defects like twin planes, grain boundaries, columnar or other linear defects such some artificial pinning centres (APC) introduce additional directional and a complex pinning anisotropy.
In magnetic-field dependent measurements all sources of anisotropy contribute to the overall response, with a difficult identification of the different contributions. Microwave measurements are able to disentangle the intrinsic from pinning anisotropy: high-frequency oscillating vortices experience the effect of vortex drag (the usual mechanism of flux-flow resistivity ρ_ff, which depends on the anisotropy through the effective vortex mass), pinning recall force (given by the pinning constant k_p), and flux-creep jumps (quantified by a normalized dimensionless parameter χ) [1]. By measuring the complex surface impedance Z=R+iX as a function of the applied dc magnetic field H and the field orientation θ, possibly at different frequencies, all these contributions can be singled out, gaining separately access to the intrinsic and pinning anisotropy [2].
We perform measurements of Z(H,θ) with different dielectric resonator setups [3], operating at 16, 27 and 47 GHz, in dc fields up to 1.3 T at different field orientations. We investigate and compare the behaviour of YBCO thin films grown by Chemical Solution Deposition [4] and Pulsed Laser Deposition (PLD) [5], with and without APC in the shape of nanoparticles and nanorods, to the results obtained in pristine FeSe_xTe_1-x thin films grown by PLD from FeSe_0.5Te_0.5 targets [6]. We extract the intrinsic anisotropy γ of the various samples, obtaining in YBCO consistent values γ = 5.0±0.5 [7] irrespectively of the kind of APC, and γ ≈ 2 in FeSeTe. On the other hand, we obtain very different pinning anisotropies in all compounds. In YBCO, the pinning anisotropy is related in a complex fashion to the different directional APC, while in FeSeTe we do not measure in the angular dependence of the pinning constant significant departures from the effective mass effect.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom programme 2014–2018 and 2019–2020 under Grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Work partially supported by Italian MIUR-PRIN project “Hibiscus”, Grant No. 201785KWLE. We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (FUNFUTURE CEX2019-000917-S) and SUMATE project RTI2018-095853-B-C21, co-financed by the European Regional Development Fund.
References
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While most applications of bulk superconductors are based nowadays on REBCO (RE: Rare earth) or BSCCO materials, the possibility to use liquid hydrogen as a fuel for airplanes and other applications on the one hand and the availability of powerful cryo-coolers on the other hand make light MgB₂ superconductors very attractive for large scale applications. The low density of this light material and its strong mechanical hardness are well suited to its possible use in electric aircrafts and windmill generators. Using the Spark Plasma Sintering technique, bulks can be manufactured in much less time than cuprates and the bulks can be given complex shapes. This is much more difficult with cuprates. These advantages make already MgB₂ a privileged material for the investigation of functional superconducting properties. As an example, we’ll report the results of our studies on i) the effect of the thickness and the diameter of cylindrical superconductors on the levitation force and the condition for stability in superconducting magnetic levitation and ii) the role of the starting material on the critical current density of MgB₂ bulks and its dependence on the applied magnetic field.

