Synergetic combination of different types of defect to optimize pinning landscape using BaZrO(3)-doped YBa(2)Cu(3)O(7)

Flame Spray Pyrolysis Synthesis of Vo-Rich Nano-SrTiO3-x

Engineering of oxygen vacancies (Vo) in nanomaterials allows diligent control of their physicochemical properties. SrTiO3 possesses the typical ABO3 structure and has attracted considerable attention among the titanates due to its chemical stability and its high conduction band energy. This has resulted in its extensive use in photocatalytic energy-related processes, among others. Herein, we introduce the use of Flame Spray Pyrolysis (FSP); an industrial and scalable process to produce Vo-rich SrTiO3 perovskites. We present two types of Anoxic Flame Spray Pyrolysis (A-FSP) technologies using CH4 gas as a reducing source: Radial A-FSP (RA-FSP); and Axial A-FSP (AA-FSP). These are used for the control engineering of oxygen vacancies in the SrTiO3-x nanolattice. Based on X-ray photoelectron spectroscopy, Raman and thermogravimetry-differential thermal analysis, we discuss the role and the amount of the Vos in the so-produced nano-SrTiO3-x, correlating the properties of the nanolattice and energy-band structure of the SrTiO3-x. The present work further corroborates the versatility of FSP as a synthetic process and the potential future application of this process to engineer photocatalysts with oxygen vacancies in quantities that can be measured in kilograms.

Enhanced critical current density in optimized high-temperature superconducting bilayer thin films

The superconducting and structural properties of bilayer thin films based on YBa2Cu3O7-x / YBa2Cu3O7-x+6%BaZrO3heterstructures have been studied. In a broad range of magnetic field strengths and temperatures, the optimal bilayer film comprises 30% YBCO at the substrate interface and 70% YBCO+6%BZO on the top. The critical current density measured for the optimal bilayer structure is shown to outperform the corresponding single layer films up to almost 60%. The obtained results are comprehensively discussed in the light of our previously published theoretical framework (Rivastoet al2023J. Phys.: Condens. Matter35075701:1-10). We conclude that the bilayering provides an efficient and easily applicable way to further increase the performance and applicability of high-temperature superconductors in various applications. Consequently, the bilayer films should be seriously considered as candidates for the upcoming generation of coated conductors.

Optimization of high-temperature superconducting bilayer structures using a vortex dynamics simulation

We argue that the current carrying properties of high-temperature superconducting thin films can be further improved, in particular under the mid-field range (B ≈ 0.1-2 T), via introduction of multilayer structures that compromise between good zero field critical current and vortex pinning performance. In this work we focus on a simple bilayer structure consisting of two adjacent layers of pure YBa₂Cu₃O6+x(YBCO) and BaZrO₃(BZO) doped YBCO under magnetic field within the mid-field range oriented parallel to thec-axis of the YBCO unit cell. We have utilized a computational model to simulate the vortex dynamics limited critical current separately from the associated zero field current, which is addressed analytically. The obtained results have allowed us to estimate the optimal layer thicknesses as a function of magnetic field. Our idealized model suggests that the thickness of the doped layer should be substantially smaller than the undoped one, that is around 30% of the total thickness of the film. We have estimated that the current carrying capability of the optimized bilayer structure can be up to 50% higher when compared with corresponding single layer films. Possible deviations from the obtained results associated with the idealized model, most prominently the effect of natural defects, are comprehensively discussed. Our results provide the foundation for the future experimental realization of the proposed bilayer structures. The comparison between the presented results and experimental realization would enable further study of the underlying primitive vortex interactions.

Progress in the Study of Vortex Pinning Centers in High-Temperature Superconducting Films

Since the discovery of high-temperature superconductors (HTSs), significant progress in the fabrication of HTS films has been achieved. In this review, we intend to provide an overview of recent progress in how and why superconductivity can be enhanced by introducing nanoscale vortex pinning centers. The comprehensive control of morphology, dimension, orientation and concentration of artificial pinning centers (APCs) and the principle of vortex pinning are the focus of this review. According to the existing literature, HTSs with the best superconductivity can be obtained when one-dimensional (1D) and three-dimensional (3D) nanoscale APCs are combined for vortex pinning.

