Symposium QT11—Superconducting Materials and Applications

A combination of broken inversion symmetry and spin-orbit coupling gives rise to a wide range of exotic superconducting states, such as mixed-parity superconductivity, superconducting Weyl state, and superconducting diode effect. Incipient ferroelectrics e.g., SrTiO₃, KTaO₃, EuTiO₃, and PbTe are near a polar instability. The emergence of superconductivity has been reported in some of these incipient ferroelectrics upon doping. Many unconventional superconductors, such as the cuprates, pnictides, and heavy fermion systems, occur in close proximity to magnetic fluctuations or magnetic orders, suggesting that these are important ingredients in the superconductivity in these materials. Here, ferroelectricity and superconductivity could be connected or accidental neighbors. Furthermore, the intersection of superconductivity and topologically nontrivial states is a fertile landscape for exciting quantum phenomena, including non-abelian excitations. Topologically nontrivial states have been reported in some of these polar superconductors. First, I will focus on the emergence of superconductivity and its interplay with ferroelectricity in tuned SrTiO₃ thin films. I will discuss the ferroelectric enhancement of superconductivity in epitaxially strained SrTiO₃ films. The ferroelectric films reveal a doubling in the critical temperature of superconductivity. We show that suppression of ferroelectricity in over-doped samples also suppresses superconductivity. Next, I will present our recent results on the emergence of 2D superconductivity in KTaO₃ (111) interfaces. Signatures of weak anti-localization are observed in both superconducting, KTaO₃ (111), and non-superconducting, KTaO₃ (100), interfaces. The observed weak anti-localization correction, however, is stronger in superconducting interfaces.
A.H. Al-Tawhid, J. Kanter, M. Hatefipour, D.P. Kumah, J. Shabani, K. Ahadi, ArXiv:2109.09786 (2021).
S Salmani-Rezaie, K Ahadi, S Stemmer Nano Letters 20 (9), 6542-6547 (2020).
K Ahadi, L Galletti, Y Li, S Salmani-Rezaie, W Wu, S Stemmer, Science advances 5 (4), eaaw0120 (2019).

The new AV₃Sb₅ (A = K, Rb, Cs) family of kagome metals are candidates for unconventional superconductivity and chiral charge density wave (CDW) order, both of which are thought to arise due to the proximity of saddle points in their band structures to the Fermi energy. In CsV₃Sb₅, a superconducting transition is observed at 2.5 K and a CDW transition at 94 K. Here we use chemical substitution in CsV₃Sb₅ to explore the relationship between the superconducting and CDW states and generate a phase diagram for CsV₃Sb_5-xSn_x for 0 ≤ x ≤ 1.5 that illustrates the impact of hole-doping the system. As the Sn content is increased, a nonmonotonic evolution of the superconduting T_C is observed, and two distinct superconductivity domes are observed, with local maxima of 3.6 K at x = 0.03 and 4.1 K at x = 0.35. By x = 1, the superconducting phase vanishes. Simultaneously, the onset temperature of the CDW instability is found to be rapidly suppressed with Sn substitution until the CDW phase vanishes by x = 0.06. The first peak in T_C coexists with the CDW state, suggesting that the SC and CDW orders are related in an unconventional manner.

A superconducting phase slip is a topological fluctuation of the order parameter typically found in one-dimensional structures such as nanowires. They are the cause of a finite resistance below room temperature and can be detrimental to superconducting device performance. Recent work has sought to not only better understand, but to make use of these excitations. Current interest in phase slip physics from an applications perspective include metrology as a quantum current standard and in the proposed phase slip qubit – that uses quantum phase slips in place of Josephson junctions and so eliminate decoherence from noise associated with the barrier. A two-dimensional analogue of the phase slip centre – the so-called phase slip line – has previously been observed in thin-film metallic samples relatively close to the transition temperature. [1]

Here, we report on nonequilibrium excitations found in three-dimensional [2] structures fabricated from nanocrystalline superconducting boron-doped diamond structures that demonstrate the key features of one-dimensional phase slip centres over a wide temperature range [3]. More remarkably, we have demonstrated control over these excitations in that we are able to switch deterministically from the superconducting state to the normal state over a wide range of bias currents. We propose that the presence of phase slip-like excitations in a three-dimensional sample is supported by the unusual morphological columnar granularity of the nanocrystalline diamond films from which they are made being akin to a ‘naturally occurring’ 2D Josephson junction array. [4]

With similarities to both phase slip centres and lines, we suggest that these excitations may find use in a variety of settings, notably as a ‘barrier-less' Josephson junction.

References

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One of the potentially transformative areas of scientific development is to achieve superconductivity at room temperature. Recently, the photochemical synthesis was carried out to prepare carbonaceous sulfur hydride (CSH) systems with room-temperature superconductivity at high pressure. In this work, we present a first-principles study aiming to unravel the photoreaction of sulfur with molecular hydrogen using the TDESMD methodology. Individual TDESMD trajectory provides details about reactions that lead to a number of allotropes of sulfur and their hydrogenated forms. Simulated mass spectra based on an ensemble of TDESMD trajectories provide the distribution of sulfanes along reaction pathways. It is found that the photoreaction starts with ring opening of cyclic S₈, which may then react with two H radicals to form S₈H₂ as a result of the homolytic dissociation of H₂. The sulfur cluster will undergo the elimination of small fragments, which can later recombine into a variety of sulfanes. The most abundant fragments generated along trajectories are H₂S, S₄H₂, and S₈H₂. The final sulfur-bearing products are a mixture of sulfanes with various chains and rings. The mechanistic and conformational information obtained from this work allows us to better understand the photoreaction, and potentially, give insights into the preparation of high T_c materials using similar reactants.

High magnetic fields have proven to be an invaluable tool for exploring magnetic and electronic properties of novel quantum materials. Extreme magnetic fields enable direct probes of the superconducting order and vortex matter in high temperature superconductors. A combination of diamond anvil cells and pulse magnets allows us to controllably tune and probe candidate systems at the extremes of pressure-field-temperature parameter envelope. We established key superconducting quantities of several hydride high-temperature superconductor families, including sulfur hydrides and rare earth “superhydrides”. We will discuss possible pathways to for maximizing temperature and reducing the pressure required to stabilize the superconductivity, such as boosting electron-phonon coupling via phonon softening and enhancing density of charge carries via Fermi velocity renormalization. Work at NHMFL-LANL was performed under the auspices of the NSF, DoE, and State of Florida.

Since the discovery of magic-angle twisted bilayer graphene (MATBG), new types of moiré superlattices have been investigated to study strongly correlated and topologically nontrivial phenomena. Several interesting states, including but not limited to correlated insulators, quantized anomalous Hall states, ferromagnetism, and correlated Chern insulators, have been observed in various moiré materials. However, during the first few years, superconductivity was reproducibly seen only in MATBG. More recently, magic-angle twisted trilayer graphene (MATTG) has shown robust superconductivity and correlated states with an additional knob for electric displacement field tunability. Interestingly, MATBG and MATTG, which are the only robust moiré superconductors known to date, are part of a hierarchy of magic-angle graphene systems exhibiting a series of twist angles for different number of twisted layers. In this talk, I will introduce the physics behind MATTG in relation to the magic-hierarchy, and its striking behaviors including strong coupling superconductivity and large Pauli limit violation. I will also discuss our current understanding about the correlated states in this system, and present some of our most recent progress on this topic.

The discovery of topological electronic states in bulk solids has drastically changed the conventional view of materials. The novel electromagnetic responses, exotic elementary excitations, and unprecedented quasiparticle in topological insulators, semimetals, and superconductors are expected to be applied to next-generation electronics. In particular, the Majorana quasiparticles, which are expected to arise in topological superconductors, have attracted a great deal of attention, because their manipulation has been theoretically proposed to realize the "error-free" quantum computers. As the first step towards this application in the field of quantum information technology, materials science is the key, in that it is essential to establish the compounds and/or structures which can generate the Majorana quasiparticles. With reviewing our studies in topological materials science [1-15], we will discuss the current status of research and development in topological superconductivity and topological quantum computation.

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The discovery of 3D topological insulator materials and topological superconductor open up a new research field in the condensed matter physics. In order to search for the topological superconductor, we have grown a large number of the single crystals of Pb-system ( Pb-Sn-In-Te) topological crystalline insulator and their topological superconductor . We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator, topological crystalline insulator and topological superconductor. We have got the bulk topological superconductor with T_c=5K.

The material IrTe₂ exhibits a transition to a modulated state featuring Ir-Ir dimers, with large associated atomic displacements. Partial substitution of Pt or Rh for Ir destabilizes the modulated structure and induces superconductivity. It has been proposed that quantum critical dimer fluctuations might be associated with the superconductivity. Here we test for such local dimer correlations and demonstrate their absence. X-ray pair distribution function (PDF) approach reveals that the local structure of Ir_0.95Pt_0.05Te₂ and Ir_0.8Rh_0.2Te₂ dichalcogenide superconductors with compositions just past the dimer/superconductor boundary is explained well by a dimer-free model down to 10 K, ruling out the possibility of there being nanoscale dimer fluctuations in this regime. This is inconsistent with the proposed quantum-critical-point-like interplay of the dimer state and superconductivity, and precludes scenarios for dimer fluctuations mediated superconducting pairing.

On the other hand, the detailed evolution of the local atomic structure across the (x, T) phase diagrams of the two superconducting families, Ir_1−xRh_xTe₂ (0≤x≤0.3) and Ir_1−xPt_xTe₂ (0≤x≤0.05) in 10K≤T≤300K range, reveal hysteretic thermal structural phase transition from a trigonal (P-3m1) to a triclinic (P-1) dimer phase for low Pt and Rh contents. This emphasizes the intimate connection between the lattice and electronic properties. For superconducting samples away from the dimer/superconductor phase boundary, structural transition is absent and the local structure remains trigonal down to 10 K. In the narrow range of compositions close to the boundary, PDF analysis reveals structural phase separation, suggestive of weak first-order character of the substitution induced dimer-superconductor quantum phase transition. Samples from this narrow range show weak anomalies in electronic transport and magnetization, hallmarks of the dimer phase, as well as superconductivity albeit with incomplete diamagnetic screening. The results suggest that the dimer and superconducting orders exist in the mutually exclusive spatial regions.

In the pursuit of epitaxially grown, lattice matched Josephson Junctions (JJ), this work investigates the growth of Nb_xTi_1-xN//AlN//Nb_xTi_1-xN tri-layer heterostructures. This all-nitride material stack offers several benefits over traditional Al//AlO_x//Al JJs. These include enhanced mechanical and chemical stability, potentially reduced two level system loss (TLS) due to the use a crystalline AlN barrier, and higher superconducting critical temperature, which may reduce the number of thermally excited quasiparticles. Engineering the ternary nitride composition to lattice match in-plane with wurtzite AlN may reduce losses caused by interface defects.
Samples were grown by plasma assisted molecular beam epitaxy (PAMBE) using a custom DCA M600 MBE system with a base pressure better than 1e-10 Torr. Samples were grown on 2” C-plane sapphire wafers which were degreased and thermally cleaned before growth. A RF plasma generator is used for nitrogen incorporation, while Nb is deposited using an electron beam evaporator. Ti is deposited using a high-temperature effusion cell. Nitrogen flux was set at 1.8 sccm at 500W, and metal fluxes were varied to compensate for changing alloy composition. Substrate temps during growth were set at between 800°C and 1000°C as determined by thermocouple.
The in-plane lattice constant of the superconducting ternary alloy can be engineered by changing the relative Ti and Nb flux during growth and is able to be shifted through alloying Nb_xTi_1-xN. Growth of these heterostructures is optimized for surface roughness, crystallinity, and compositional uniformity. Characterization of each layer is performed with high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), and reflection high-energy electron diffraction (RHEED).
Growing the ternary alloy at 800°C is shown to possess a lattice constant that shifts from TiN to Nb₄N₃, which is a tetragonal nitrogen deficient phase of NbN_x. By increasing the growth temperature to 1000°C, more nitrogen can be incorporated in the film, bringing the high Nb content film’s lattice constant closer to the atomic spacing of wurtzite AlN. Trends of achievable lattice constants versus composition will be discussed, along with observations of phase uniformity as Ti content in the alloy decreases.
Co-planar waveguide resonators are fabricated from the Nb_xTi_1-xN material to determine quality factor, and the role of alloying on the creation and performance of Josephson junctions will be discussed in detail. Devices are tested in a High Precision Devices adiabatic de-magnetization refrigerator (ADR) down to 50mK.