Despite the promises of superconducting qubits, their performance is presently limited by short coherence times due to defects intrinsic to materials. As a result, future quantum computers would require massive error correction circuits, which seem to be very challenging to build. Another more promising path would be to improve this coherence time, which would relax the constraints on the quantum error correction circuits and would thus make a quantum computer more feasible. This task is considered one of the main challenges in the field, and we contribute to its solution by measurements on high quality factor superconducting resonators [1,2].
Our approach gave vital clues to the long-standing problem of noise and decoherence in superconducting devices: a technique for on-chip Electron Spin Resonance (ESR) allowed to identify the chemical species responsible for the flux noise in superconducting circuits [3]. Furthermore, the noise measurements in superconducting resonators point to the link between charge and flux noise in superconducting circuits [4]: a mild sample treatment has led to tenfold reduction of the density of the surface spins, responsible for the flux noise, as evidenced by ESR, and this treatment has also lead to tenfold reduction of the low frequency noise in superconducting resonator, usually associated with charge noise.
Another major challenge comes from non-equilibrium quasiparticles (QPs) that result in qubit relaxation and dephasing. We reveal a previously unexplored decoherence mechanism in the form of a new type of Two-Level Systems (TLS) originating from trapped QPs, which can induce qubit relaxation. Using spectral, temporal, thermal and magnetic field mapping of TLS-induced fluctuations in frequency tunable resonators [2], we identify a highly coherent subset of the general TLS population with a low reconfiguration temperature around 300 mK, and a non-uniform density of states. These properties can be understood if the TLS are formed by QPs trapped in shallow subgap states formed by spatial fluctuations of the superconducting order parameter. This implies that even very rare QP bursts will impact coherence over exponentially long timescales. Summarizing this part, some spurious two-level systems observed in superconducting devices are attributed to long-lived quasiparticles trapped in gap inhomogeneities [5].
Finally, we study a single highly coherent environmental two-level system. We trace the in-time spectral diffusion of this individual TLS and demonstrate that it originates from the TLS coupling to a countable number of low energy incoherent fluctuators (TLF). From the analysis of these fluctuations, we can access relevant parameters of these otherwise elusive low energy TLFs including TLF-TLF interaction energies. Our approach opens up for deriving the macrosocopic observables of glassy media from direct measurements of local TLF dynamics at microscopic level - a route towards substantiating commonly accepted models for a decohering environment.
In summary, the use of high-Q superconducting resonators contributes to the construction of full microscopic model for interacting TLS and will then present a solid foundation for building up an ensemble level description of a glassy TLF media. Such a development, in turn, will provide so much needed understanding of decoherence mechanisms at microscopic level and indicate approaches for mitigation of decoherence.

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[4] Nature Comms. 9, 1143, (2018)
[5] Sci. Adv. 2020, accepted

Non-equilibrium quasiparticles are possible sources for decoherence in superconducting qubits because their tunneling across the Josephson junction can lead to relaxation. In this talk, we will present the impact of intrinsic properties of our transmons on quasiparticle tunneling (QPT) and discuss how this affects the device quality and performance. We find that the QPT rate to be sensitive to the choice of materials and to the geometry of the capacitive pads shunting the Josephson junctions in transmons. In some of our devices, we observe an anomalous temperature dependence of the QPT rate below ~ 100 mK that deviates from a flat background associated with non-equilibrium quasiparticles. One hypothesis for this trend which will be discussed is that sites of high transparency in the Josephson junction give rise to such behavior.

Improving the coherence of superconducting circuits is one of the biggest challenges in superconducting quantum computing. Superconducting quantum circuits are built from materials such as dielectrics and superconductors. The microwave dissipations from these materials limit the coherence of the circuits. Since the dissipated power of the constituent materials combines in parallel, the materials’ quality of all the dominating loss channels needs to be improved simultaneously to enhance the coherence. Therefore, identifying and quantifying materials’ microwave losses in superconducting circuits is crucial to developing quantum circuits with very high coherence. In this talk, we will present how to use superconducting resonators to characterize and measure microwave losses of materials in superconducting circuits.

The Josephson effect is the consequence of coupling between two superconducting electrodes and consists of a Cooper pair current flowing between the two superconductors at zero bias voltage V=0 and coherent emmission of photons with energy eV at a finite bias voltage V>0. Here we show that ultra small Josephson junctions embedded in a circuit with a feedback oscillate between zero and finite voltage states. We measure the resulting AC oscillations in Pb-Pb, Al-Al and Pb-NbSe$_2$ atomic size Josephson junctions with a millikelvin Scanning Tunneling Microscope (STM). We discuss prospects and applications of the newly found AC behavior.