Vortex dynamics simulation for pinning structure optimization in the applications of high-temperature superconductors

We introduce a molecular dynamics based simulation model that enables the efficient optimization of complex pinning structures in unpresented wide magnetic field and angular ranges for high-temperature superconductor applications. The fully three-dimensional simulation allows the modeling of the critical current and the associated anisotropy in the presence of any kinds of defects despite their size and orientation. Most prominently, these include artificial defects such as nanorods along with intrinsic weak-links orab-plane oriented stacking faults, for example. In this work, we present and analyze the most fundamental results of the simulation model and compare them indirectly with a wide range of previous experimental and computational observations. With the provided validation for the proposed simulation model, we consider it to be an extremely useful tool in particular for pushing the limits of ampacity in the coated conductor industry.

Modification of Critical Current Density Anisotropy in High-Tc Superconductors by Using Heavy-Ion Irradiations

The critical current density Jc, which is a maximum value of zero-resistivity current density, is required to exhibit not only larger value but also lower anisotropy in a magnetic field B for applications of high-Tc superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-Tc superconducting materials, which can modify both the absolute value and the anisotropy of Jc in a controlled manner: the unique structures of CDs, which significantly affect the Jc properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the Jc anisotropy in high-Tc superconductors using CDs installed by heavy-ion irradiations. The direction-dispersion of CDs, which is tuned by the combination of the plural irradiation directions, can provide a variety of the magnetic field angular variations of Jc in high-Tc superconductors: CDs crossing at ±θi relative to the c-axis of YBa2Cu3Oy films induce a broad peak of Jc centered at B || c for θi < ±45°, whereas the crossing angle of θi ≥ ±45° cause not a Jc peak centered at B || c but two peaks of Jc at the irradiation angles. The anisotropy of Jc can also modified by tuning the continuity of CDs: short segmented CDs formed by heavy-ion irradiation with relatively low energy are more effective to improve Jc in a wide magnetic field angular region. The modifications of the Jc anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.

High Jc and low anisotropy of hydrogen doped NdFeAsO superconducting thin film

The recent realisations of hydrogen doped LnFeAsO (Ln = Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density Jc. The superconducting transition temperature Tc is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field Jc of over 10 MA/cm2 was recorded at 5 K, which is likely to be caused by a short London penetration depth. The anisotropic Ginzburg-Landau scaling for the angle dependence of Jc yielded temperature-dependent scaling parameters γJ that decreased from 1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F). Additionally, γJ of NdFeAs(O,H) is smaller than that of NdFeAs(O,F). Our results indicate that heavily electron doping by means of hydrogen substitution for oxygen in LnFeAsO is highly beneficial for achieving high Jc with low anisotropy without compromising Tc, which is favourable for high-field magnet applications.

Development and large volume production of extremely high current density YBa2Cu3O7 superconducting wires for fusion

The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate remarkable engineering current density (averaged over the full wire), JE, at very high magnetic fields, driving progress in fusion and other applications. The key challenge for HTS wires has been to offer an acceptable combination of high and consistent superconducting performance in high magnetic fields, high volume supply, and low price. Here we report a very high and reproducible JE in practical HTS wires based on a simple YBa2Cu3O7 (YBCO) superconductor formulation with Y2O3 nanoparticles, which have been delivered in just nine months to a commercial fusion customer in the largest-volume order the HTS industry has seen to date. We demonstrate a novel YBCO superconductor formulation without the c-axis correlated nano-columnar defects that are widely believed to be prerequisite for high in-field performance. The simplicity of this new formulation allows robust and scalable manufacturing, providing, for the first time, large volumes of consistently high performance wire, and the economies of scale necessary to lower HTS wire prices to a level acceptable for fusion and ultimately for the widespread commercial adoption of HTS.

Suppression of superconductivity at the nanoscale in chemical solution derived YBa2Cu3O7-δ thin films with defective Y2Ba4Cu8O16 intergrowths

The analysis of the microstructure and superconducting behavior of chemical solution deposited epitaxial YBa₂Cu₃O_7-δ films, with thickness going down to 5 nm has been carried out with the purpose to disclose the behavior of the most common intergrowth in these films, the Y₂Ba₄Cu₈O₁₆. The analysis of ultrathin films is a unique opportunity to investigate the superconducting behavior of these nanoscale defects because of the high concentration created as a consequence of the elastic energy associated to the misfit strain. Magnetic susceptibility and X-ray diffraction measurements evidence a strong decrease of the superconducting volume correlated with an increase of the intergrowth volume fraction. We demonstrate that these intergrowths are non-superconducting nanoscale regions where Cooper pair formation is disrupted, in agreement with their key role as artificial pinning centers for vortices in YBa₂Cu₃O_7-δ films and coated conductors.

