After many years of evaluating MgB 2 films prepared with various techniques for the application to superconducting radio-frequency (SRF) cavities, we have decided to build a system to coat full-size 1.3-GHz elliptical cavities. This paper describes the design and construction of the system. Additionally, we briefly describe experimental results with a small system and first tests with the new large system. We were able to obtain superconducting samples with a T c of up to 38 K with the small system, but we have not been able to get any superconducting samples with the new system yet.

The addition of artificial pinning centers has led to an impressive increase in the critical current density ( J c ) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The J c of superconductors has reached ~0.2–0.3 J d , where J d is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase J d . For (Y 0.77 Gd 0.23 )Ba 2 Cu 3 O y ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe 2 (As 1− x P x ) 2 films. The variation in λ and ξ leads to an intrinsic improvement in J c via J d , allowing extremely high values of J c of 130 MA/cm 2 and 8.0 MA/cm 2 at 4.2 K, consistent with an enhancement in J d of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe 2 (As 1− x P x ) 2 films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m 3 at 4.2 K and 18 T ( H || c ), the highest values ever reported for any superconductor.

Ca3Co2O6 is a frustrated magnet consisting of a triangular arrangement of chains of Ising spins. It shows regular magnetization steps vs magnetic field every 1.2 T that are metastable with very slow dynamics. This has puzzled the community for many years and given rise to numerous potential theories. Here we approach the problem by seeking the elusive magnetic equilibrium state at T = 2 K. To this end, we explore two approaches: (1) bypassing the slow dynamics produced by changing fields by instead field-cooling directly to the target temperature, and (2) quantum annealing in transverse magnetic fields. While we observe no measurable effect of the quantum annealing in fields up to 7 T, which is likely due to the large Ising anisotropy of Co spins in this material, we find that for the field cooling in longitudinal fields we achieve the predicted equilibrium 1/3 magnetization. We perform Monte Carlo simulations of the ground state phase diagram and we also simulate the quantum annealing process and find good agreement between experiment and theory. Thus we present an investigation of the elusive ground state properties of the canonical frustrated triangular system Ca3Co2O6.

YBa2Cu3O7-based coated conductors (CCs) achieve the highest critical current densities (J(c)) of any known superconductor and are a key technology for applications such as rotatory machines, high-field magnets and power transmission. Incorporation of nano-sized non-superconducting second phases as additional vortex pinning centers has been considered the most amenable route to further enhance J(c) at an industrial scale, and has been successfully used in commercial CCs. The resulting pinning landscape is quite complex, with both synergistic and competing interactions among the various types of defects. Particle irradiation, on the other hand, allows for a controlled post-processing incorporation of a well-defined defect morphology. We have previously shown that irradiation with protons and other light ions can further enhance the in-field J(c) in commercial state-of-the-art CCs. Here we develop a combined irradiation process that increases J(c) above values previously achieved by irradiating with only one species. Our new approach involves sequentially irradiating with 250 MeV Au ions and 4 MeV protons. For example, at T similar to 27 K (liquid neon) and mu H-0 similar to 4 T, a region of interest for rotatory machines applications, we obtain J(c) similar to 5 MA cm(-2), which is about 40% higher than the values produced by the individual irradiations. Finally, we conclude that this is due to the synergistic pinning effects of the introduced splayed, non-uniform columnar defects and small clusters.

temperature dependence of dielectric constant also shows strong ν-dependence with similar robustness to H. The isothermal H-dependent dielectric results at low temperatures establish anisotropic MDE coupling. It is intriguing to note that there is a “step” roughly at one-third of saturation values as in the case of isothermal magnetization curves for same temperatures (for orientation along spin-chain), a correlation hitherto unrealized for geometrically frustrated systems.

We report the effect of Co2N impurity on the superconducting properties of delta-MoN thin films grown by polymer-assisted deposition on c-cut sapphire (Al2O3). The films show a superconducting transition temperature of 10.4 K and an upper critical field H-c2(0) perpendicular to the film surface around 3 T. The latter corresponds to a relatively large coherence length xi, which enhances the two-dimensional limit when the magnetic field is applied parallel to the film surface. In comparison with pure delta-MoN films, the inclusion of Co2N impurity in the delta-MoN films could significantly modify the critical current density at the vortex-free state. The ability to tune the superconducting properties of metal-nitride superconductors by introducing chemically and structurally compatible impurity may find potential applications for superconducting single-photon detectors.