Direct writing of Josephson junctions using focused ion beam was recently demonstrated in YBCO [1] and MgB2 thin films [2]. This method can simplify and extend the fabrication of Josephson junctions for superconducting devices. We use this approach for superconducting nitride thin films, and show how disorder-driven superconductor-insulator transition (SIT) can be used in Josephson junction fabrication using a helium ion beam. We use high quality superconducting thin films of NbN, TiN and NbTiN that we grow with an infrared pulsed laser deposition technique [3], and demonstrate that in all of the studied nitrides superconductivity can be suppressed by helium ion irradiation, with a higher helium ion fluence resulting in a lower critical temperature Tc. This controllable Tc suppression combined with the high spatial resolution of the helium ion microscope enables us to successfully fabricate SNS-type Josephson junctions with highly tunable weak links. First results indicate that the most promising candidate among the three superconducting nitrides studied is NbTiN, because of its high Tc (~ 15 K), low resistivity and easily controllable SIT transition. In addition, we demonstrate that NbTiN thin films can be pushed to the insulating side of the SIT transition with a high enough helium ion fluence. Thus, we also managed to fabricate insulating barrier SIS Josephson junctions with NbTiN, in addition to SNS- type devices.
[1] S. Cybart et al., Nature Nanotech. 10, 598 (2015)
[2] L. Kasaei et al., AIP Advances 8, 075020 (2018)
[3] S. Chaudhuri, M. R. Nevala, T. Hakkarainen, T. Niemi and I. J. Maasilta, IEEE Trans. Appl. Supercond. 21, 143 (2011); A Torgovkin et al., Supercond. Sci. Technol. 31, 055017 (2018)

At microwave frequencies, Josephson parametric amplifiers (JPA), based on a resonant architecture, have demonstrated quantum-limited noise and are currently used in emerginng fields like axion search or solid state qubits read out. Despite significant advances allowing improved bandwidth and saturation power JPAs are however still insufficient for addressing the needs of the above-mentioned applications. Traveling Wave Parametric Amplifiers (TWPAs) represent a potential solution. At microwave frequencies, a TWPA is designed as a nonlinear metamaterial that exploits the signal response of reactive parts (typically inductance) in a superconducting circuit. A large pump tone modulates this inductance, coupling the pump ( f_p ) to a signal ( f_s ) and idler ( f_i ) tone via frequency mixing such that 2f_p =f_s +f_i (4-wave mixing, 4WM) or f_p =f_s +f_i (3-wave mixing, 3WM).
Parametric amplifiers are currently the only known method to achieve quantum-limited sensitivity to microwave signals but achieving a large enough and sufficiently flat bandwidth over the entire needed band is still a challenging task. Worldwide, research efforts are under way to solve these problems, by developing several prototype functional amplifiers. TWJPA appear as the ultimate microwave amplifiers since they promise large gain, wide bandwidth, high saturation power and low noise approaching the quantum limit, making it robust, easily integrable into experimental setups, and cost-effective application in detectors read out and in quantum information sciences.
In recent times, the concept of a parametric amplifier with microwaves travelling along a transmission line with embedded Josephson junctions (TWJPA) was developed in several groups. These devices have already demonstrated quantum-limited noise and are used to readout superconducting qubit. TWJPA are classically designed for operating only in the 4WM regime. In this approach, the necessary phase matching of the travelling microwaves requires sophisticated dispersion engineering which still cannot ensure perfect phase matching in a sufficiently wide frequency range. This results in undesirable resonance dips and considerable ripple in the gain versus frequency dependence. Recently, a wide-band parametric amplifier operating in the regime of 3WM, has been proposed. The proposed design consists of a 3WM TWJPA based on microwave transmission line formed by a serial array of non-hysteretic rf-SQUIDs. This one-dimensional metamaterial possesses large quadratic nonlinearity and nominally zero (unwanted) cubic (Kerr-like) nonlinearity. Proof-of-principle measurements performed at a temperature T = 4.2 K on Nb/AlOx/Nb trilayer samples have demonstrated the validity of the concept of this practical TWJPA with great promise for quantum-limited performance. It is expected that TWJPA devices operating in the three-wave mixing mode will outperform state-of-the-art parametric amplifiers with respect to simultaneously achieving large gain and high bandwidth.
In the present work several characterizations of TWJPAs based on Al standard Josephson junctions technology will be presented. The DC current tunability of such a rf-SQUIDs based metamaterial will be investigated by focusing on the 4WM and 3WM regimes. Single tone transmission spectroscopy and signal gain evaluation by means of the so-called pump-on-pump-off method will be discussed with particular attention to real devices non-idealities that introduces a complex behaviour of the amplifier response.