Nanosized Pinning Centers in the Rare Earth-Barium-Copper-Oxide Thin-Film Superconductors

Since the discovery of high-temperature superconductivity, significant progress in the fabrication of REBCO-based (Rare Earth Barium Copper mixed Oxides) thin-films superconductors has been achieved. In our review, we described the approaches and possibilities of the improvement of superconducting properties by the introduction of nanosized pinning centers. We focused on the synthesis and viability of the material for artificial pinning centers and methods used for the introduction of the pinning centers into superconducting REBCO-based thin-films. This article summarizes available materials and procedures regardless of the financial cost of the individual method. According to available literature, the most significant superconducting REBCO tapes can be obtained when a combination of 1D and 0D nanoparticles are used for nanoscale pinning.

Self-assembled nanorods in YBCO matrix - a computational study of their effects on critical current anisotropy

In order to understand how the doping with self-assembled nanorods of different sizes and concentrations as well as applied magnetic fields affect the critical current anisotropy in YBa₂Cu₃O_7−x (YBCO) thin films close to YBCO c-axis, we present an extensive and systematic computational study done by molecular dynamics simulation. The simulations are also used to understand experimentally measured J_c(θ) curves for BaHfO₃, BaZrO₃ and BaSnO₃ doped YBCO thin films with the help of nanorod parameters obtained from transmission electron microscopy measurements. Our simulations reveal that the relation between applied and matching field plays a crucial role in the formation of J_c(θ)-peak around YBCO c-axis (c-peak) due to vortex-vortex interactions. We also find how different concentrations of different size nanorods effect the shape of the c-peak and explain how different features, such as double c-peak structures, arise. In addition to this, we have quantitatively explained that, even in an ideal superconductor, the overdoping of nanorods results in decrease of the critical current. Our results can be widely used to understand and predict the critical current anisotropy of YBCO thin films to improve and develop new pinscapes for various transport applications.

Enhanced flux pinning isotropy by tuned nanosized defect network in superconducting YBa2Cu3O6+x films

Striving to improve the critical current density Jc of superconducting YBa₂Cu₃O_6+x (YBCO) thin films via enhanced vortex pinning, the interplay between film growth mechanisms and the formation of nanosized defects, both natural and artificial, is systematically studied in undoped and BaZrO₃ (BZO)-doped YBCO thin films. The films were grown via pulsed laser deposition (PLD), varying the crystal grain size of the targets in addition to the dopant content. The microstructure of the PLD target has been observed to have a great impact on that of the deposited thin films, including the formation of vortex pinning centers, which has direct implications on the superconducting performance, especially on the isotropy of flux pinning properties. Based on experimentally measured angular dependencies of Jc, coupled with a molecular dynamics (MD) simulation of flux pinning in the YBCO films, we present a quantitative model of how the splay and fragmentation of BZO nanorods artifically introduced into the YBCO film matrix explain the majority of the observed critical current anisotropy.

Control of nanostructure and pinning properties in solution deposited YBa2Cu3O7-x nanocomposites with preformed perovskite nanoparticles

Solution deposited YBa₂Cu₃O_7−x (YBCO) nanocomposites with preformed nanoparticles represent a promising cost-effective approach for superior critical current properties under applied magnetic fields. Nonetheless, the majority of YBCO nanocomposites with high nanoparticle loads (>20%) suffer from nanoparticle coalescence and degraded superconducting properties. Here, we study the influence of nanoparticle concentration (0–25% mol), size (5 nm–10 nm) and composition (BaHfO₃, BaZrO₃) on the generation of structural defects in the epitaxial YBCO matrix, key parameter for vortex pinning. We demonstrate that flash-heated superconducting nanocomposites with 20 mol% preformed BaHfO₃ or BaZrO₃ perovskite secondary phases feature discrete and small (7 nm) nanoparticles and high density of YBa₂Cu₄O₈ (Y248) intergrowths. We identify a synergy between Y248 intergrowth density and small nanoparticles to increase artificial vortex pinning centers. Also, we validate the multideposition process to successfully increase film thickness of epitaxial nanocomposites with competitive critical currents I_c at 77 K.