Since their discovery, superconductivity in cuprates has motivated the search for materials with analogous electronic or atomic structure. We have used soft chemistry approaches to synthesize superconducting infinite layer nickelates from their perovskite precursor phase, using topotactic reactions. We will present the synthesis and transport properties of the nickelates, observation of a doping-dependent superconducting dome, and our current understanding of the electronic structure.

Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconductors. One prime materials candidate has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd_0.8Sr_0.2NiO₂. Undoped NdNiO₂ belongs to a series of layered square-planar nickelates with chemical formula Nd_n+1Ni_nO_2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here, we present the superconducting and electronic properties of molecular beam epitaxy-synthesized Nd₆Ni₅O₁₂, the quintuple-layer (n = 5) member of this series, which achieves optimal cuprate-like electron filling (3d^8.8) without chemical doping. We observe a superconducting transition beginning at ~13 K. [1] Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd₆Ni₅O₁₂ interpolates between cuprate-like and infinite-layer nickelate-like behavior. By engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality. In part II of this talk, we further expound on the synthetic strategies of the layered nickelates including Nd₆Ni₅O₁₂.
[1] GA Pan et al. Superconductivity in a quintuple-layer square-planar nickelate. arXiv:2109.09726.
[2] D Ferenc Segedin et al. Superconducting Quintuple-layer Square-planar Nickelates: Synthetic Strategies and Challenges (Part II). MRS Bulletin Spring 2022.

Since the discovery of superconductivity in Sr_0.2NdNiO₂, square-planar nickelates have become a prime testbed to study superconductivity in a cuprate-like materials analog. The Ruddlesden-Popper (RP) square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to further tune the electronic properties with dimensionality. Our group recently discovered superconductivity in Nd₆Ni₅O₁₂ [1-2]. The layered nickelates, Nd_n+1Ni_nO_2n+2, and their parent Nd_n+1Ni_nO_3n+1 oxygenated forms, have yet to be synthesized in bulk for n > 3. Here, we use atomically-precise molecular beam epitaxy and subsequent oxygen de-intercalation to stabilize RP square-planar nickelate thin films. We explore competing growth requirements for the epitaxial stabilization of the as-grown Nd_n+1Ni_nO_3n+1 material and the subsequent reduction to Nd_n+1Ni_nO_2n+2. The precursor Nd_n+1Ni_nO_3n+1 films synthesized with more than 1% tensile strain form strain-relieving vertical NdO faults. That said, a high-quality reduced phase can be achieved only with tensile strain in the as-grown state due to the expansion of the in-plane lattice constant upon reduction. We demonstrate the synthesis of n = 3 and n = 5 RP square-planar nickelates on NdGaO3(110), which provides sufficient (~ 1%) tensile strain for reduction while minimizing the formation of vertical NdO faults in the as-grown state. Further synthetic optimization of the square-planar RP nickelates could lead to future investigations of nickelate superconductivity with dimensionality as a tuning knob in addition to chemical doping.
[1] GA Pan et al. Superconductivity in a quintuple-layer square-planar nickelate. arXiv:2109.09726.
[2] GA Pan et al. Superconducting Quintuple-layer Square-planar Nickelate Nd6Ni5O12: Superconducting and Electronic properties (Part I). MRS Bulletin Spring 2022.

Sitting right next to copper in the periodic table of the elements, nickel-based oxides have long been considered a candidate for hosting cuprate-like unconventional superconductivity.¹ However, enforcing a cuprate-like d⁹ electronic configuration in nickelates results in the thermodynamically unstable nickel formal valence of Ni⁺. Consequently, only few materials with square-planar NiO₂ structure satisfying this requirement have been successfully synthesized, none of which showed signs of superconductivity.^2–6 It was only recently that superconductivity was finally observed in infinite-layer nickelate LnNiO₂ (Ln = La, Pr, Nd) thin films, sparking large interest in the community.^7–9
Initial experimental studies on the infinite-layer nickelates have already revealed many interesting observations. Notably, despite structural and electronic similarities, the nickelates show several important distinctions from the cuprates. In particular, unlike the cuprates – which have antiferromagnetic insulator and Fermi liquid metal ground states of the underdoped and overdoped regions of the phase diagram, respectively – in nickelates both of these regions exhibit a weakly insulating upturn.¹⁰ In conjunction with the observation of additional electron pockets in the calculated Fermi surface of the nickelates, these differences raise an important question: to what degree can we consider nickelates as analogs to the cuprates?^11,12
Meanwhile, the doping dependence studies show an interesting correlation where superconductivity is observed only when the magnitude of the normal state resistivity is below the resistance quantum per NiO₂ plane.¹⁰ This naturally motivates the question of whether the observed superconducting dome in the nickelates is influenced by extrinsic factors arising from crystalline imperfections. Indeed, achieving high and consistent crystallinity in this material has been challenging, and nontrivial variations in the crystallinity and electrical resistivity have been observed.^10,13 To resolve uncertainties arising from crystalline imperfections and to probe the intrinsic properties of the infinite-layer nickelates, further improvements in the crystallinity of this material are strongly called for.
In continuation of our efforts to optimize the growth and stabilization of this material,¹³ we have recently made significant progress in improving the crystallinity of infinite-layer nickelates, enabling us to reproducibility stabilize Nd_1–xSr_xNiO₂ (x = 0~0.3) with high and consistent crystallinity throughout the explored range of x. X-ray diffraction and scanning transmission electron microscopy measurements confirm that the Ruddlesden-Popper-type faults, the dominant extended defect in this material,¹³ are essentially eliminated. With these new advances in sample quality, we have revisited the doping dependent study of the infinite-layer nickelates and have found important distinctions in the phase diagram compared to previous work,¹⁰ the details of which will be discussed.
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* supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract No. DE-AC02-76SF00515, and the Packard Foundation (B.H.G and L.F.K).

The recently discovered nickel-based oxide superconductors [1] provide a close analogue and long-sought opportunity for experimental comparison to the high-T_c cuprates. Despite their nominally isostructural infinite-layer forms and isoelectronic metal 3d⁹ configurations, however, several key distinctions between the two systems have been observed, including differences in the hybridization, charge-transfer energy, and pairing mechanism [2-4]. More thorough understanding of the physical mechanisms driving superconductivity in the nickelates will therefore require additional advanced experimental characterization.
One of the main barriers to experimental measurements of the nickelates has been the challenge of sample synthesis. So far, the only demonstrated route to superconducting samples involves initial perovskite-phase growth followed by topochemical reduction to infinite-layer phase [5]. The details of this growth and reduction process are still being explored, and the impacts of lattice defects, minority phases, and other inhomogeneities on bulk measurements of superconductivity are still being unconvered (see K. Lee, et al., this program). Still, while the family of superconducting infinite-layer nickelates continues to grow in thin films [6-10], superconductivity in bulk samples of the same compounds has remained perplexingly elusive [11]. This apparent discrepancy raises key questions about the origin of superconductivity in the films, particularly regarding the importance of different aspects of the thin film geometry such as the role played by crystalline strain or the atomic interface. For example, the charged atomic planes in the RNiO₂ (R = rare earth) nickelate film and the neutral planes of the SrTiO₃ substrate have been predicted to form a strongly polar interface hosting a two-dimensional electron gas (2DEG) [12] with carrier density even higher than the superconducting LaAlO₃-SrTiO₃ polar interface [13].
To directly probe the electronic nature of the interface between infinite-layer nickelate thin films and SrTiO₃ substrate, we leverage high-resolution electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). We extract a quantitative picture of the cation arrangement and oxygen occupancy. Comparison between the experimentally measured lattice structure and charge distribution at the nickelate-substrate interface with systematically varied theoretical models provides a more complete understanding of the role played by this interface for superconductivity. We also discuss insights which will inform future growth and heterostructuring [14] of these compounds.
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*Supported by DOD AFOSR (FA 9550-16-1-0305), DOE BES MSD (DE-AC02-76SF00515), the Moore Foundation (GBMF9072), the Packard Foundation, and the German Research Foundation (DFG) within CRC/TRR 80 (Projects G3, G8) with computational time at magnitUDE granted by the Center for Computational Sciences and Simulation of the University of Duisburg-Essen (DFG grant nos. INST 20876/209-1 FUGG).

Since the discovery of superconductivity in the palladium-hydrogen system [1] it was found that alloying Pd with noble metals (Au, Ag and Cu) with a particular concentration of H increased the superconducting transition temperatures with respect to the pure Pd-H system [2]. The amorphous palladium-gold alloys Pd_100-xAu_x, x=13, 16, 19 at %, were studied with a computational method. We start with the process of amorphization of 216-atom supercells of these alloys by using ab initio molecular dynamics and the undermelt-quench approach [3], then their structure is analized with the Pair Distribution Function (PDF) which shows the characteristic splitting of the second peak, feature of an amorphous metal. Hydrogen was inserted interstitially in each supercell until it reached a ratio H/M=0.9 (M=Au+Pd atoms) in order to study the effect of this process on the topology and the electronic Density of States (eDoS) . Results will be reported and an analysis of their possible implications for superconductivity will be discussed.
[1] T. Skoskiewicz. Superconductivity in the palladium-hydrogen and palladium-nickel-hydrogen systems. physica status solidi (a) (1972), 11(2), K123-K126. https://doi.org/10.1002/pssa.2210110253
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[3] A. A. Valladares. in Glass Research Progress, edited by: Jonas C. Wolf and Luka Lange, Nova Science Publishers, Inc., 2008. Pp. 61-123

Bismuth is a semimetal that has interesting properties depending on the structure in which it is found; one of these properties is superconductivity. At ambient pressure, superconductivity has been observed in the amorphous phase with a critical temperature, T_c, of 6 K, while for the crystalline phase it has a value below 0.53 mK [1], a result predicted by our group in 2016 [2]. A superconducting transition temperature of 2.61 K was also predicted by our group in 2018 for the bismuth bilayers [3]. Furthermore, we have studied the appearance of this phenomenon in bismuth under pressure and some results agree with experiments [4] while others are awaiting experimental verification [5, 6]. In this work we will investigate the effect of negative pressures on the superconductivity of the amorphous bismuth by ab initio calculations in supercells of 216 atoms with densities of 9.81 g/cm³, 9.34 g/cm³ and 8.53 g/cm³. Amorphous structures will be obtained through molecular dynamics and our undermelt-quench process that has given specimens in accord to the experiment [2]. We shall report electronic properties of such samples and draw conclusions.

[1] O. Prakash, A. Kumar, A. Thamizhavel, S. Ramakrishnan. Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure. Science 355 (2017), pp. 52-55. DOI: 10.1126/science.aaf8227.
[2] Z. Mata-Pinzón, A. A. Valladares, R. M. Valladares, A. Valladares. Superconductivity in Bismuth. A New Look at an Old Problem. PLoS ONE 11 (2016), e0147645. DOI:10.1371/journal.pone.0147645.
[3] D. Hinojosa-Romero, I. Rodriguez, A. Valladares, R. M. Valladares, A. A. Valladares. Possible superconductivity in Bismuth (111) bilayers. Their electronic and vibrational properties from first principles. MRS Advances 3 (2018), pp. 313-319. DOI: 10.1557/adv.2018.119.
[4] I. Rodríguez, D. Hinojosa-Romero, A. Valladares, R. M. Valladares, A. A. Valladares. A facile approach to calculating superconducting transition temperatures in the bismuth solid phases. Scientific Reports 9 (2019) 5256 DOI:10.1038/s41598-019-41401-z
[5] D. Hinojosa-Romero, I. Rodriguez, Z. Mata-Pinzón, A. Valladares, R. M. Valladares, A. A. Valladares. Compressed Crystalline Bismuth and Superconductivity - An ab initio computational Simulation. MRS Advances 2 (2017), pp. 499-506. DOI: 10.1557/adv.2017.66
[6] A. A. Valladares, I. Rodríguez, D. Hinojosa-Romero, A. Valladares, R. M. Valladares. Possible superconductivity in the Bismuth IV solid phase under pressure. Scientific Reports 8 (2018) 5946 DOI:10.1038/s41598-018-24150-3.