As the relaxation time T₁ of transmons has increased in recent years, apparently so has the size of the fluctuations in T₁. To investigate the cause of these fluctuations we measured the T₁ of transmons made with an electrode layer of pure aluminum and a counter-electrode layer made with either pure Al or oxygen-doped granular Al. The superconducting energy gap of the counter-electrode depends on the layer’s thickness and the grain size, which depends on the oxygen doping. At 20 mK an oxygen-doped device showed T₁ variations between about 80 μs and a maximum of 300 μs, while an un-doped device on the same chip showed uncorrelated T₁ variations between about 50 and 100 μs. Measurements of the fluctuations versus temperature reveal that the standard deviation of T₁ is proportional to T₁, even above 150 mK, where the transmon relaxation is dominated by thermally generated quasiparticles. We discuss why this behavior is not consistent with the two most commonly proposed mechanisms, fluctuations in two-level-system dielectric loss or fluctuations in the density of non-equilibrium quasiparticles and propose an alternative mechanism that is consistent with this behavior.

On the 19th of June 2020, the European Strategy Group announced the 2020 update of the European Strategy for Particle Physics, identifying the ambition to operate a proton-proton collider (FCC-hh) at the highest achievable energy as long term future priority. CERN’s FCC-hh is the most ambitious scenario for a post LHC machine. It will operate as an 80-100 km acceleration ring where 16-18 T magnets will steer proton bunches producing center-of-mass collision energies of 100 TeV. In order to protect the superconducting magnets, the 35.4 W/m/beam synchrotron radiation emitted by the protons will be absorbed by a stainless steel tube, the so called beam-screen chamber, held at a temperature window of 40-60 K. Image currents will be induced into the steel walls of the beam-screen endangering the beam stability. To counteract this effect, the interior of a beam-screen chamber has to be coated with a highly conductive material. Our consortium explores the possibility to replace the conventional beam-screen coating Cu with REBa₂Cu₃O_7-x (RE = rare earth) Coated Conductors (CCs) in order to increase the beam stability margin.
In this contribution, we demonstrate why commercially available REBCO CCs are promising candidates for the FCC-hh beam screen coating. Critical current densities measured up to 9 T at 50 K, which is the operating reference temperature of the beam screen chamber, point towards a superconducting performance that can sustain the induced peak image currents of 25 A at FCC conditions. We present the surface resistance R_s of CCs and Cu colaminated on stainless steel at 8 GHz and in a wide range of cryogenic temperatures and magnetic fields. We find that CCs outperform Cu under operating conditions close to those to be found in the FCC-hh. In addition, high frequency vortex parameters like the depinning frequency are extracted from the data by fits with the Gittleman-Rosenblum model. This allows a discussion on the limits of the mean-field model and the correlation between surface resistance and microstructure of different REBCO CCs. The next steps towards an aspect ratio study of Cu and CCs in order to minimize trapped fields in the beam screen chamber, their assembly in the beam screen and the expansion of measurement conditions to the more realistic 16 T and 1 GHz will be discussed.

The past years have witnessed major advancements in all-electrical doping control on cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations. In this work, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in YBa₂Cu₃O_7−δ superconducting bridges. The underlying methodology is rather simple and avoids sophisticated nanofabrication process steps and complex electronics. A patterned multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ is probed in situ by optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photoinduced conductivity and Raman scattering investigations, reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping of cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content would permit a systematic study of complex phase diagrams and the design of electrically addressable devices.

Most investigations on the functional properties of bulk superconductors, such as the trapped field and the levitation force, are done nowadays by numerical simulations combining the finite element method and the A, H or T-Ω formulations of the differential Maxwell equations. By comparison, analytical models require generally a regular distribution of the applied field and are practical with simple superconductor shapes with a high level of symmetry only. They have however the advantages to require much less computation time than numerical simulations and to show physical laws that are not always obvious in the results of the simulations. In this contribution we’ll present analytical mean field models of superconducting magnetic levitation (SML) reproducing the levitation force on the one hand and the guidance force on the other hand. We’ll show that these models give very useful information for the design of the applications of SML concerning: i) the localization of the shielding currents in the superconductors and ii) the condition for stability of axisymmetric levitating systems.