Electronic structure and high-temperature thermochemistry of BaZrO3−δ perovskite from first-principles calculations

Accurate first-principles calculations predict electronic structure and high-temperature thermochemical properties of oxygen-deficient BaZrO₃.

Glassy Dynamics in a heavy ion irradiated NbSe2 crystal

Fascination with glassy states has persisted since Fisher introduced the vortex-glass as a new thermodynamic phase that is a true superconductor that lacks conventional long-range order. Though Fisher's original model considered point disorder, it was later predicted that columnar defects (CDs) could also induce glassiness - specifically, a Bose-glass phase. In YBa2Cu3O7-x (YBCO), glassy states can cause distinct behavior in the temperature (T ) dependent rate of thermally activated vortex motion (S). The vortex-glass state produces a plateau in S(T ) whereas a Bose-glass can transition into a state hosting vortex excitations called double-kinks that can expand, creating a large peak in S(T ). Although glass phases have been well-studied in YBCO, few studies exist of other materials containing CDs that could contribute to distinguishing universal behavior. Here, we report on the effectiveness of CDs tilted ~30° from the c-axis in reducing S in a NbSe2 crystal. The magnetization is 5 times higher and S is minimized when the field is parallel to the defects versus aligned with the c-axis. We see signatures of glassiness in both field orientations, but do not observe a peak in S(T ) nor a plateau at values observed in YBCO. Finally, we discuss the possibility that competing disorder induces a field-orientation-driven transition from a Bose-glass to an anisotropic glass involving both point and columnar disorder.

Pushing the limits of applicability of REBCO coated conductor films through fine chemical tuning and nanoengineering of inclusions

An outstanding current carrying performance (namely critical current density, Jc) over a broad temperature range of 10-77 K for magnetic fields up to 12 T is reported for films of YBa2Cu3O7-x with Ba2Y(Nb,Ta)O6 inclusion pinning centres (YBCO-BYNTO) and thicknesses in the range of 220-500 nm. Jc values of 10 MA cm-2 were measured at 30 K - 5 T and 10 K - 9 T with a corresponding maximum of the pinning force density at 10 K close to 1 TN m-3. The system is very flexible regarding properties and microstructure tuning, and the growth window for achieving a particular microstructure is wide, which is very important for industrial processing. Hence, the dependence of Jc on the magnetic field angle was readily controlled by fine tuning the pinning microstructure. Transmission electron microscopy (TEM) analysis highlighted that higher growth rates induce more splayed and denser BYNTO nanocolumns with a matching field as high as 5.2 T. Correspondingly, a strong peak at the B||c-axis is noticed when the density of vortices is lower than the nanocolumn density. YBCO-BYNTO is a very robust and reproducible composite system for high-current coated conductors over an extended range of magnetic fields and temperatures.

Angular flux creep contributions in YBa2Cu3O7-δ nanocomposites from electrical transport measurements

The shape of the electric-field—current-density (E-J) curve is determined by flux pinning and also by dynamics of vortices. Here, we propose a novel methodology to study the normalized flux creep rate S in YBa₂Cu₃O_7−δ measured from E-J curves obtained by electrical transport measurements that provides a fast and versatile way to foresee the flux magnetic relaxation in films and disentangle angular flux creep contributions by the scaling of the isotropic contribution of S. After a detailed comparison of various pristine and nanocomposite films with differentiated nanostructures, we focus on the roles that intrinsic pinning and stacking faults (YBa₂Cu₄O₈-intergrowths) play when the magnetic field is applied parallel to the superconducting CuO₂ planes. This study reveals that the emerging intergrowths provide advanced pinning properties that additionally reduce the thermal activated flux magnetic relaxation. For this purpose, creep analysis becomes a very appropriate tool to elucidate the dominance of the different pinning sites at different regions of the magnetic-field—temperature diagram.

Enhanced flux pinning in YBCO multilayer films with BCO nanodots and segmented BZO nanorods

The flux pinning properties of the high temperature superconductor YBa₂Cu₃O_7−δ (YBCO) have been conventionally improved by creating both columnar and dot-like pinning centres into the YBCO matrix. To study the effects of differently doped multilayer structures on pinning, several samples consisting of a multiple number of individually BaZrO₃ (BZO) and BaCeO₃ (BCO) doped YBCO layers were fabricated. In the YBCO matrix, BZO forms columnar and BCO dot-like defects. The multilayer structure improves pinning capability throughout the whole angular range, giving rise to a high critical current density, J_c. However, the BZO doped monolayer reference still has the most isotropic J_c. Even though BZO forms nanorods, in this work the samples with multiple thin layers do not exhibit a c axis peak in the angular dependence of J_c. The angular dependencies and the approximately correct magnitude of J_c were also verified using a molecular dynamics simulation.