Cuprates as a class of high T_c superconductors host multiple unconventional phases that require further studies to fully understand. Studying the time dependent response of these materials can enhance our understanding of excitations, light-matter interaction, electron-phonon coupling and the electronic response in different phases. Here we focus on the relationship between the pseudogap and superconducting phases. To study this, we employ time and angle resolved photoemission spectroscopy (trARPES). We directly probe the Fermi surface and electron dispersion, giving us access to the spectral function at different momenta and the dynamics of quasi particles. Relating this information about the quasi particles to the pseudogap and superconducting phase respectively can help us understand the underlying mechanism of superconductivity in these compounds.
The pseudogap phase is characterized by a low density of states (DOS) at the Fermi level, a loss of states in the Fermi surface compared to the superconducting phase and the loss of one hole contributing to the charge transport. Doping dependent studies on the bi-layer high T_c superconductor Bi2212 reveal a change in the topology of its Fermi surface [1,2,3]. At a hole doping of 22% the topology changes from a hole- to electron-like Fermi surface, which is usually called a Lifshitz transition. We find that for the optimally doped Bi2212 (16% hole doping), we are able to induce this Lifshiftz transition by irradiating the sample with high fluence femtosecond laser excitation. The induced transition persists for multiple picoseconds and is of reversible nature.
Exploiting this effect could allow for a controlled manipulation of the phase diagram through the use of laser radiation.

[1] A. Kaminski, et al., Phys. Rev. B 73, 174511 (2006)
[2] S. Benhabib, et al., PRL 114, 147001 (2015)
[3] N. Doiron-Leyraud, et al., Nat Commun 8, 2044 (2017)

Pure copper is a metal with a high conductivity which is normally attributed to its low electron-phonon coupling strength. This characteristic result indicates that if copper undergoes a superconducting transition at all, then it must be at a very low temperature, according to the conventional theories of superconductivity. This premise is confirmed by the fact that pure copper does not superconduct at temperatures so far studied. However, many materials that include copper in their composition have been found to be superconducting, including transition metal alloys. In particular, amorphous alloys in the Cu_xZr_1-x system have been found to be superconducting while their crystalline counterparts have not [1, 2]. Here we combine the McMillan approach [3] and our computational results for the amorphous alloys to revisit superconductivity in the Cu_xZr_1-x system at various concentrations and to calculate some of its fundamental superconducting properties. Ab initio DFT molecular dynamics was used, together with the undermelt-quench approach developed within our group [4], to generate amorphous supercells with 216 atoms. Electronic and vibrational properties were obtained for these structures and used to estimate the transition temperatures of some of the specimens studied through the McMillan approach. An analysis of the results obtained will be presented and conclusions put forth.
References
[1] Arias, D., & Abriata, J. P. (1990). Cu-Zr (copper-zirconium). Journal of Phase Equilibria, 11(5), 452–459. https://doi.org/10.1007/bf02898260
[2] Garoche, P., & Veyssie, J. J. (1981). Superconductivity in amorphous versus Crystalline Cu_0.33Zr_0.66 alloys. Journal De Physique Lettres, 42(15), 365–368. https://doi.org/10.1051/jphyslet:019810042015036500
[3] McMillan, W. L. (1968). Transition temperature of strong-coupled superconductors. Physical Review, 167(2), 331–344. https://doi.org/10.1103/physrev.167.331
[4] Valladares, A. A. (2008). In J. C. Wolf & L. Lange (Eds.), Glass Materials Research Progress (pp. 61–123). Nova Science Publishers, Inc.

Magnetotransport measurements on superconducting samples of CaMg₂-H were performed at high pressures of up to 310 GPa and magnetic fields of up to 14 T. We determine key superconducting properties, including upper critical field, superconducting coherence length, and critical currents. Tc ranges from 50-170K. Magnetic field strongly suppresses the superconducting phase, which is well described by the Werthamer-Helfand-Hohenberg model. We find that the suppression of superconducting magnetic field is consistent with previously studies hydrates.

Superconductivity in topological materials is reported to host various unconventional properties. Due to the limited number of topological superconducting materials, the pairing mechanism is not clearly understood. Recent studies suggest that bulk superconductors can possess a topologically nontrivial band structure [1]. Among these, A15 superconductors, a well-known family of metal-based compounds, are promising candidates to realize topological superconductivity. Reports suggest that the standard crystal structure symmetry of A15 compounds, along with spin-orbital coupling (SOC), creates a gapped crossing near the Fermi level. The unnoticed topological surface states near the Fermi surface of the A15 compounds were unveiled in recent theoretical calculations, indicating the potential candidacy to host topological superconductivity [2, 3]. The topological bulk band structures of these compounds can be characterized by nontrivial Z2 invariants, with topological surface bands appearing near the Fermi surface as dictated by the bulk boundary correspondence. Though the surface state of the A15 - Ti₃X (X = Ir, Sb) compound has been studied theoretically, to the best of our knowledge, none of the early works had a follow-up regarding the microscopic investigation of their superconducting properties. We investigated the superconducting ground state of topological superconducting material candidates Ti₃Ir and Ti₃Sb, coupled with magnetization, heat capacity, and muon spin rotation and relaxation measurements [4].
Magnetization measurements suggest that both the compounds are type-II superconductors with bulk transition temperatures of 4.3 and 6.7 K, respectively. The upper critical field for Ti₃Ir is higher than the Pauli limit indicating a possible unconventional pairing mechanism in the superconducting ground state. Muon spin rotation and relaxation (μSR) measurement in transverse field (TF) mode was done to reveal the field distribution across the sample and find temperature dependence of magnetic penetration depth to a high degree of accuracy. However, specific-heat and TF-μSR measurements rule out any possibility of a nodal or anisotropic superconducting gap but reveal a conventional isotropic s-wave gap structure in Ti₃X (X = Ir, Sb). Zero-field μSR measurement is an excellent means to detect spontaneous magnetization that can be associated with spin-triplet superconductivity [5-7]. The measured zero-field-μSR spectra below and above superconducting transition T_C were best fitted by the static Kubo-Toyabe function multiplied by an exponential decay [8]. There is no change in asymmetry spectra for the samples at temperatures above and below Tc. This suggests the absence of a spontaneous magnetic field below the superconducting transition temperature, categorizing Ti₃X (X = Ir, Sb) compounds into time-reversal-symmetry-preserved topological superconducting candidates [4]. In future, it is vital to study more A15 superconductors to understand the role of the nontrivial band topology and topological surface states in the superconducting ground state.
References:
[1] S. Yonezawa, AAPPS Bull. 26, 3 (2016).
[2] M. Kim, et al., Phys. Rev. B 99, 224506 (2019).
[3] E. Derunova, et al., Sci. Adv. 5, eaav8575 (2019).
[4] M. Mandal, et al., Phys. Rev. B 103, 054501 (2021).
[5] G. M. Luke, et al., Nature (London) 394, 558 (1998).
[6] Y. Maeno, et al., J. Phys. Soc. Jpn. 81, 011009 (2012).
[7] G. M. Luke, et al., Phys. Rev. Lett. 71, 1466 (1993).
[8] R. S. Hayano, et al., Phys. Rev. B 20, 850 (1979).

The introduction of defects in a material can profoundly affect its mechanical and electronic properties. Defects often cause a deleterious effect on the superconducting properties of a material, decreasing the critical temperature and critical field. In this talk, we present our recent experimental and theoretical investigation on WB2 that starts superconducting above 50GPa, which we believe is due to the introduction of stacking fault and twin boundary defects. Our high-pressure X-ray diffraction data show that the ground state hP12 phase of WB2 persists up to 145GPa. Theoretically calculated Tc of the hP12 phase fails to explain the observed superconducting critical temperature of 17K at 70GPa. Our defect energy calculations show that stacking faults and twin boundaries can plausibly form under pressure in the hP12 phase. The local atomic arrangement of these planar defects resembles the arrangement of atoms in the P6/mmm MgB2. The P6/mmm WB2 show the highest calculated critical temperature among the competing phase of WB2. However, P6/mmm WB2 is thermodynamically stable only at pressures above 130GPa, much higher than the pressures where our sample displays superconducting behavior. The spatial projected density of states (DOS) at the planar defects mirrors the DOS of the bulk P6/mmm WB2 phase. Thus, the formation and percolation of superconducting stacking fault and twin boundary defects at high pressure can explain the observed Tc of WB2.

Since the discovery of High Temperature Superconductors (HTS) a possible cryogen-free scenario has been always wished. Nowadays liquid helium is running out, and it is likely that the cooling by will be a large part of the costs of any superconducting system. Bi-2212 wires at temperature higher than 4.2 K still show a very high irreversibility field and thus a deep investigation of their properties in such a range of temperature is very useful in order to assess the applicability in high field cryogen-free magnets. Here electrical transport and magnetic properties characterization at variable temperature and magnetic field of Bi-2212 wires are reported together with a well described original approach to calculate irreversibility field Hirr [1]. From the magnetic and transport measurements comparison it has also been studied the properties of the network of bridges between the filaments formed by grains grown through the Ag matrix during the partial-melt heat treatment process. This is a feature unique in Bi-2212/Ag wires and, although these interconnections favor a redistribution of the current among the filaments allowing high critical current density, they represent a strong electrical coupling between the filaments themselves. Such a coupling increases the AC losses, present also in case of charge and discharge of DC magnets, principal applications of this kind of superconductor. Here we report on the behavior of these bridges as a function of applied magnetic field and temperature and the implications they have on the electrical coupling. The results show that the effective length scale on which the filaments are coupled is dependent on the field and temperature, passing from the filaments-bundle diameter at low field and temperature to single filament diameter at high field and temperature.
All the experiments have been performed on two multifilamentary wires prepared by GDG-PIT process [2] starting from two commercial Bi-2212 precursor powders: Nexans and Engi-Mat. These studies are devoted to provide reference data on the behaviour of the only isotropic wire for high field application with an eye to the performances at temperatures above 4.2 K. We believe that such findings are very useful in magnet design.
[1] A. Leveratto et al, Scientific Reports 11 (2021), 11660
[2] A. Leveratto et al, Supercond. Sci. Technol. 29 (2016), 045005

Since the discovery of high temperature superconductivity 35 years ago the superconducting tapes of second generation (2G HTS) or so-called coated conductors (CC) have proved to be the only material which was actually developed and scaled up to a practical level and can be utilized for various applications. During 30 years of CC development several alternative approaches were proposed, introduced and examined by different companies worldwide. At present the most advanced CC are manufacturing by combination of ion beam assisted deposition with pulsed laser deposition techniques (IBAD/PLD).
In a past 10 years our group, which consists of SuperOx and S-Innovations (both located in Russia) and SuperOx Japan, became the world leader in IBAD/PLD production, customization and application of 2G HTS and recently reached a level of 1000 km per year – the maximal production capacity reported for companies working in this field.
In this presentation we introduce the new SuperOx manufacturing facilities mainly focusing on fabrication of coated conductors for 3 different applications: Superconducting fault current limiters (SFCL), superconducting motors for aircrafts and superconducting coils for fusion reactors.
The SuperOx research and development of new 2G HTS products based on combination of various physical, chemical and technical approaches including: chemical substitutions, oxygen nonstoichiometry, artificial pinning centers, ion irradiation and fabrication of multilayers etc. Our record state of the art tapes demonstrating extremely attractive properties, for instance critical current Ic >1000A/12mm (s.f, 77K), engineering current Je >1500A/mm² at 20K-20T.
We carried out statistical analysis of large number of industrially produced tapes and systematic examination of various sample batches by XRD, SEM-EDX, TEM, micro-Raman techniques to identify the processing parameters and microstructural features responsible for superior Ic-B characteristics. Special attention was paid to improve the reproducibility, increase the production yield and finally to reduce the product cost - a key factor which determine the future of coated conductors and potential size of the whole superconductivity market.