The most powerful remnant-magnetic-field magnets are stacks of REBCO high-temperature superconducting tapes, which can trap up to 17.7 T. However, relatively low alternating magnetic fields perpendicular to the trapped magnetic field can demagnetize, or even fully demagnetize, the stacks [1]. This is a problem for many power devices, such as rotating machines with stacks of tapes in the rotor. Promising applications are aircraft propulsion motors for hybrid or electric airplanes [2], where ripple fields are of the order of 1 kHz or higher. At this frequency, a flight of only 1 hour corresponds to more than 3 million ripple-field cycles, which can fully demagnetize the stack, if the ripple field amplitude is high enough [1]. In this contribution, we present a solution to reduce cross-field demagnetization for many cycles, based on soldering the tapes. We base our study in both measurements and modeling. For modeling, we use the Minimum Electro-Magnetic Entropy Production (MEMEP) [3] to find the screening currents, taking into account the effect of soldering the tapes with a finite resistivity [4]. For the measurements, we magnetize the stack in liquid nitrogen with a superconducting magnet and later apply a transverse alternating field by means of a copper coil for frequencies of up to 500 Hz. Modeling shows that soldering the tapes with low enough resistance can drastically reduce the cross-field demagnetization. In order to achieve sufficiently small tape-to-tape resistance, we solder the tapes face-to-face in the experiments, confirming the findings from modeling. With this contribution, we show that soldered stacks with low resistance between tapes are very interesting for applications, and hence it is worth to dedicate efforts in this direction.

[1] A Dadhich, E Pardo. Modeling cross-field demagnetization of superconducting stacks and bulks for up to 100 tapes and 2 million cycles, Scientific Reports 10, 19265 (2020)
[2] F Grilli et al. Superconducting motors for aircraft propulsion: the Advanced Superconducting Motor Experimental Demonstrator project. Journal of Physics: Conference Series 1590, 012051 (2020)
[3] E Pardo and M Kapolka. 3D computation of non-linear eddy currents: Variational method and superconducting cubic bulk. J. Comp. Phys. 344, 339-363 (2017)
[4] S Li, J Kováč, E Pardo. Coupling loss at the end connections of REBCO stacks: 2D modelling and measurement. Supercond. Sci. and Technol. 33, 075014 (2020)

The understanding of copper oxide superconductors, and in particular YBa₂Cu₃O_7-δ, is inextricably tied to the complex electronic phases of those materials in their normal state. Here various nanoscale ordering phenomena occur as a function of doping and temperature and thus two key factors need to be addressed: 1) study the material at the nanoscale and thus the characteristic length scales of the phenomena under study, 2) precise control of the charge doping, which determines its properties.
Nanowires with different oxygen content are the ideal platform to study the electronic properties of YBCO as a function of doping. Alas, their fabrication is challenging, and each doping level requires a separate fabrication process. Moreover, the nanowire properties at different doping levels might vary due to different fabrication conditions. As an alternative solution, we use electromigration (EM) to control ex-situ the doping level of YBCO nanowires.
This process involves the displacement of the dopant oxygen atoms by applying a strong electric current. Pulsed EM can be used to gradually reduce the doping of a YBCO nanowire, while DC biased EM can be conversely used to restore the oxygen content of the same nanowire. These EM processes result in high-quality YBCO nanowires and can be used to fine tune their doping level. Indeed, with a single nanowire we were able to recover most of the YBCO phase diagram, with critical temperatures ranging from 90 K to 45 K.
The ex-situ control of the doping level in YBCO nanowires is a milestone towards an in-depth study of the phase diagram of cuprates. The EM bypasses the challenges of different fabrication techniques and opens the way for using the same nanowire to study the entire phase diagram. Moreover, this technique is interesting for the technological applications of YBCO weak links, where the tuning of the nanowire properties, such as critical current and kinetic inductance, is required.

Coated conductor (CC) tapes with functional layer of high-temperature superconductor REBCO (RE = Y, Gd, Er,…) exhibit impressive capability of transporting large amounts of electricity when the critical current, I_c, could commonly reach 1000 A in a tape 12 mm wide when cooled down to 77 K. However, when checking this property along the tape length, it quite often reveals the existence of locations with suddenly reduced I_c value. It is then not obvious which value of current should be taken as the limitation for a steady DC transport. We expect that a current slightly surpassing the local critical value could be sustained, but at its further increase the temperature will rise quickly creating the “hot spot” with enormous local dissipation causing an irreparable damage.
We present the model allowing to investigate the evolution of temperature in the location with reduced critical current analytically. It was found, that at surpassing the local I_c this temperature could stabilize at an elevated value, provided the dissipation is balanced by the cooling capability of surrounding environment. However, for a given tape architecture and cooling conditions, there is always the value of transported current that triggers the development of a hot spot. Advantage of our approach is that it allows in rather simple way to predict the result of DC transport at currents larger than the local minimum of critical current. Experiments with the tapes optimized for use in a resistive fault current limiter confirmed the validity of our theoretical formulas.