Imaging of super-fast dynamics and flow instabilities of superconducting vortices

Quantized magnetic vortices driven by electric current determine key electromagnetic properties of superconductors. While the dynamic behavior of slow vortices has been thoroughly investigated, the physics of ultrafast vortices under strong currents remains largely unexplored. Here, we use a nanoscale scanning superconducting quantum interference device to image vortices penetrating into a superconducting Pb film at rates of tens of GHz and moving with velocities of up to tens of km/s, which are not only much larger than the speed of sound but also exceed the pair-breaking speed limit of superconducting condensate. These experiments reveal formation of mesoscopic vortex channels which undergo cascades of bifurcations as the current and magnetic field increase. Our numerical simulations predict metamorphosis of fast Abrikosov vortices into mixed Abrikosov-Josephson vortices at even higher velocities. This work offers an insight into the fundamental physics of dynamic vortex states of superconductors at high current densities, crucial for many applications.Ultrafast vortex dynamics driven by strong currents define eletromagnetic properties of superconductors, but it remains unexplored. Here, Embon et al. use a unique scanning microscopy technique to image steady-state penetration of super-fast vortices into a superconducting Pb film at rates of tens of GHz and velocities up to tens of km/s.

Structural Evolution Induced by Interfacial Lattice Mismatch in Self-Organized YBa2Cu3O7-δ Nanocomposite Film

Intriguing properties of self-organized nanocomposites of perovskite oxides are usually derived from the complex interface of constituent material phases. A sophisticated control of such a system is required for a broad range of energy and device applications, which demand a comprehensive understanding of the interface at the atomic scale. Here, we visualized and theoretically modeled the highly elastically strained nanorod, the interface region with misfit dislocations and heterointerface distortion, and the matrix with strain-induced oxygen vacancies in the self-organized YBa₂Cu₃O_7-δ nanocomposite films with Ba perovskite nanorods. Large misfit strain was elastically accommodated in the nanocomposites, but since the elastic strain was mainly accommodated by the nanorods, the concentration of strain-induced oxygen vacancies was small enough for the matrix to keep high critical temperature (>85 K). The interfacial bonding distorted the atomic structure of YBa₂Cu₃O_7-δ, but the thickness of distortion was limited to a few unit cells (less than the coherence length) due to the electron screening. The effect of volume fraction on elastic strain and the electron screening are crucial for strong vortex pinning without significant degradation of both the elementary pinning force and critical temperature in the nanocomposites. Thus, we comprehensively clarified the self-organized nanocomposite structure for on-demand control of superconductivity and oxide functionality in the nanocomposite engineering of perovskite oxides.

Vortices in high-performance high-temperature superconductors

The behavior of vortex matter in high-temperature superconductors (HTS) controls the entire electromagnetic response of the material, including its current carrying capacity. Here, we review the basic concepts of vortex pinning and its application to a complex mixed pinning landscape to enhance the critical current and to reduce its anisotropy. We focus on recent scientific advances that have resulted in large enhancements of the in-field critical current in state-of-the-art second generation (2G) YBCO coated conductors and on the prospect of an isotropic, high-critical current superconductor in the iron-based superconductors. Lastly, we discuss an emerging new paradigm of critical current by design-a drive to achieve a quantitative correlation between the observed critical current density and mesoscale mixed pinning landscapes by using realistic input parameters in an innovative and powerful large-scale time dependent Ginzburg-Landau approach to simulating vortex dynamics.

A route for a strong increase of critical current in nanostrained iron-based superconductors

The critical temperature T_c and the critical current density J_c determine the limits to large-scale superconductor applications. Superconductivity emerges at T_c. The practical current-carrying capability, measured by J_c, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce T_c. Simultaneous increase of T_c and J_c in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both T_c and J_c together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe_0.5Te_0.5 films. T_c is enhanced due to the nanoscale compressive strain and proximity effect, whereas J_c is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of J_c enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.

Simultaneous increase of critical temperature and critical current in superconductors is desirable for application purpose, but very difficult to realize. Here, Ozaki et al. report a simultaneous enhancement of T_c and J_c in FeSe_0.5Te_0.5 films with cascade defects produced by low-energy proton irradiation.