The Second Generation (2G) High Temperature Superconducting (HTS) wire is being used in a range of applications including cables, FCL’s, rotating machines and high field magnets. The basic wire used in all these applications is based on a composite architecture consisting of a thin YBCO superconducting layer deposited on one side of a thin, flexible metal substrate. The HTS/substrate composite is typically surrounded by an electrochemically deposited Cu layer or laminated between two thin metal strips. This basic composite architecture has been virtually unchanged since the initial development of the 2G HTS wire technology. AMSC has been examining alternate composite architectures which incorporate two HTS layers bonded to each side of a thin metal foil. This double HTS layer composite can be surrounded by an electroplated Cu layer or laminated between two metal strips, resulting in a wire package nearly indistinguishable from the today’s typical HTS wire, but with up to twice the critical current.
Pinning enhancement in today’s state-of-the-art 2G HTS wire is typically achieved through the incorporation of rare earth nanoparticles or columnar structures based on materials such as BaZrO₄. Although both approaches are successful in enhancing the pinning over a range of temperatures and magnetic fields, slight variations on the chemical composition or structure can result in variations in the pinning enhancement between wires or even along the length of a wire. AMSC has been examining an alternate approach which significantly enhances pinning and is highly reproducible. The approach is based on the introduction of a uniform defect structure generated during a reel-to-reel ion irradiation of a fully processed YBCO layer by ions such as Au⁵⁺. By controlling the irradiation parameters, the pinning enhancement can be tuned for specific temperatures and magnetic fields.
In this presentation, we will update progress on the development of the novel wire architectures and pinning enhancements under development at AMSC.
This work is supported by EERE under contract DE-EE0007870.

Investigations carried out up to now on superconducting Maglev trains have always planned to use YBCO or REBCO oxides as superconducting materials^1,2. While MgB₂ enters more and more applications³, bulk MgB₂ has not yet been seriously considered for Maglev projects. Its main drawback is its T_c that is low as compared to that of HTS oxides. As a consequence, the cooling costs are assumed to be much higher, although cryo-coolers able to provide a high cooling power at 20K are now available⁴. Bulk MgB₂ presents, however, advantages that could offset this inconvenient. Magnesium and Boron are abundant and available worldwide and MgB₂ is a light, low cost and high hardness material. The fabrication process of MgB₂ bulks by Spark Plasma Sintering-SPS⁵ is simpler, takes much less time and is much less expensive than the techniques used for the growth of YBCO or REBCO bulks⁶. In addition, MgB₂ bulks can be made with a priori no limitation on their dimensions and shape.
The non-conventional SPS process is based on the combination of a high current and a mechanical high pressure applied directly to the powder material. This technique presents the advantage to provide dense materials in a few minutes while mastering their microstructure. A lot of works have been reported since the introduction of SPS in research laboratories, and many groups have tried to understand the densification mechanisms involved. After a brief history of the SPS technique, the characteristics of this process will be detailed and compared to those of the other sintering techniques.
The densification, and the functionnal properties of superconducting MgB₂ cryo-magnets sintered by SPS will be discussed. Levitation forces up to 400 N at 25 K measured on a 70 mm diameter MgB₂ disc will be reported. The effect of the thickness and the diameter of cylindrical superconductors on the levitation force and the condition for stability in superconducting magnetic levitation will be also analysed.
References
¹G. G. Sotelo, D. H. N. Dias, E.S. Motta, F.Sass, et al., Physics Procedia 36 (2012) 943 – 947
²F N Werfel, U Floegel-Delor, R Rothfeld, T Riedel, et al., Supercond. Sci. Technol. 25 (2012) 014007
³Yukikazu Iwasa, Physica C 445–448 (2006) 1088–1094
⁴ https://www.cryomech.com/products/
⁵J. G. Noudem, M. Aburras, P. Bernstein, X. Chaud, M. Muralidhar and M. Murakami, J. Appl. Phys. 116, 163916 (2014).
⁶X. Chaud, S. Meslin, J-G. Noudem, C. Harnois, et al., Journal of Crystal Growth 275 (2005) 855-860.

Vortex dynamics is a fundamental issue both, concerning superconducting applications at the macroscopic level (wires, magnets, …) ant at the small scale (superconducting microelectronics). Generally speaking, the basic conceptual scheme where the theory of vortex interactions is established, has been mainly associated to the configuration with the electric current density locally oriented perpendicular to the magnetic field. Thus, the physical framework that quantifies the appearance of resistive states builds upon the concept of a ``Lorentz-like’’ drag force on the vortex lines.
In this work, thin nanometric layers of YBCO deposited by pulsed laser deposition (PLD) have been microstructured by means of optical lithography as microwires of 4um width, 1mm length and from 60 to 100nm thick. Isothermal magnetoresistance curves have been measured at low magnetic fields from 0 to 0.8 T for different configurations depending on the orientation between applied magnetic field, current density and sample dimensions at temperatures close to the critical value (). According to our observations, in these conditions, dissipation may be related to the flux dynamics through the interplay of: (i) conventional drag forces, (ii) thermoelectric effects, (iii) pinning by structural defects, and (iv) dimensional effects induced by flux quantization and the size of the sample. The enhancement of one phenomenon or another may be tuned through the relative orientation of field, current density, and sample. In particular, the experimental magnetoresistance curves show oscillations (figure 1) which can be related to the entrance of individual vortex arrays, as shown by their periodicity in terms of characteristic fields. This shows that microstructuration acquires a fundamental role for the investigation of vortex physics.

A detailed understanding of vortex pinning and dynamics is imperative for technological applications of type II superconductors. In addition, it is of fundamental interest to explore vortex solids at elevated fields only accessible in pulsed magnets. One particularly powerful approach to probe vortex physics is through nonlinear electrical transport measurements. However, such characterization in high-temperature superconductors requires high magnetic fields owing to the large critical fields of these materials. Pulsed-field magnets are often used to access the magnetic fields (H) necessary to study high-temperature superconductors, but their short pulse durations (~50 ms) present technical complications for nonlinear current measurements and their large dH/dt (~1000 T/s) values raise questions about the vortex dynamics regime. In this talk, we use the recently developed non-destructive, nonlinear electrical transport capabilities in pulsed fields [1] to explore critical current and critical fields in YBa₂Cu₃O_7-x thin films with different, artificial pinning centers, and demonstrate the latest advances in this technique. We also explore the influence of the irreversibility line on critical currents at low temperatures and in fields up to 60 T.

[1] M. Leroux, F.F. Balakirev, M. Miura, K. Agatsuma, L. Civale, and B. Maiorov, Phys. Rev. Appl. 11, 054005 (2019).

In recent years, the impressive performance improvement and commercial production capability increase of Rare-Earth Barium-Copper Oxides (REBCO) High-Temperature Superconductor (HTS) tapes is triggering a great interest in the perspective of applications for extremely high magnetic field generation as in nuclear fusion and high-energy physics sectors. In particular, for nuclear fusion, since the ‘50s proposed as one of the most exciting energy sources due to its nominally unlimited, efficient, and eco-sustainable characters, HTS based technologies can play a revolutionary role. The concrete perspective of high-performing HTS conductors operating in a wide temperature range and extremely high fields, inaccessible by other traditional Nb-based superconductors, is promoting new concepts of plasma-magnetic confined fusion devices to emerge. It is generally recognized that the commercial exploitation of fusion energy will only be feasible if the magnetic system will use a technology based on HTS materials. Hence, the development of a suitable HTS magnet technology is a key step towards the successful achievement of these goals.
In this presentation, the current status of the HTS conductor development for fusion applications will be firstly reviewed and discussed taking into account the conductors developed for both large (DEMO-like) and compact (SPARC) tokamak machines and other concepts of fusion devices [1, 2]. Then, a high current REBCO Cable-In-Conduit (CIC) conductor, conceived for large fusion magnet applications based on aluminum slotted-core incorporating a number of HTS tape stacks, will be presented in detail. The conductor layout, designed aiming at the industrial feasibility of the manufacturing process, has shown promising electrical, thermo-hydraulic, and mechanical properties assessed in several experimental studies of cable samples. More recently, the thermal stability of such a cable has been addressed in dedicated experiments for the investigations of quench phenomena. The designing and the manufacturing of CIC conductor samples (targeting more than 15 kA at 11 T at 4.2 K), aimed at the investigations of thermal stability and quench behavior under fusion relevant conditions to be performed at the SULTAN facility (Swiss Plasma Center), will be presented. The development of a new HTS CIC conductor targeting 60 kA at 18 T at 4.2 K will be also described and the first experimental test campaigns will be presented. The perspectives of this CIC conductor for DEMO central solenoid insert coil applications as well as for DTT (Divertor Tokamak Test facility) [3] applications will be at the end drawn and discussed.
References
[1] K. Sedlak, et al., Advance in the conceptual design of the European DEMO magnet system, Supercond. Sci. Technol. 33 (2020) 044013 (9pp).
[2] Z. S Hartwig, et al., VIPER: an industrially scalable high-current high temperature superconductor cable, Supercond. Sci. Technol. 33 (2020) 11LT01 (8pp).
[3] L. Giannini, et al., Conceptual Design studies of an HTS insert for the DTT Central Solenoid, presented at EUCAS 2021 (virtual conference September 5^th – 9^th).

To achieve improved accelerating gradients operating above the temperature of liquid helium, efforts are underway to coat Nb superconducting radiofrequency cavities with high-quality Nb₃Sn films. The enhanced performance of Nb₃Sn coated cavities is contingent upon the growth of smooth, homogeneous A15 Nb₃Sn grains. Nb-Sn growth studies probe the interplay between the underlying Nb oxide morphology, Sn coverage, and substrate heating conditions on Sn wettability, intermediate surface phases, and Nb₃Sn grain growth dynamics. Using a well-characterized (3×1)-O/Nb(100) single crystal substrate, Sn was deposited with sub-monolayer precision and the Sn/Nb interface was analyzed and heated in situ. Scanning tunneling microscopy and spectroscopy data detail thermally induced Sn diffusion behavior leading to the formation of electronically and structurally distinct Sn adlayers. Photoelectron spectroscopy data further elucidate the impact of growth conditions on the near-surface intermetallic binding motifs with oxygen. This experimental work, supported by concomitant adsorption energy calculations of Sn adatoms sites on the (3×1)-O, detail morphological and electronic consequences of the surface-mediated intermetallic dynamics driving optimal Nb₃Sn formation.

Superconducting radio-frequency (SRF) cavities, the key component in particle accelerators, require alternative materials and nanostructured surfaces beyond niobium. This would open new opportunities for high electric field, low surface resistance, and high operation temperature for a wide range of future accelerator applications. At Cornell University, we design and fabricate active niobium surfaces through alloying, doping, and oxide control inspired by theory.

First, we demonstrated Nb₃Sn films with low surface roughness and pure stoichiometry (two critical parameters for SRF cavities) through an electrochemical method both in sample studies and at cavity scale. We will report the chemical, structural, and superconducting RF properties of the electrochemically deposited Nb₃Sn films together with the growth mechanism analysis. Also, we will compare these results with the state-of-the-art vapor diffused films.