One of the most promising routes for achieving high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce T c . Consequently, an optimal balance must be achieved between the nanoparticle density n p and size d . Determining this balance requires garnering a better understanding of vortex–nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y 0.77 , Gd 0.23 )Ba 2 Cu 3 O 7 − δ films in magnetic fields of up to 35 T and compare the trends to recent results from time-dependent Ginzburg–Landau simulations. We identify consistency between the field-dependent critical current J c ( B ) and expectations from strong pinning theory. Specifically, we find that J c ∝ B − α , where α decreases from 0.66 to 0.2 with increasing density of nanoparticles and increases roughly linearly with nanoparticle size d / ξ (normalized to the coherence length). At high fields, the critical current decays faster ( ∼ B − 1 ), sug-gesting that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in J c ( B ) . Due to a spread in defect sizes, this novel peak effect remains unresolved here. Finally, we reveal that the dependence of the vortex creep rate S on nanoparticle size and density roughly mirrors that of α , and we compare our results to low- T nonlinearities in S ( T ) that are predicted by strong pinning theory.

We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa 2 CuO 4+ δ , a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization M measurements over a wide range of temperatures T and fields H , we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field H p ( T ), second magnetization peak H smp ( T ), and irreversibility field H irr ( T ) all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temperatures T / T c >~ 0.5. By measuring the rates of thermally activated vortex motion (creep) S ( T , H ) = | d ln M ( T , H )/ d ln t |, we reveal glassy behavior involving collective creep of bundles of 2D pancake vortices as well as temperature- and time-tuned crossovers from elastic (collective) dynamics to plastic flow. Based on the creep results, we show that the second magnetization peak coincides with the elastic-to-plastic crossover at low T , yet the mechanism changes at higher temperatures.

Room-temperature magnetoelectric (ME) coupling is developed in artificial multilayers and nanocomposites composed of magnetostrictive and electrostrictive materials. While the coupling mechanisms and strengths in multilayers are widely studied, they are largely unexplored in vertically aligned nanocomposites (VANs), even though theory has predicted that VANs exhibit much larger ME coupling coefficients than multilayer structures. Here, strong transverse and longitudinal ME coupling in epitaxial BaTiO3:CoFe2O4 VANs measured by both optical second harmonic generation and piezoresponse force microscopy under magnetic fields is reported. Phase field simulations have shown that the ME coupling strength strongly depends on the vertical interfacial area which is ultimately controlled by pillar size. The ME coupling in VANs is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. The revealed mechanisms shed light on the physical insights of vertical interface coupling in VANs in general, which can be applied to a variety of nanocomposites with different functionalities beyond the studied ME coupling effect.

The word “superconductor” (SC) evokes the best-known and most-impressive characteristic of these materials, namely their capability to transport electrical current without dissipation. Zero resistance, though, is not the most fundamental property of an SC and by no means has a trivial explanation. The defining phenomenon in an SC is the Meissner effect, which dictates that, in the presence of a low external magnetic field ( H ), the field inside the SC is zero (1). The Meissner effect implies that if H is high enough the SC phase will be destroyed, i.e., there is a critical field. At intermediate fields, a broad variety of SCs, called “type II,” let most of the field penetrate in the form of “vortices,” while most of the material remains superconducting. This trick allows the critical field to reach dramatically higher values, making these materials technologically useful (1, 2). But there is one problem: In a homogeneous SC, electric currents move the vortices, producing dissipation (resistance R ≠ 0). This technologically detrimental motion can be precluded by the presence of material disorder, which produces “pinning centers” that trap the vortices, as long as the current density does not exceed a critical value, J c (1⇓–3). For five decades, the art and science of improving vortex pinning in SCs has progressed through educated guesses, theoretical modeling, and resource-intensive experimental optimization. Recently, enabled by more powerful computational capabilities and inspired by the “materials by design” new paradigm, an effort to advance toward a systematic “critical-currents-by-design” approach has been underway (4). In PNAS, Sadovskyy et al. (5) continue moving on that path but incorporate a radically different strategy. Starting from a “seed” pinning landscape, they apply a genetic algorithm to allow it to evolve toward a configuration with optimum J c. By informing the engineering … [↵][1]1Email: lcivale{at}lanl.gov. [1]: #xref-corresp-1-1