Toward Superconducting Critical Current by Design

A new critical-current-by-design paradigm is presented. It aims at predicting the optimal defect landscape in superconductors for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To this end, critical current measurements on commercial high-temperature superconductors are combined with large-scale time-dependent Ginzburg-Landau simulations of vortex dynamics.

Large pinning forces and matching effects in YBa2Cu3O(7-δ) thin films with Ba2Y(Nb/Ta)O6 nano-precipitates

The addition of mixed double perovskite Ba₂Y(Nb/Ta)O₆ (BYNTO) to YBa₂Cu₃O_7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m³ at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

Upward shift of the vortex solid phase in high-temperature-superconducting wires through high density nanoparticle addition

We show a simple and effective way to improve the vortex irreversibility line up to very high magnetic fields (60T) by increasing the density of second phase BaZrO3 nanoparticles. (Y0.77,Gd0.23)Ba2Cu3Oy films were grown on metal substrates with different concentration of BaZrO3 nanoparticles by the metal organic deposition method. We find that upon increase of the BaZrO3 concentration, the nanoparticle size remains constant but the twin-boundary density increases. Up to the highest nanoparticle concentration (n ~ 1.3 × 10(22)/m(3)), the irreversibility field (Hirr) continues to increase with no sign of saturation up to 60 T, although the vortices vastly outnumber pinning centers. We find extremely high Hirr, namely Hirr = 30 T (H||45°) and 24 T (H||c) at 65 K and 58 T (H||45°) and 45 T (H||c) at 50K. The difference in pinning landscape shifts the vortex solid-liquid transition upwards, increasing the vortex region useful for power applications, while keeping the upper critical field, critical temperature and electronic mass anisotropy unchanged.

Formation energies and electronic structure of intrinsic vacancy defects and oxygen vacancy clustering in BaZrO3

Thermodynamically stable oxygen vacancy clustering in the ZrO₂ layer of BaZrO₃ gives rise to n-type behaviour which originates from the charge delocalization of the in-plane unbound Zr₁-4d electrons.

Stroboscopic phenomena in superconductors with dynamic pinning landscape

Introducing artificial pinning centers is a well established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain without dissipation. In case of spatially periodic pinning, a clear enhancement of the superconducting critical current arises when commensurability between the vortex configurations and the pinning landscape occurs. With recent achievements in (ultrafast) optics and nanoengineered plasmonics it has become possible to exploit the interaction of light with superconductivity, and create not only spatially periodic imprints on the superconducting condensate, but also temporally periodic ones. Here we show that in the latter case, temporal matching phenomena develop, caused by stroboscopic commensurability between the characteristic frequency of the vortex motion under applied current and the frequency of the dynamic pinning. The matching resonances persist in a broad parameter space, including magnetic field, driving current, or material purity, giving rise to unusual features such as externally variable resistance/impedance and Shapiro steps in current-voltage characteristics. All features are tunable by the frequency of the dynamic pinning landscape. These findings open further exploration avenues for using flashing, spatially engineered, and/or mobile excitations on superconductors, permitting us to achieve advanced functionalities.

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)

Maximizing the sustainable supercurrent density, J C, is crucial to high-current applications of superconductivity. To achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high-current applications of iron-based superconductors because J C amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical temperature. However, for these compounds, virtually nothing is known about the atomic-scale interplay of the crystal damage from the high-energy ions, the superconducting order parameter, and the vortex pinning processes. We visualize the atomic-scale effects of irradiating FeSe x Te1-x with 249-MeV Au ions and find two distinct effects: compact nanometer-sized regions of crystal disruption or "columnar defects," plus a higher density of single atomic site "point" defects probably from secondary scattering. We directly show that the superconducting order is virtually annihilated within the former and suppressed by the latter. Simultaneous atomically resolved images of the columnar crystal defects, the superconductivity, and the vortex configurations then reveal how a mixed pinning landscape is created, with the strongest vortex pinning occurring at metallic core columnar defects and secondary pinning at clusters of point-like defects, followed by collective pinning at higher fields.