Second, we designed an inhomogeneous layer within the field penetration depth through doping of the niobium surface. Our theory collaborators at the Center for Bright Beams have demonstrated a significant improvement of superheating field via this doping method. We have achieved desirable doping profiles and promising DC superconducting properties on the sample scale using electrochemical and physical vapor methods. We will report the doping profiles and phase transformation under different doping conditions, and expect to obtain their corresponding RF results by the time of the conference.

Third, we investigated the surface oxide structures on niobium in order to generate an approach to control the surface nanostructure inspired by [T. Kubo and A. Gurevich, 2019]. We will discuss updates to our recent study on the oxide structural analysis under different processing conditions.

Lastly, it is challenging to achieve high quality films and surfaces on large-scale, intricately-structured cavity devices. To do this, we developed customized electrochemical, chemical vapor, and physical vapor deposition systems. The deposition system development will be covered throughout the presentation.

In the last decade, rapid advances in the materials science of niobium surfaces have pushed the performance of superconducting radio-frequency (SRF) cavities far beyond what were thought to be the limits of this humble elemental superconductor. At the same time, an improving fundamental understanding of SRF has made clear that further advances are possible. We present theoretical results on new “surfaces by design” under active development by our collaborators: using density-functional theory (DFT) and time-dependent Ginzburg-Landau (TDGL) theory, we investigate novel doping treatments which can alter the electronic structure of the surface, inhibit the development of weakly-superconducting phases, or promote the growth of new phases with enhanced superconducting properties.

Our two-pronged theoretical approach explores firstly the dependence of fundamental properties like Fermi-level density of states, electron-phonon coupling strength, and elastic constants on the composition and structure of niobium-based materials of interest, and secondly the effect of processing conditions such as temperature and chemical potential of different species on the kinetics and thermodynamics that determine the ultimate structure of the surface. Specifically, we employ Wannier function techniques to precisely characterize the electronic states at the Fermi level and smoothly integrate over reciprocal space to determine scattering amplitudes for electron-phonon and electron-defect scattering processes. These fundamental material properties determine phase stability, superconducting properties and, as shown in TDGL simulations, SRF performance.

In addition to inspiring new sample preparation recipes, we show encouraging agreement between our calculations and experimental measurements, in particular explaining the significantly improved properties of our sample coupons relative to similar samples documented in the literature. As applications for SRF cavities continue to expand, from a growing demand for electron beams to the construction of large particle accelerators to exotic uses in particle detection and quantum computing, a richer understanding of niobium-based superconductors and how to optimize their properties for SRF stands to pay great dividends.

The tremendous computation advantages promised by quantum processors are increasingly of interest due to the necessity of simulating complex quantum systems. One of the main issues for practical implementation of fault tolerant quantum processors based on superconducting qubits is the quantum decoherence processes emerging from materials-inherent defects. These impurities couple with microwave fields in quantum circuitry and result in loss of information by noise and dephasing. The origin of these defects is yet to be understood, posing great urgency with the rise of quantum information science. Dielectric losses introduced by formation of native oxide on Al and Nb are hypothesized to be one of the most predominant decoherence sources. In addition, thin films of Nb and Al with non-uniform thicknesses or impurities induce variations in the superconducting gap that are prone to trapping dissipative quasiparticles, breaking cooper pairs and therefore inducing decoherence in quantum states. Nitride-based compounds such as TiN and NbN with transition temperatures above 10K ideal for the replacement of archetypal Nb and Al to mitigate these effects in superconducting qubits.
Plasma assisted atomic layer deposition is potentially a powerful growth method these nitrides, their alloys, and multilayers with excellent control of composition and uniformity providing a playground for material and device design for superconducting qubit platforms. The caveat for this process is potential variations in the growth phase which depends on the substrate crystallographic orientation, voltage bias, and temperature of the deposition technique. In addition, residual AC resistivity induced by oxygen incorporation during material growth can be critical. This is especially significant with quartz or sapphire plasma tubes which self-etch during long material growth process, introducing oxygen (~3%) into the nitrides. In this work, we address this problem by developing thermal ALD processes for NbN using hydrazine (N₂H₄) for nitridation. Sequential exposures of anhydrous N₂H₄ and the organometallic Nb precursor t-Butylimido tris(diethylamino) niobium(V) (TBTDEN) were performed at various process temperatures and conditions. Optimal conditions were found at 300C with a growth rate of 0.15 A per ALD cycle. XPS analysis indicates stoichiometric NbN with minimal oxygen content in comparison to plasma assisted ALD process. STEM-EELS analysis shows polycrystalline structure with grains up to 5nm and smooth interfaces with SiO₂. The bulk resistivity of 1.5 μΩ-m is calculated from 17 nm thick NbN films which presents transition temperature comparable to Nb ideal for superconducting qubits.

Doped Fe-based superconductors show broken-symmetry phases coexisting with superconductivity, and determined by structural distortions of the local tetrahedral geometry. The tetragonal-to-orthorhombic phase transition of the parent materials however make these structural deformations too complex to be predictably controlled by chemical doping or hydrostatic pressure in bulk systems. We demonstrate a systematic approach to manipulate the local structural configurations in epitaxial thin films of the nominally non-superconducting parent material BaFe₂As₂ by controlling two independent structural factors: orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance). We generate and tune superconductivity without chemical doping by using these to control the local tetrahedral coordination in designed thin film heterostructures with substrate clamping and bi-axial strain. We further show that this allows control the structural phase transition, associated magnetism, and superconductivity in the parent material BaFe₂As₂. This approach will advance the development of tunable thin film superconductors in reduced dimension.
This work has been done in collaboration with J. H. Kang, P. J. Ryan, J.W. Kim, J. Schad, J. P. Podkaminer, N. Campbell, J. Suttle, T. H. Kim, L. Luo, D. Cheng, Y. G. Collantes, E. E. Hellstrom, J. Wang, R. McDermott, M. S. Rzchowski.
This work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, (DE-FG02-06ER46327).

A challenging material for high magnetic field applications has to face a high current-carrying capability as well as a very low anisotropy. So far this was a motivation in favor of YBCO that is known as a 3D superconductor with a γ factor lower than 10 and critical current density above some MA/cm². The race of Iron-Based Superconductors is driven by anisotropy, dimensionality and material defects acting as pinning centers. Our experience tells us that the Fe(Se,Te) chalcogenide superconductor can compete with High Temperature Superconductors due to its rather simple crystal structure, huge upper critical fields with respect to LTS, lower anisotropy compared to HTS. By the way, the vortex physics in the mixed state of this compound is rich of phenomena [1], with a surprising anisotropy in the flux pinning energy [2] and the observation of Flux Flow Instability induced by an electronic mechanism at very high bias currents [3,4]. We also present a pioneering behavior of the upper critical field that is accounted for by Pauli paramagnetic and orbital pair-breaking effects with a non-zero spin-orbit coupling emerging from the angular dependence of the H-T phase diagram. Beyond the very low anisotropy of Fe(Se,Te) superconductor and its multiband nature, the additional degree of freedom in our angular measurements shows emerging anisotropic trends of flux pinning energy U₀(H,T,θ), irreversibility field H_irr(H,T,θ) and critical current density J_c(H,T,θ) as well. A deep investigation of Fe(Se,Te) microstructure at nanoscale reveals indeed extended defects elongated preferentially parallel to ab plane [5], thus mimicking the layered anisotropy usually observed in YBCO. Nevertheless, by looking at anisotropy, dimensionality and pinning, Fe(Se,Te) films grown on CaF₂ substrate are able to struggle against YBCO films, particularly at high magnetic fields, those relevant for practical applications.
[1] A. Galluzzi et al., Sci. Rep. 11, 7247 (2021)
[2] M. R. Khan et al., Materials 14, 5289 (2021)
[3] G. Grimaldi et al., Sci. Rep. 8, 4150 (2018)
[4] A. Leo et al., Supercond. Sci. Technol. 33, 104005 (2020)
[5] M. Scuderi et al., Sci. Rep. 11, 20100 (2021)
Acknowledgments
Work partially supported by MIUR-PRIN project "HiBiSCUS" - grant no. 201785KWLE and PON Research and Competitiveness 2007-2013 under grant agreement PON NAFASSY, PONa3_00007

Iron-based superconductors (IBS) with critical temperatures up to 55 K, nearly isotropic upper critical field exceeding 50 T, high critical current density, can represent a breakthrough to overcome the limits of low-T_c Nb₃Sn and the extreme material complexity of cuprate high-T_c superconductors (HTS), which leads to articulated and expensive processes for conductor fabrication.
We are developing new fabrication processes for Coated Conductors (CCs) based on Fe(SeTe), relying on the advanced technologies developed in the last decades for HTS-CCs, but focusing on the possibility of simplifying the template to drastically reduce fabrication costs and times. Fe(Se,Te) can be grown as an epitaxial film on a metallic oriented substrate with few and loose requirements on the template microstructure, that allow for the design of a simplified CC architecture. This design requires the use of a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template [1]. In this work, 5% Zr-doped CeO₂ (CZO) based buffer layers are prepared on single crystals via metal organic decomposition (MOD) and polymer assisted deposition (PAD). Fe(Se,Te) films are deposited on these templates via laser deposition, and excellent samples are obtained on a Fe(Se,Te) seed layer, used to favour chemical matching with the buffer [2]. We then focused on the possibility to deposit such buffer layers on home-made biaxially textured Fe-Ni substrate [3], in order to develop a completely original template to fabricate Fe(Se,Te) CCs.
A detailed analysis and correlation of the transport properties and microstructure of the thin films are carried out in order to understand the flux pinning mechanisms in such systems [4].
[1] G. Sylva et al., Supercond. Sci. Technol. 33 (2020) 114002.
[2] A. Vannozzi et al., Supercond. Sci. Technol. 33 (2020) 084004.
[3] G. Sylva et al., Supercond. Sci. Technol. 32 (2019) 084006.
[4] M. Scuderi et al., Sci. Rep. 11 20100 (2021)
This work was partially supported by MIUR-PRIN Project “HIBiSCUS” under Grant 201785KWLE

The ability of bulk superconducting materials to generate higher magnetic fields than permanent magnets enables their potential application in portable, medium-field magnet systems. Both MgB₂ and (RE)BCO are promising materials for these applications. MgB₂ has a reasonably high transition temperature (T_c = 39 K) and can be fabricated in bulk form by simple and cheap powder processing methods. However, the superconducting performance of MgB₂, especially its macroscopic critical current density (J_c), requires further improvement in order to provide a suitable magnetic field for practical applications. On the other hand, (RE)BCO has much better high field properties, and can ultimately trap much higher fields, but bulk pellets need to be processed as single grains to avoid grain boundaries that are weak links. This means that a slow melt-processing method is required that is less readily scalable to larger samples and mass production. Here, we will report a summary of recent work on MgB₂ and (RE)BCO bulks that has resulted from a collaboration between Oxford and Cambridge Universities, particularly focusing on understanding how microstructure can be controlled to improve performance in these materials.
Regarding MgB₂ bulks, we have investigated a variety of novel strategies for improving the macroscopic J_c in ex-situ MgB₂ bulks processed by Field Assisted Sintering Technology and ultra high pressure synthesis. The work focuses on two elements: (i) increasing the connectivity between MgB₂ particles, and (ii) improving the intrinsic J_c of the MgB₂ by optimizing the pinning landscape. The first of these traditionally requires the use of higher processing temperatures that results in the coarsening of the microstructure and a decrease in intrinsic J_c. Therefore, we have developed a new method for improving connectivity at relatively low temperatures. In addition, we have explored the effects of a variety of additions, such as Y₂O₃, hBN and cBN together with the high energy ball-milling process on the microstructure and superconducting properties of MgB₂.
We will also report recent work on understanding the microstructural evolution in the melt-growth process used to fabricate high performance (RE)BCO bulks. In particular, the choice of the RE element plays a key role in determining the growth rate of single grains, the precise microstructure, mechanical properties and final superconducting properties of the bulk samples, and also the likelihood of RE substitution onto the Ba site which can degrade the performance. A number of rare elements including Nd, Sm, Eu, Gd, Dy and Y have been incorporated into (RE)BCO single crystals, and the superconducting properties of the bulks carefully studied, and it is known that the final microstructure varies with the RE element as a result of changes in the melting temperatures of the RE–123 phase and the resulting growth rates. In this work, we have studied the growth and microstructure of (RE)BCO single crystals with RE = Gd and Eu, where the degree of Ba substitution is known to be very different. We have carried out detailed microstructural characterization of the phase distribution and composition using high resolution electron microscopy to understand the effects of Gd and Eu on the uniformity of the samples, the distribution of the secondary 211 phase, porosity and chemical variations in different regions of the melt-grown single crystals. In addition to discussing processing innovations to improve uniformity and increase the size of the bulks, we will report on new experiments using Atom Probe Tomography aimed at understanding the role of grain refiners used in the melt-processing of (RE)BCO bulks.