Over the last few years, it has been shown that an internal oxidation process can be used to more than double the layer Jc of state-of-the-art Nb3Sn conductors, with ternary level Bc2 values and practical conductor layouts. Part of the reason for this large increase is that the grain size is reduced, and since conventional Nb3Sn pins flux exclusively at the grain boundaries, this leads to large transport increases. This grain boundary refinement is cause by Zener pinning associated with oxide nanoparticles generated by the oxygen which has diffused in during the internal oxidation process. However, the oxide nanoparticles themselves pin flux directly. As the size of the nanoparticles decreases and their density increases, we can shift the character of the pinning from grain boundary to point pinning, as quantified by the shape of the pinning curve. In this work we compare various members of the group-IVB elements (Ti, Zr, Hf) as internal oxidants, showing that the size decreases while density increases as we move from Ti to Zr to Hf. We also confirm that internal oxidation is required to obtain the largest grain reductions and required for any level of peak shift. We use TEM to show the size and distribution of the precipitates, as well as map their distribution across the growth layer. We see that the TiO particles are typically 100 nm, while those of the ZrO are 5-10 nm, and those of the HfO are below 5 nm. We discuss the role of classic nucleation and growth in determining these factors. We find that the Fp curve shifts from 0.2 Birr for unoxidized Hf and control samples to 0.25 Birr for ZrO precipitates to 0.27 Birr for HfO. Further consideration of the underlying mechanisms should allow yet further improvements in conductor performance. Our results, in particular the influence of the precipitates on peak shift, suggests that internal oxidation is presently the most promising route for high field performance increases in Nb3Sn.

Josephson junctions with ferromagnetic layers where the ground-state phase difference can be reliably controlled are a potential candidate for applications in cryogenic memory devices, which can greatly reduce the ever-growing energy requirements for large-scale computing. Phase control has been successfully demonstrated with junctions containing a Ni fixed layer and a NiFe free layer [1,2]. However, there are still several improvements that can be made to increase the efficiency and reliability of these junctions. We present work on improving the transmission efficiency of single layer NiFe junctions by “dusting” the NiFe with thin layers of Ni on both sides. We also present a study of the 0-Pi transitions in these Ni “dusted” NiFe junctions.

This work is supported by Northrop Grumman Corporation.

[1] E. C. Gingrich, B. M. Niedzielski, J. A. Glick, Y. Wang, D. L. Miller, R. Loloee, W. P. Pratt, and N. O. Birge, Nat. Phys. 12, 564 (2016).
[2] I. Dayton, T. Sage, E. Gingrich, M. Loving, T. Ambrose, N. Siwak, S. Keebaugh, C. Kirby, D. Miller, A. Herr, Q. Herr, and O. Naaman, IEEE Magn. Lett. 9, 3301905 (2018).