Probing dynamics and pinning of single vortices in superconductors at nanometer scales

The dynamics of quantized magnetic vortices and their pinning by materials defects determine electromagnetic properties of superconductors, particularly their ability to carry non-dissipative currents. Despite recent advances in the understanding of the complex physics of vortex matter, the behavior of vortices driven by current through a multi-scale potential of the actual materials defects is still not well understood, mostly due to the scarcity of appropriate experimental tools capable of tracing vortex trajectories on nanometer scales. Using a novel scanning superconducting quantum interference microscope we report here an investigation of controlled dynamics of vortices in lead films with sub-Angstrom spatial resolution and unprecedented sensitivity. We measured, for the first time, the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement, revealing a far more complex behavior than has previously been recognized, including striking spring softening and broken-spring depinning, as well as spontaneous hysteretic switching between cellular vortex trajectories. Our results indicate the importance of thermal fluctuations even at 4.2 K and of the vital role of ripples in the pinning potential, giving new insights into the mechanisms of magnetic relaxation and electromagnetic response of superconductors.

Development of very high Jc in Ba(Fe1-xCox)2As2 thin films grown on CaF2

Ba(Fe_1-xCo_x)₂As₂ is the most tunable of the Fe-based superconductors (FBS) in terms of acceptance of high densities of self-assembled and artificially introduced pinning centres which are effective in significantly increasing the critical current density, J_c. Moreover, FBS are very sensitive to strain, which induces an important enhancement in critical temperature, T_c, of the material. In this paper we demonstrate that strain induced by the substrate can further improve J_c of both single and multilayer films by more than that expected simply due to the increase in T_c. The multilayer deposition of Ba(Fe_1-xCo_x)₂As₂ on CaF₂ increases the pinning force density (F_p = J_c × µ₀H) by more than 60% compared to a single layer film, reaching a maximum of 84 GN/m³ at 22.5 T and 4.2 K, the highest value ever reported in any 122 phase.

Strongly enhanced flux pinning in one-step deposition of BaFe₂(As₀.₆₆P₀.₃₃)₂ superconductor films with uniformly dispersed BaZrO₃ nanoparticles

The high upper critical field and low anisotropy of the iron-based superconductor BaFe₂As₂ make it promising for its use in the construction of superconducting magnets. However, its critical current density in high magnetic fields needs to be improved. Here we demonstrate a simple, one-step and industrially scalable means of achieving just this. We show that introducing controlled amounts of uniformly dispersed BaZrO₃ nanoparticles into carrier-doped BaFe₂As₂ significantly improves its superconducting performance without degrading its structural or superconducting properties. Our BaFe₂(As_0.66P_0.33)₂ films also exhibit an increase in both the irreversibility line and critical current density at all magnetic-field orientations. These films exhibit nearly isotropic critical current densities in excess of 1.5 MA cm⁻² at 15 K and 1 T—seven times higher than previously reported for BaFe₂As₂ films. The vortex-pinning force in these films reaches ~59 GN m⁻³ at 5 K and 3–9 T, substantially higher than that of the conventional Nb₃Sn wire.

Iron-based superconductors are promising for its use in building superconducting magnets; however, their high-field critical current density needs to be improved. Miura et al. show that this can be achieved with a one-step growth process that uniformly incorporates BaZrO₃ nanoparticles into BaFe₂As₂ films.

Strong pinning of propagation fronts in adverse flow

Reaction fronts evolving in a porous medium exhibit a rich dynamical behavior. In the presence of an adverse flow, experiments show that the front slows down and eventually gets pinned, displaying a particular sawtooth shape. Extensive numerical simulations of the hydrodynamic equations confirm the experimental observations. Here we propose a stylized model, predicting two possible outcomes of the experiments for large adverse flow: either the front develops a sawtooth shape or it acquires a complicated structure with islands and overhangs. A simple criterion allows one to distinguish between the two scenarios and its validity is reproduced by direct hydrodynamical simulations. Our model gives a better understanding of the transition and is relevant in a variety of domains, when the pinning regime is strong and only relies on a small number of sites.

Explore or exploit? A generic model and an exactly solvable case

Finding a good compromise between the exploitation of known resources and the exploration of unknown, but potentially more profitable choices, is a general problem, which arises in many different scientific disciplines. We propose a stylized model for these exploration-exploitation situations, including population or economic growth, portfolio optimization, evolutionary dynamics, or the problem of optimal pinning of vortices or dislocations in disordered materials. We find the exact growth rate of this model for treelike geometries and prove the existence of an optimal migration rate in this case. Numerical simulations in the one-dimensional case confirm the generic existence of an optimum.