Among high temperature superconductors (HTS), YBa₂Cu₃O_7-δ (YBCO) based thin films and coated conductors (CCs) offer an unparalleled opportunity to be used in large-scale superconducting power applications and high field magnets due to their outstanding ability to carry high critical currents at high magnetic fields. In an essential need of high-performance and low-cost manufacturing, chemical-solution deposition (CSD) has become a very competitive cost-effective and scalable methodology to grow epitaxial YBCO films [1]. However, their growth rates are rather slow (0.5-1 nm/s). For this purpose, we have developed a novel growth approach, entitled, Transient Liquid Assisted Growth (TLAG) [2], which is able to combine CSD methodologies with ultra-fast growth rates (100-1000 nm/s) by facilitating a non-equilibrium liquid-mediated approach. Critical current densities up to 5 MA/cm² at 77K are already realized in TLAG-CSD grown thin films, but in order to further improve the current carrying properties, understanding of initial nano phases in pyrolysis process and fine tuning of growth parameters are essential to define a robust process, including thicker layers. Therefore, the microstructure of multi-deposited pyrolyzed and grown YBCO films, investigated via high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX), will be presented.
Furthermore, the critical current density 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 metal-organic initial inks [2]. Therefore, using aberration corrected STEM combined with high angle annular dark field (HAADF) and EELS, the detailed microstructure of grown TLAG-CSD YBCO films, nanocomposites, and coated conductors, with a focus on new defects landscape at the atomic level, secondary phases, and strain effects, will be presented. Finally, the recent progress in TLAG-CSD films and nanocomposites, in terms of microstructure correlation with growth mechanisms and growth rate will be discussed.
[1] J. Gutierrez, A. et al., Nat. Mat., 6, 367 (2007)
[2] L. Soler et al, Nat. Commun., 11, 344 (2020)

The climate crisis due to the constant evolution of global warming is nowadays a concerning reality, making the development of alternative energy sources together with efficient energy transport of vital importance. Undoubtedly, the discovery of high temperature superconductors (HTS) proved to be an appealing solution to notably enhance clean electrical energy, but the high production costs of the currently available technology are keeping it from its worldwide spread. Chemical solution deposition (CSD) techniques implemented towards the production of nanostructured epitaxial cuprate superconductors have demonstrated the feasibility for a low cost and high throughput method. Thus, we present a versatile and tunable CSD method suitable for the preparation of high performance epitaxial YBa₂Cu₃O₇ superconducting films. By concentrating our focus on fluorine-free solutions, not only we are meeting the global requirement for greener chemical processes compared to the renowned TFA route, but at the same time we are able to achieve ultrafast growth rates beyond 100 nm/s by using transient liquids during the epitaxial growth of the superconducting layer in the innovative framework of TLAG (transient liquid assisted growth), a high-throughput scalable process [1], making it interesting for industrial applications. Hence, solutions of different Ba/Cu molar ratio are prepared with the aim of modifying the supersaturation and characteristics of the transient liquid, using a facile and fast method from home-synthesized Y, Ba, and Cu propionates precursors, reaching endured solution stability, thickness tunability and high homogeneity. Employing in-situ XRD synchrotron radiation and HRTEM/STEM, the distinct liquid compositions provide us with a broad comprehension of the TLAG process opportunities, kinetic phase diagram, and a strong correlation between superconducting performance properties and unique characteristics of each liquid composition, easily tunable through solution chemistry.

[1] Soler, L., Jareño, J., Banchewski, J. et al. Ultrafast transient liquid assisted growth of high current density superconducting films. Nat Commun 11, 344 (2020). https://doi.org/10.1038/s41467-019-13791-1

The growth of Chemical Solution Deposition (CSD) nanocomposites from colloidal precursor solutions containing preformed nanoparticles has recently proven to be a very good pathway developing low cost YBa₂Cu₃O_7-δ (YBCO) superconducting layers carrying high currents at high magnetic fields by pinning vortices at none superconducting nanoscale defects [1][2].
Metal oxide nanoparticles such as BaMO₃ (M= Zr and Hf) and BaM₂O₆ (M= Nb and Ta) are the most interesting choices due to their compatibility with YBCO precursor solutions and growth. Using the H2S2 hybrid method, which combines aqueous sol-gel and solvothermal approaches, we have synthesized small-sized, crystalline, monodisperse and stable BaMO₃ NPs. We are able to tune their size in a range of 4 - 20 nm, controlling the hydrolysis step involved in the reaction mechanism [3]. Moreover, small-sized (4-6 nm) and agglomeration-free BaM₂O₆ NPs have been obtained, for the first time, using a surfactant-free solvothermal method and a post-synthetic surface functionalization.
We have used the non-equilibrium ultrafast Transient Liquid Assisted Growth (TLAG-CSD) method to grow YBCO nanocomposite films with the above preformed nanoparticles. We have developed a fluorine-free colloidal YBCO precursor solution adapted to TLAG-CSD nanocomposites able to achieve very homogenous and reproducible multideposited films. We have demonstrated the compatibility and stabilization of preformed NPs in the YBCO precursor solution, obtaining homogenous pyrolyzed films with crystalline NPs, homogenously distributed without coarsening. Finally, we have further proved the compatibility and chemical/thermal stability of NPs with the TLAG approach [4], and determined the parameters to achieve epitaxial YBCO multideposited TLAG nanocomposites obtaining the first promising Jc results of 2 MA/cm² at 77 k for 400 nm of thickness.
[1] A. Llordés et al., Nat Mater. 6, 2012, 329-36
[2] Z. Li et al., Scientific Reports 9, 2019, 5828
[3] N. Chamorro et eal., RSC Adv. 10, 2020, 28872-28878
[4] L. Soler et al., Nat Commun. 11, 2020, 344

Superconductivity presents a huge interest worldwide for its opportunities for cleantech energy as superconductors allow transporting electric power with minimal losses. However, the widespread use of high temperature superconductors in large scale applications is limited by the high cost/performance ratio of existing manufacturing processes (1/3 of the final cost of the device is the price of the superconductor). We propose a new solution which consists of an innovative method for manufacturing High Temperature Superconductors Coated Conductors (CC). This method is based on YBCO thin films which are produced via a Transient Liquid Assisted Growth (TLAG) process combined with Chemical Solution Deposition (CSD). While CSD is a relatively mature and low cost process, TLAG technology is disruptive combining low cost Chemical Solution Deposition (CSD), high throughput and ultrafast growth Transient Liquid Assisted Growth (TLAG) process and high performance (nanocomposites). All together allows a commercially and industry scalable manufacturing process for 2G High Temperature Superconductors which can revolutionize superconductivity sector.
TLAG-CSD enables ultrafast growth rates of YBCO films (up to1000 nm/s) thanks to the kinetic preference of Ba-Cu-O to form transient liquids prior to crystalline thermodynamic equilibrium phases. The process enables HTS films fabrication with high critical current densities (5MA/cm² at 77K) [1].This presentation is devoted to the technology transfer of TLAG-CSD from single crystal substrates to metallic tapes. Characterizations by XRD, SEM, TEM, Optical microscopy, T_c, J_c and Hall microscopy have enabled us to determine the parameters to be modified and optimized. Liquid reactivity could be avoided with the use of a manganite buffer layer. CSD deposition by spin coating has been used to prepare 5 x 5 mm² samples on metallic substrates while slot die coating was used for the 3-15 cm tapes. In addition, nanocomposites have been prepared from precursor solutions with stabilized preformed nanoparticles (BaHfO₃) for enhanced superconductor performance at high magnetic fields.
[1] Nature Communications article: 11, 344 (2020). https://doi.org/10.1038/s41467-019-13791-1
We acknowledge funding from EU-ERC_AdG-2014-669504 ULTRASUPERTAPE and EU-PoC-2020-IMPACT projects, and the Excellence Program Severo Ochoa SEV2015-0496

Superconducting materials find valuable application in the fabrication of magnetic shields [1]. In this regard, MgB₂ has been proved to be a very promising option [2]. Moreover, it has been shown how suitable simulation tools guiding the optimization process can be a successful approach to produce devices with high shielding ability [3].
This work focuses on the exploitation of a 3D modeling procedure based on a vector potential formulation [4] implemented by means of the finite-element software COMSOL Multiphysics® [5]. By this approach, we calculated the shielding properties of tube- and cup-shaped MgB₂ samples with a height/diameter aspect ratio close to unity.
The shielding efficiency of these layouts was analysed in both the axial-field (AF) and the transverse-field (TF) orientations, studying the effect of superimposing ferromagnetic vessels of different sizes around the superconductor [3, 6]. Then, focusing on the most efficient hybrid arrangements, we investigated their shielding performances for different angles of the applied magnetic field and compared the results obtained on the hybrid layouts with the corresponding superconducting configuration.
Finally, a multiphysics analysis is presented as a preliminary modelling approach to predict the flux jumps phenomena that were observed during the experimental measurements [7].
References
[1] K.Hogan et al., Supercond. Sci. Technol., 31, 015001 (2018).
[2] D.Barna et al., IEEE Trans. Appl. Supercond., 29, 4101310 (2019).
[3] COMSOL Multiphysics®5.4 software (http://www.comsol.com).
[4] L. Gozzelino, M. Fracasso et al., Supercond. Sci. Technol. Accepted (2021).
[5] L. Gozzelino et al., Supercond. Sci. Technol., 33, 044018 (2020).
[6] L. Gozzelino, M. Fracasso et al., Supercond. Sci. Technol. Accepted (2021).
[7] L. Gozzelino et al., Supercond. Sci. Technol., 33, 044018 (2020).

In numerical models of high-temperature superconductor (HTS) applications, the electrical behavior of the superconductor material is often modeled as a material with a nonlinear resistivity, which is derived from a power-law E-J constitutive law. In that case, the critical current density J_c is determined as the value corresponding to a threshold electrical field, usually set equal to 1 μV/cm for HTS. This approach has proved its reliability for simulating phenomena such as the AC losses caused by time-varying magnetic fields, created by transport currents, externally applied magnetic fields, or a combination of the two. The reason of such success is that, in those conditions, the currents flowing in the superconductor are very close to the critical current density J_c. There are however situations where this condition is not met: on one side, when the current flowing in the superconductor exceeds its current-carrying capability – a typical case is represented by fault current limiters; on the other side, when the excitation is small or when relaxation phenomena are involved – for example in certain magnetization processes. In all these cases, the use of the power-law can become questionable and alternative E-J constitutive laws might be preferable.
In this work, we present the result of electrical characterizations aiming at exploring the behavior of the superconductor material at electric fields considerably higher or lower than 1 μV/cm. For over-critical characterization, we use a fast pulsed current measurement system that allows injecting high currents in the superconducting sample on very short time scales. The E-J constitutive law of the HTS material is then extracted from the experimental data by means of an electro-thermal numerical model. For under-critical characterization, we measure the voltage-current characteristic with the usual four-point technique on long wire samples, by placing the voltage taps several meters apart. Both types of experiments are performed by placing the samples in liquid nitrogen at atmospheric pressure.
We use the so-determined alternative E-J constitutive laws in finite-element simulations of HTS applications. We compare the results with those obtained by using the power-law and discuss the reasons behind the observed differences.