Currently, the record J_c achieved in Bi-2212 short samples is around 9600 ampere per mm² at 5 T in 4.2 K. However, this performance has not yet been reproduced in long length coils and to date, the maximum J_c achieved in a coil is much lower compared to short samples. One possible reason for this lower J_c in coils is because the Bi-2212 wire in the coils has more interfilamentary bonding than in short Bi-2212 wires. This difference in amount of bonding can be explained by the amount of time the Bi-2212 spends in the melt state (t_melt) during the heat treatment. Short samples are heat treated in small overpressure (OP) furnaces and the heat treatment parameters can be controlled precisely. Large coils, which have large thermal masses, have to be heat treated in large OP furnaces where precise temperature control is not straight forward and small variations can cause significant changes in t_melt. We find that J_c depends on t_melt and this dependence is more prominent in wires with shorter distance between the Bi-2212 filaments compared to wires with larger filament separation. For a given t_melt, wires with smaller filament separation have more filament bonding than wires with larger filament separation. We believe the amount of filament bonding impacts the degree of Bi-2212 alignment, with longer t_melt resulting in less aligned Bi-2212 and therefore lower J_c. We are studying the underlying mechanism of how interfilament bonding impacts the electromagnetic properties of Bi-2212 wires. In our studies, we have found that interfilament bonding causes fluctuations in Bi-2212 filament cross-sections along the length of the wire, which possibly reduces the critical current. We have begun studying the kinetics of interfilament bond formation so we can quantitatively describe the amount of bonding that occurs in different wires as a function of time during the OP heat treatment. Our goal is to use this kinetic data to optimize the distance between Bi-2212 filaments to minimize the impact t_melt has on J_c, which should make it easier to OP heat treat coils in large OP furnaces, and will improve coil performance.

The rapidly growing field of compact nuclear fusion technology is driving the need for HTS-equipped electromagnets. The amount of tape needed just to build a single demonstrator reactor outstrips – by an order of magnitude – the present worldwide capacity to produce tape. Thus, higher speed tape manufacturing is a critical need. One of the bottlenecks in the production of buffered tape is the epitaxial MgO layer deposited on amorphous oxide. In the Stanford process, ion beam assisted deposition (IBAD) produces the biaxially textured template for subsequent addition of epitaxial MgO (EPI) by electron beam evaporation on heated substrates. The IBAD growth rate is limited by the available ion beam flux, which must match the arriving flux of MgO molecules at the tape substrate.
PVD Products has developed and commercialized a dual-chamber system for high speed production of IBAD/EPI layers. The system can process 1 kilometer of 100 µm thick tape at a speed of 200 meters/hour. Longer lengths of thinner tape are possible due the high capacity electron beam sources. The system features extended length deposition zones with multiple tape passes and, most importantly, a unique IBAD geometry pairing one evaporator with two large rectangular RF ion sources. The EPI layer chamber is likewise equipped with dual evaporation sources to support growth of thick EPI MgO at high processing speed. Multi-pass deposition handles 12 mm wide tape, and the tool is also designed to handle a single lane of 10 cm wide tape, albeit at a lower tape speed of ~50 m/hour. Two dedicated, differentially pumped RHEED chambers provide real-time feedback on film quality of both the IBAD and EPI processes without exposing the RHEED system to evaporant or high gas pressure. The tool has demonstrated processing 1 km of tape in a continuous 5-hour run. Due to the robust design, initial testing has also been extended to web speed up to 350 m/hr while still producing good RHEED patterns from both chambers. We will give an overview of the technology and present results from initial characterization the films deposited on metal tapes, including post-processing of LMO and ReBCO to complete the HTS stack.

Recent efforts to develop REBCO wire at Shanghai Superconductor Technology will be presented. Mainly two kinds of wire, GdBCO and EuBCO with/without pinning center are constantly produced at a high-speed deposition rate of 50nm/sec. Monthly production of the wire achieved in 2020 achieved about 50 to100km/month. Not only mass-production but also R&D for high-field application are also being developed. For example, we achieved high critical current, I_c, and pinning force at 4K, 18T of 430 A/4 mm-width and 806.2 GN/m³, respectively. In addition to these main R&D, efforts to improve production system are being done for wire precise size control, Hastelloy substrate thickness, and so on.