Epitaxial growth of YBa2Cu3O7−x films on Ce0.9La0.1O2−y buffered yttria-stabilized zirconia substrates by an all-chemical-solution route

A high critical current density of 3 MA cm⁻² (77 K, self-field) was achieved on a YBa₂Cu₃O₇ film deposited on Ce_0.9La_0.1O₂ buffered yttria-stabilized zirconia substrate, which is an all-chemical derived configuration.

Long-range and short-range structures of cube-like shape SrTiO3 powders: microwave-assisted hydrothermal synthesis and photocatalytic activity

We report herein a detailed study on the influence of microwave-assisted hydrothermal (MAH) treatment time on both long and short range structures around Ti atoms of SrTiO3 powders. Few studies have been carried out on short-order structural properties as well as the relationship between the local order and the SrTiO3 photocatalytic properties. We use X-ray diffraction to determine the long-range structure, while the local environment around the Ti atom is probed with X-ray absorption spectroscopy and the vibration frequencies are investigated by Raman spectroscopy. The faster crystallization of SrTiO3 powders provided by the MAH system resulted in large distortions of Ti-O bond lengths which remain unchanged even for a longer MAH treatment time. Despite the long-range structure being associated with ideal TiO6 clusters, X-ray absorption spectroscopy measurements identified the presence of undercoordinated TiO5 clusters. Compared with the reference bulk SrTiO3, the hierarchical SrTiO3 cube-like shape showed enhanced photocatalytic activity, which was associated with the presence of these TiO5 clusters. Field emission scanning electron microscopy (FE-SEM) revealed that the superstructures based on a cube-like shape are formed by an assembly process, becoming well defined as a function of MAH treatment time.

A new general synthetic strategy for phase-pure complex functional materials

The ability of ionic liquids to solvate inorganic salts completely has to date never been employed in the synthesis of complex inorganic materials. Here, we demonstrate that complex functional oxides, even those traditionally considered extremely difficult to synthesize in bulk, such as quinternary superconductors, are produced with no impurity phases and on timescales that are much shorter than other synthetic techniques.

Nanorod Self-Assembly in High Jc YBa₂Cu₃O₇-x Films with Ru-Based Double Perovskites

Many second phase additions to YBa₂Cu₃O_7−x (YBCO) films, in particular those that self-assemble into aligned nanorod and nanoparticle structures, enhance performance in self and applied fields. Of particular interest for additions are Ba-containing perovskites that are compatible with YBCO. In this report, we discuss the addition of Ba₂YRuO₆ to bulk and thick-film YBCO. Sub-micron, randomly oriented particles of this phase were found to form around grain boundaries and within YBCO grains in bulk sintered pellets. Within the limits of EDS, no Ru substitution into the YBCO was observed. Thick YBCO films were grown by pulsed laser deposition from a target consisting of YBa₂Cu₃O_y with 5 and 2.5 mole percent additions of Ba₂YRuO₆ and Y₂O₃, respectively. Films with enhanced in-field performance contained aligned, self-assembled Ba₂YRuO₆ nanorods and strained Y₂O₃ nanoparticle layers. A 0.9 µm thick film was found to have a self-field critical current density (J_c) of 5.1 MA/cm² with minimum J_c(Θ, H=1T) of 0.75 MA/cm². Conversely, J_c characteristics were similar to YBCO films without additions when these secondary phases formed as large, disordered phases within the film. A 2.3 µm thick film with such a distribution of secondary phases was found to have reduced self-field J_c values of 3.4 MA/cm² at 75.5 K and J_c(min, Θ, 1T) of 0.4 MA/cm².

Controllable synthesis and photoluminescence of single-crystalline BaHfO3 hollow micro- and nanospheres

This paper presents a facile hydrothermal route to synthesize monodispersive and single-crystalline BaHfO(3) hollow micro- and nanospheres in a concentrated basic environment. The hollow spheres were size tunable from submicrometer to nanoscale by simply adjusting the base concentration at a suitable temperature. The base concentration played the key role on forming BaHfO(3) hollow spheres. Detailed investigations on base concentration, reaction temperature, and duration indicated that the formation of BaHfO(3) hollow spheres was driven by Ostwald ripening process. Because of the abundance of defects, the as-prepared BaHfO(3) hollow nanospheres exhibited a blue light emission under UV-light excitation at room temperature. Calcination led to the photoluminescence declination due to the decrease of defects.