I discuss ways of decreasing the power losses in thick superconducting films under strong radio-frequency (rf) field by optimizing pinning of vortices, tuning the thickness of a suboxide metallic later at the surface, and by multilayer nanostructuring. The surface resistance R_s(T) in the Meissner state can be optimized and even reduced below its value for an ideal surface by engineering the quasiparticle density of states at the surface using pairbreaking mechanisms, for instance, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal oxide layer and its contact resistance. These issues can determine the behavior of R_s(T) at low temperatures and the residual surface resistance which controls the lower limit of rf losses in superconductors at T<< T_c. I also discuss recent numerical simulations of vortices driven by strong rf field in a film with randomly - distributed pinning centers which have shown complex dependencies of R_s(ω,H) on frequency ω and the field amplitude H, particularly, a non-monotonic field dependence of R_s(H). The ways of engineering an optimal density of states by surface nano-structuring and impurities by materials treatments to reduce losses in superconducting micro-resonators, thin film striplines, and rf resonant cavities for particle accelerators are addressed.

Disorder is a powerful approach to elicit and control superconducting properties, as evidenced from record Tc's, quantum phase transitions and exotic quasiparticles, predicted or evidenced in the presence of disorder. Here we will categorize disorder in notoriously heterogeneous scanning tunneling microscopy (STM) images of unconventional superconductors. From the information centric point of view, understanding the structure of these datasets amounts to effective and physically meaningful data compression. We will discuss compressibility of various kinds of STM data setting up the context for techniques of compressive sensing, machine learning and information theory as a way to understand the results of the experiments and improve data acquisition. Similarity learning emerges as a consistent strategy to categorize disorder with minimum prior knowledge. We also apply disorder analysis to identify Josephson and Andreev currents in on the atomic scale. Research sponsored by Materials Science and Engineering Division, US DOE and Center for Nanophase Materials Sciences, Oak Ridge National Lab, a DOE Office of Science user facility.
1. P. Maksymovych, J. Yang, B. C. Sales, J. Wang, arxiv:2106.13691
2. B. Lerner, A. Flores-Garibay, B. J. Lawrie, P. Maksymovych, Phys. Rev. Research 3 (2021) 43040
3. W. Ko, E. Dumitrescu, P. Maksymovych, Phys. Rev. Research 3 (2021) 033248

Being the only high-T_c superconductor available in multifilamentary round wires with multiple architectures, Bi-2212 is a very promising conductor for the realization of high-field applications, like accelerator, NMR, and research magnets. In fact, solenoid magnets, racetracks made with Rutherford cable, and canted-cosine-theta coils have been already demonstrated. Despite their relatively simple wire fabrication by the powder-in-tube technique, Bi-2212 wires require a tightly controlled overpressure heat treatment with a multi-parameter schedule to achieve high J_c. Changes in the wire design, wire diameter, powder quality, and processing conditions can lead to strong variations in both the microstructure and the superconducting performance. Particularly noticeable are the variations in filament bridging, previously observed for instance by J. Jiang et al. [1] and by T.A. Oloye et al. [2], which inevitably affects J_c as well as the AC losses. In this presentation, we will focus on how, using conventional magnetic characterizations, we can gain valuable insights to estimate for instance the variation of the effective filament diameter, the level of bridging and inter/intragrain properties in wires that underwent different processing. These results will be correlated to transport properties and microstructures in order to obtain a deeper understanding on the cause of performance variation.
[1] J. Jiang et al. IEEE trans. Appl. Supercond. 31(5), 6400206 (2021).
[2] T.A. Oloye et al., in preparation.
Acknowledgement
This work was supported by the U.S. DOE Office of High Energy Physics under Grant DE-SC0010421, by the NHMFL NSF under Award DMR-1644779, and by the State of Florida, and is performed under the purview of the U.S. Magnet Development Program (MDP).

Soft matter enabled quantum materials constitute an emergent area of research bringing together the fields of soft and hard condensed matter science. Soft matter enabled superconducting quantum materials in particular hold great scientific as well as technological promise. This is based, respectively, on the observation of substantial modulations of superconductor properties, e.g. as a result of soft matter directed mesostructure formation, as well as on the availability of new form factors enabled via solution processing, e.g. via spin-coating, roll-to-roll (R2R) processing, or additive manufacturing (3D printing), foreign to traditional synthesis approaches often based on ultrahigh vacuum techniques. In this contribution, block copolymer (BCP) self-assembly based approaches to mesostructured superconductors will be discussed. BCP self-assembly, a hallmark of soft condensed matter physics, will be introduced to the formation of type-I and type-II superconductors in the form of metals and nitrides, respectively. Particular focus will be on the formation of three-dimensionally periodic cubic co-continuous structures such as the double gyroid and alternating gyroid, respectively, and how the resulting mesoporous structures substantially alter macroscopic superconductor behavior via modification of intrinsic superconductor materials properties including the Cooper-pair correlation length. Via shimming through various mesostructures while keeping the intrinsic superconductor atomic lattices unaltered, the formation of superconducting metamaterials will be introduced. Extension of materials synthesis approaches from the bulk to thin films is another focus area, allowing integration of superconductor mesostructure formation with microelectronics processing, e.g. including photolithography-based definition of thin film patterns. Results of these studies suggest exciting opportunities for opening up the limited traditional synthesis space of superconductors via the combination of crystalline hard materials formation with solution-based approaches known from the soft matter sciences. In return, this promises a range of novel materials structures and form factors that may lead to advanced superconducting materials properties not accessible via traditional routes, thereby substantially broadening the superconducting materials application space.

A-site doped ABO₂ layered-oxide perovskites have been known to host high temperature superconductivity since the discovery of the cuprates. Compounds with similar characteristics have long been searched for with little progress. The parent three-dimensional perovskite structure offer a range of properties through chemical substitution of A and B site cations with the possibility of facile reduction to ABO₂ compounds through routes like topochemical reduction. Due to the vast number of possible combinations on A and B cations and a host of doping elements, it is impractical to effectively study the reducibility of all the possible compounds. However, since the compounds share a general framework, a rational trend may be expected. Here, we report trends in successive O vacancy formation energies obtained from density functional theory calculations of the pristine and A-site doped double perovskites. Using this information, we are able to build a descriptor for the rational design of layered oxide compounds based on correlation of reduction energetics and the difference in the expected charge state of the B-site cation in pristine and reduced compound. Coupled with predictions of the corresponding magnetic transition temperatures, this work opens the door for the future discoveries of high temperature superconductors.

Vortex pinning should be controlled to enhance critical current density (Jc) in YBa2Cu3O7 (YBCO) films for coated conductor application. Improvement of vortex pinning has been achieved by the nanocomposite structure. Nanorods are one of the most effective pinning centers that improve the Jc especially for the magnetic field parallel to the c-axis. An angular dependence of Jc has been measured in the YBCO nanocomposite films to understand the vortex pinning anisotropy, and a complicated angular dependence of Jc has been observed. The Jc anisotropy is a very important parameter, and the Jc should be improved isotropically. Incorporation of multiple types pinning centers, namely the hybrid pinning is very effective for this purpose. For the design of the hybrid pinning structure, understanding of the Jc angular dependence is needed. Also, the fabrication of the hybrid pinning is required. In this study, the angular dependence of Jc was measured in the YBCO films containing BaMO3(BMO, M=Zr, Sn, Hf) nanorods (YBCO+BMO films). The hybrid pinning structure comprising of the nanorods and the nanoparticles was formed in the YBCO films. The nanorods were self-organized in the pulsed laser deposition using the mixed target, and the nanoparticles were incorporated into the films with the artificial surface modified target method and the self-organized manner.
The c-axis Jc peak and the ab-plane Jc peak were observed in the angular dependence of Jc, but the peak height and width strongly depended on magnetic field and temperature. In addition, an off-axis peak was observed in the moderate magnetic field of 4-7 T at 77 K. While the c-axis Jc peak originated from the nanorod pinning, the ab-plane Jc peak was due to the crystalline anisotropy, the intrinsic pinning, and/or the ab-plane correlated pinning such as stacking faults. When the magnetic field was lower than the matching field, the nanorods were high-density enough to pin all vortices. On the other hand, the nanorods were pinned by the vortex interaction and matric uncorrelated defects. As the magnetic field was tilted from the c-axis, the vortices were pinned by the vacant portion of nanorods, namely the vortex retrapping occurred. The vortex retrapping resulted in the off-axis peak in the moderate magnetic fields. The vortex phase diagram in the YBCO+BMO films will be discussed.
The YBCO films containing BaHfO3(BHO) nanorods and Y2O3 nanoparticles (YBCO+BHO+Y2O3) were fabricated using the mixed target and the surface modified target method. Very high Jc (Fp,max=1.57TN/m3 at 4.2 K) was successfully achieved. The Jc for the magnetic field parallel to the ab-plane was improved by the Y2O3 nanoparticles, demonstrating the hybrid pinning. In the YBCO+ Ba2LuNbO6(BLNO) films, the Y2O3 nanoparticles in addition to the BLNO nanorods were formed only by the mixed YBCO+BLNO target without using the surface modified target method. Thus, the self-organized formation of the hybrid pinning was demonstrated in the YBCO+ BLNO films. The c-axis correlated pinning of the BLNO nanorods was observed in the films, and the Jc was improved also for the magnetic field parallel to the ab-plane. The hybrid pinning design will be discussed by comparing the pinning properties of the hybrid pinning structures.
This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant No. 18H01478).

The recent success in the fabrication of superconducting nickelate thin film has opened a path to understand the origin of unconventional superconductivity in high-temperature cuprates. However, the challenging material growth, complicated strain and interface effects from the substrate, and a complex multiorbital picture of nickelates have obstructed our understanding of important physics questions such as the superconducting gap symmetry. Here we perform in-plane London penetration depth measurement on optimally doped (Nd,Sr)NiO₂ and (La,Ca)NiO₂ to determine the pairing symmetry of the infinite-layer nickelate superconductor family. In contradiction to the cuprate analog picture, our result concretely disproves a dominant d_x²-y²-wave pairing symmetry which was predicted in theoretical calculations while suggesting a dominant s-wave pairing with two gaps of different T_c in nickelates. Such observation cannot be convincingly described by the existing theoretical viewpoints and calculations. Our findings could revolutionize understanding of the role of the copper oxide plane and 3d_x²-y² electronic band in the origin of unconventional superconductivity in cuprates.