During 10 years since it foundation SuperOx Japan has been successfully developing IBAD/PLD approach for manufacturing of 2G HTS wires and in collaboration with SuperOx and S-Innovations (Russia) delivered hundreds of kilometers of REBCO based coated conductors for R&D and commercial projects worldwide.
SuperOx strategy based on customization of 2G HTS superconducting wires for application in particular temperature and field ranges. Based on the operating conditions in terms of magnetic field strength and temperature the company outlined five target areas for the development of new 2G HTS wires including:
1) Superconducting fault current limiters (SFCL, operating in self-field at 77K);
2) Rotating machines, particularly motors for aircrafts (1-3T and 65-70K);
3) Magnets for induction heaters, flywheels and MagLev (3-5T, 30-40K);
4) Coils for fusion reactors (12-20T, 20K);
5) Inserts for high-field NMR (>25T, 4.2K).
The R&D strategy for new line of SuperOx 2G HTS products relied on the combination of following approaches:
Variation of composition by chemical substitutions;
Optimizing the overall superconducting material stoichiometry;
Controlling of hole doping level (oxygen content);
Introduction of pinning centers including fabrication of multilayers;
Heavy ion irradiation of the HTS wires.
By the end of 2020 SuperOx companies reached production capacity of 300 km of 2G HTS wires per year and in 2021 we are planning to approach the level of 1000 km/year. In the course of production, it was revealed that new tapes demonstrate extremely attractive properties, for instance critical current (Ic) up to 1000A/12mm (s.f, 77K) record high engineering current (Je) exceeding 1000A/mm2 at 20K-20T and over 2000A/mm2 at 4.2K and 20T was demonstrated.
In this work we carried out systematic analysis of large number of industrially produced tapes studied by XRD, SEM-EDX, TEM, micro Raman techniques and their connection with final Ic-B-theta characteristics. Several important features responsible for superior Ic-B characteristics were identify which was very helpful to stabilize the production of tapes with improved Ic-B characteristics and to increase the production yield.

Significant progress on the thin film growth of iron-based superconductors (FBS) has been achieved over the last decade. As a result, high quality, epitaxial thin films of the technological important FBS [e.g., Fe(Se,Te), doped LnFeAsO (Ln: lathanoide) and doped AeFe₂As₂ (Ae: alkari earth elements)] are realised on different kinds of single crystalline substrates and technical substrates except for (Ba,K)Fe₂As₂.
Recently, we have successfully fabricated (Ba,K)Fe₂As₂ epitaxial thin films on fluoride substrates[1], which gives a great opportunity to investigate electrical transport properties. Here, we report on a high critical current density J_c of 14.6 MA/cm² at self-field and 4 K, corresponding to the pinning efficiency η=J_c/J_d of 8.8% (J_d: depairing current density). This value is much higher than the single crystal with artificial isotropic defects (η=3.5%) [2]. The angular-dependent J_c always shows a large c-axis peak below 25 K, which is the opposite behaviour to the expected one from the upper critical field anisotropy. Microstructural analysis by transmission electron microscope revealed the presence of many grain boundaries located almost parallel to the crystallographic c-axis. These results suggest that low-angle grain boundaries work as strong correlated pinning centres.

This work was supported by JST CREST Grant Number JPMJCR18J4. A portion of work was performed at the National Magnetic Field Laboratory, which was supported by National Science Foundation Cooperative Agreement No. DMR-1644779, US Department of Energy Office of High Energy Physics under the grant number DE-SC0018750, and the State of Florida. This work was also partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

[1] D. Qin, K. Iida, T. Hatano, H. Saito, Y. Ma, C. Wang, S. Hata, M. Naito, A. Yamamoto, submitted.
[2] V. Mishev, M. Nakajima, H. Eisaki, M. Eisterer, Sci. Rep. 6, 27783 (2016).

A recent paper (https://arxiv.org/abs/1702.05001) suggests that the D-wave gap symmetry of cuprate YBCO may be changed to S-wave gap symmetry by simultaneous doping of an electron doping atom and a hole doping atom into the parent compound. An S-wave gap symmetry creates better Josephson junctions across grain boundaries in poly-crystalline YBCO and would lead to an increase of its critical current density, Jc. A low Jc in poly-crystalline cuprates has been a major technological roadblock to fabricating high-Jc cuprate wires. We synthesized Y(1-x-y)Ca(x)Ce(y)Ba2Cu3O(7-d), where the Ca and Ce atoms substitute onto Y sites as hole and electron donors, respectively. We find that over a broad range of Ca and Ce dopings (x and y each varying from 0.0 to 0.32), the Tc of the cuprates is in the range of 72-77 K. This Tc stability to a large quantity of impurities is incompatible with a D-wave superconducting gap and suggests the appearance of S-wave gap symmetry. In order to study the Jc of our samples, we have designed a non-contact radiofrequency (RF) resonator that measures changes in the AC resistance and inductance in order to extract the Jc. In this talk, we will present our results for Jc as a function of temperature and Caand Ce doping.