The intrinsic and effective anisotropies, both in the liquid and solid vortex regimes, of YBa2Cu3O7 (YBCO) pristine and nanocomposite thin films have been investigated. Angular resistivity measurements under varying fields and temperatures were performed to characterize the intrinsic vs effective anisotropy of samples in the regime of long-range vortex displacements. The effective anisotropy gamma(eff) was determined from the scaling of the irreversibility line, applying the Blatter approach developed for uniaxial anisotropic superconductors. Resistive measurements in flux flow were utilized, in addition to H-c2 measurements in ultrahigh-fields, to determine the intrinsic anisotropy mass gamma, enabling the study of a large number of samples with varied nanoparticle compositions. In order to access the intrinsic anisotropy in the vortex solid phase, complex impedance measurements at high microwave frequencies were performed, allowing us to access the flux-flow intrinsic anisotropy in the regime of very short vortex oscillations within the pinning potential wells. Results show that while the effective anisotropy gamma(eff) decays as the nanoparticles-induced nanostrain in the YBCO films increases, the intrinsic anisotropy gamma (determined both in dc and at microwave frequency) remains unaltered.

Non-linear electrical transport studies at high-pulsed magnetic fields, above the range accessible by DC magnets, are of direct fundamental relevance to the physics of superconductors, domain-wall, charge-density waves, and topological semi-metal. All-superconducting very-high field magnets also make it technologically relevant to study vortex matter in this regime. However, pulsed magnetic fields reaching 100 T in milliseconds impose technical and fundamental challenges that have prevented the realization of these studies. Here, we present a technique for sub-microsecond, smart, current-voltage measurements, which enables determining the superconducting critical current in pulsed magnetic fields, beyond the reach of any DC magnet. We demonstrate the excellent agreement of this technique with low DC field measurements on Y$_{0.77}$Gd$_{0.23}$Ba$_2$Cu$_3$O$_7$ coated conductors with and without BaHfO$_3$ nanoparticles. Exploring the uncharted high magnetic field region, we discover a characteristic influence of the magnetic field rate of change ($dH/dt$) on the current-voltage curves in a superconductor. We fully capture this unexplored vortex physics through a theoretical model based on the asymmetry of the vortex velocity profile produced by the applied current.

We present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10 below the ferromagnetic transition T-C as a double step in the magnetization at two critical fields H-c1 and H-c2. We map the H - T phase diagram with special focus on the temperature range 50 K <= T <= TC. We find that the critical field H-c1(T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8 T for the lowest temperatures and ends in a critical endpoint at (1 T; 80 K). We conclude from the temperature dependence of the ratio H-c1/H-c2(T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

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Disorder can have remarkably disparate consequences in superconductors, driving superconductor–insulator transitions in ultrathin films by localizing electron pairs and boosting the supercurrent carrying capacity of thick films by localizing vortices (magnetic flux lines). Though the electronic 3D-to-2D crossover at material thicknesses d ~ ξ (coherence length) is well studied, a similarly consequential magnetic crossover at d ~ L c (pinning length) that should drastically alter material properties remains largely underexamined. According to collective pinning theory, vortex segments of length L c bend to adjust to energy wells provided by point defects. Consequently, if d truncates L c , a change from elastic to rigid vortex dynamics should increase the rate of thermally activated vortex motion S . Here, we characterize the dependence of S on sample thickness in Nb and cuprate films. The results for Nb are consistent with collective pinning theory, whereas creep in the cuprate is strongly influenced by sparse large precipitates. We leverage the sensitivity of S to d to determine the generally unknown scale L c , establishing a new route for extracting pinning lengths in heterogeneously disordered materials.