Since the discovery of superconductivity in LaFeAs(O,F) in 2008 [1], extensive studies have been performed on iron-based superconductors (IBSs) for exploration of both fundamental physics and applications. One of salient features of IBSs is that they have large upper critical field over 500 kOe at low temperatures [2] and have good potential to replace conventional Nb-based superconductors. To apply superconducting materials into practical use at low temperatures and high fields, good critical current characteristics is indispensable. In single crystalline form, critical current density (J_c) in IBSs at low temperatures exceeds 0.1 MA/cm² [3,4] and it can be enhanced further by introducing artificial defects [5,6]. With such excellent characteristics, developments of superconducting wires and tapes have been conducted. Among various materials systems in IBSs, such as 1111, 122, and 11 systems, 122 system with only one kind of anion with modest T_c turns out to be an excellent candidate for such purpose. J_c of the wire exceeded 1x10⁴ A/cm² at 4.2 K under self-field quickly [7], and it exceeds 1x10⁴ A/cm² even at 100 kOe by introducing hot-isostatic-press (HIP) process [8].
In the present study, we fabricated small magnets using 10 m-class (Ba,A)Fe₂As₂ (A: Na, K) round wires prepared by powder-in-tube (PIT) method. Magnets are processed under high pressure using HIP technique after glass-fiber insulations are installed [9]. Critical current (I_c) of the coils at 4.2 K reach 60 A and 66 A under the self-field, and magnetic fields of 2.6 kOe and 2.5 kOe are successfully generated at the center of the coil, using (Ba,Na)Fe₂As₂ and (Ba,K)Fe₂As₂ wires, respectively. After the characterization of the coils, wires are unwound and short segments are picked out from different locations of the wire for further characterizations. The largest transport I_c at 4.2 K under the magnetic field of 100 kOe for (Ba,Na)Fe₂As₂ wire reaches 51.8 A, which corresponds to J_c = 54 kA/cm². This J_c value is larger than the record value of previous IBS superconducting round wires by more than 20 %. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of J_c. After this success, we attempted to increase I_c further by making the cross section of the core larger. We also prepared such wires as long as 10 m, and constructed small coils using react-and-wind method. However, the resulting I_c of the wire was fairly low. Inspection of the wire core after unwinding the coil makes it clear that high density cracks are introduced in the wire due to small bending diameter in spite of large core of the wire.
References
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008).
[2] N. Ni et al., Phys. Rev. B 78, 014507 (2008).
[3] R. Prozorov et al., Phys. Rev. B 78, 224506 (2008).
[4] Y. Nakajima, T. Taen, and T. Tamegai, J. Phys. Soc. Jpn. 78, 023702 (2009).
[5] Y. Nakajima et al., Phys. Rev. B 80, 012510 (2009).
[6] T. Tamegai et al., Supercond. Sci. Technol. 25, 084008 (2012).
[7] K. Togano, A. Matsumoto, and H. Kumakura, Appl. Phys. Express 4, 043101 (2011).
[8] J. D. Weiss et al., Nat. Mater. 11, 682 (2012).
[9] S. Pyon et al., Supercond. Sci. Technol. 34, 105008 (2021).

Bilayer composed of parent and doped compounds creates a clean and sharp interface due to the chemically inert each layer. Herein, we report on the sharp interface between K-doped BaFe₂As₂ (Ba122) and the parent compound Ba122 grown by molecular beam epitaxy. Structural and microstructural analysis revealed that K did not diffuse into the parent Ba122. The bilayer K-doped Ba122/Ba122 exhibited a high superconducting transition temperature T_c of 40 K, which is higher than K-doped Ba122 grown on CaF₂(001) (T_c~33 K) [1-2] due to the suppression of strain. We also demonstrate that K-doped Ba122/Ba122 is grown on symmetric [001]-tilt MgO bi-crystal substrates having different misorientation angles θ_GB. We will discuss the microstructure and superconducting properties of the bilayer K-doped Ba122/Ba122.
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, Phys. Rev. Mater. 5, 014801 (2021).
[2] K. Iida, D. Qin, C. Tarantini, T. Hatano, C. Wang, Z. Guo, H. Gao, H. Saito, S. Hata, M. Naito, A. Yamamoto, NPG. Asia Mater. 13, 68 (2021).

High-temperature superconductors (HTS) based on REBa2Cu3Oy (REBCO, RE = rare earth) have become powerful materials for high field applications such as rotation machines, generators for wind turbine and magnet use in medical imaging machines. Optimizing the critical current density J_c of REBCO coated conductors (CCs) at a given temperature and magnetic field is particularly important for these applications. The in-field J_c can be enhanced by the introduction of precipitates and defects with nano-meter size, which can pin the vortices. The desirable pinning structures could be provided by ion irradiation which enable the creation of a variety of defects, such as points, clusters and tracks, by choosing appropriate ion species and energy. Therefore, ion irradiation would be a promising approach to realize high performance CCs by engineering the vortex pinning landscape for the specific needs of each application. Recently, ion irradiation in a low energy range has been revisited as a practically feasible approach to improve in-field J_c, due to compact accelerator, less radioactivation and less expensive to operate. We demonstrated an enhancement of J_c by using proton and Au-ion irradiation with low-energy in iron-chalcogenide FeSe_0.5Se_0.5 superconducting films.[1] In this presentation, we present systematically the effect of low-energy ion irradiation on flux pinning and microstructure in REBCO CCs. Upon 10 MeV Au ion irradiation (10 MeV Au ions would penetrate through the GdBCO layer), over 70% J_c enhancement was achieved around 3 T at 30 K, indicative of effective pinning defects by the irradiation. In the case of 2 MeV Au ion irradiation (2 MeV Au ions would stop in the GdBCO layer), a nearly 50% increase in J_c is observed over 3 T at 30 K. The superconducting transition temperature T_c’s of the GdBCO CCs irradiated by both 2 and 10 MeV Au ions decrease gradually with increasing flucence up to around 8.0×10¹¹ /cm^–2 and then significantly started to drop. Positron annihilation measurements reveal that irradiation-induced vacancy-type defects caused a reduction in T_c [2]. We will also report superconducting properties and flux pinning in GdBCO CCs irradiated by various doses and ion species.
We would like to thank Sumitomo Electric Industries, Ltd. for providing the GdBCO CCs.
1) T. Ozaki et al., Supercond. Sci. Technol. 33, 094008 (2020).
2) A. Yabuuchi et al., Appl. Phys. Express 13, 123004 (2020).

In this contribution, we present results obtained in the new configuration of magnets and superconductors permitting to strongly increase the levitation force of Superconducting Magnetic Levitating (SML) systems, previously described [1]. We compare levitation force measurements in this configuration to measurements carried out in the classical configuration. We investigate the stability of the proposed system by measurements of the restoring force. We show how to calculate the levitation and the restoring force from i) that measured in the classical configuration and ii) the repulsive force between the magnets included in the system. We discuss the conditions of stability of the proposed system as well as the consequences that its introduction could have on the future of SML MAGLEV transportation.

[1] P. Bernstein, L. Colson and J. Noudem, "A New Magnetic Levitation System With an Increased Levitation Force," in IEEE Transactions on Applied Superconductivity, vol. 29, no. 5, pp. 1-4, Aug. 2019, Art no. 3602204, doi: 10.1109/TASC.2019.2899950.

High-temperature superconductors (HTS), combined with other functional materials offer unique opportunities to tune their physical properties with multiple external inputs, thus providing the basis for realizing energy-efficient electronic devices for information and communication technologies (sensors, logic and memory devices, filters antennas).
In spite of great technological advancements in the recent years, electronic devices based on HTS are still in the early stage with respect to those based on conventional low-temperature superconductors. An important drawback is their complexity and high sensitivity on doping which imposes extreme demands on the micro-, nano-fabrication.
In this talk, I will present different strategies to design nanostructured high temperature YBa₂Cu₃O_7-x (YBCO) superconducting films, with local reversible carrier density modulation through field-induced oxygen migration [1]. We demonstrate that non-volatile volume metal-insulating phase transitions can be locally modulated to generate transistor-like devices with free-resistance channels, or create reconfigurable pinning landscapes for fluxtronic device applications. Moreover, I will show that by combining YBCO with ferromagnetic structures in hybrid systems, one can manipulate magnetic textures, through superconducting stray fields or transport super-currents. Multiple volatile and non-volatile magnetic states with different magnetoresistance signal can be stabilized at remanence, and modified by applying small loss-less magnetic fields or currents. The proposed approaches open up new venues for energy-efficient information storage and manipulation [2].
[1] Gonzalez-Rosillo et al. Small 2001307 (2020), Marinkovíc et al. ACS Nano, 14, 11765 (2020), Palau et al. ACS Appl. Mater. Interfaces 10, 30531(2018).
[2] Alcalà et al. Surfaces and interfaces of metal oxide thin films, multilayers nanoparticles and nanocomposites. Springer Nature Book, ed. By A. Gómez et al. Chapt. 6. 167-182 (2021); Rouco et al. Sci. Rep. 7, 5663 (2017); A. Palau et al. Adv. Sci. 3,1600207 (2016)

In recent years, the field of Fe-based superconductors (FBS) has consolidated to a steady progress in sample qualities, fundamental understanding of their unconventional superconductivity, as well as in optimizing their properties with regard to possible applications.
The high-field application-relevant FBS usually show relatively high critical temperatures and critical fields in combination with low electronic anisotropies. Furthermore, grain boundaries in these materials show larger transparencies than those in cuprates, which eases the production of wires and tapes. Several projects worldwide aim to fabricate high-quality and/or long-length wires and tapes, may it be via powder-in-tube or coated-conductor technology. However, the class of Fe-based superconductors is very rich in crystal structures (despite the common feature of FeAs or FeSe layers), hence dimensionality and anisotropy, kind of doping (electron, hole, isovalent, undoped, direct/indirect etc.), and chemical composition and stoichiometry. Comparing these different phases in their (micro)structural and magneto-electrical properties can help elucidate the specifics of these compounds with regard to nature of superconductivity and their vortex matter, as well as help optimizing the transport properties of application-relevant FBS.
This talk will give an overview over recent developments in growth, measurements, and optimization of Fe-based superconductors with focus on thin films, vortex matter, and pinning enhancement.

Superconducting coplanar waveguide (CPW) microwave resonators are very sensitive to defects in their surfaces mainly due the presence of two-level system (TLS) oxides and non-TLS quasiparticles that significantly reduce coherence in quantum circuits. Quality factor of CPW resonators is directly related to quantum coherence of superconducting circuits. Long coherence time is one of the key factors in realizing a commercial scale quantum computer and other related devices. The unique coupling of CPW resonators to other elements in quantum circuits is what forms the base of circuit quantum electrodynamics (cQED) architecture. While extensive research has explored techniques to reduce coherent losses of such devices, the precise structure of amorphous dielectric layers on surfaces and interfaces and their associated losses mechanism remain topics of active discussion. In this work we present the design, fabrication and characterization of Niobium CPW resonators with a particular surface treatment using self-assembled monolayers (SAMs) that result in reducing superconducting losses. We show resonator samples with more than 10⁶ internal quality factors at single-photon-excitation power, measured at 100 mK, that have been probed using a suite of structural characterization tools (SEM, XPS and TEM) to correlate the efficiency our surface treatment. We finally compare the improvements in quality factors to our numerical simulations.

122 phase iron-based superconductors show high upper critical field (H_c2 > 50 T) with small electromagnetic anisotropy (γ ~ 1-2)¹⁾ and large critical grain boundary angle (θ_c ~9deg)²⁾, and therefore is a promising material for applications in polycrystalline form. Foreseeing high field magnet applications, Weiss et al. have reported demonstration of trapped field of 1 T for K-doped BaFe₂As₂ (Ba122) polycrystalline bulks synthesized by hot isostatic pressing³⁾. In this study, we synthesized K-doped Ba122 bulks by Spark Plasma Sintering (SPS). In a highly pure Ar glove box, mechanically alloyed precursor powder was prepared by high-energy ball-milling of elemental metals with the molar ratios of Ba:K:Fe:As = 0.6:0.4:2:2^{4), 5)}. The precursor powder was filled into a graphite mold and sintered using SPS apparatus. Two approaches to optimizing the process conditions were considered: optimization by researchers’ experience and intuition, based on the knowledge of nanostructural analysis and multi-scale observations^{6), 7)}, and optimization by process machine learning⁸⁾. Bayesian optimization was performed to find the best input parameters which maximize the target output property, critical current density (J_c), on the experimentally available range of processing conditions. High J_c value exceeding 10⁵ A/cm², which is among the highest in Ba122 bulk samples, was developed by both experiments guided by researchers and such an adaptive experimental optimization process. In order to demonstrate the rapped field properties, relatively large size, disk-shaped bulk samples with a diameter of 30 mm were synthesized using the optimized processing parameters. The obtained bulk samples were dense with relative density of ~90%. Detailed trapped field characteristics will be reported in the presentation.
Acknowledgement
This work was supported by JST CREST (JPMJCR18J4), JSPS KAKENHI (JP21H01615), and Nanotechnology Platform (A-18-TU-0037, A-21-KU-1001) of the MEXT, Japan.
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