Interfacial Modulation of Spin-Orbit Torques Induced by Two-Dimensional van der Waals Material ZrSe3

Two-dimensional van der Waals (2D vdW) materials are widely used in spin-orbit torque (SOT) devices. Recent studies have demonstrated the low crystal symmetry and large spin Hall conductivity of 2D vdW ZrSe3, indicating its potential applications in low-power SOT devices. Here, we study the interfacial contribution of SOTs and current-induced magnetization switching in the ZrSe3/Py (Ni80Fe20) and ZrSe3/Cu/Py heterostructures. SOT efficiencies of samples are detected by the spin-torque ferromagnetic resonance (ST-FMR), and out-of-plane damping-like torque (τB) is observed. The ratio between τB and the field-like torque (τA) decreases from 0.175 to 0.138 when inserting 1 nm Cu at the interface and then drops to 0.001 when the thickness of Cu intercalation is 2 nm, indicating that Cu intercalation inhibits the τB component of SOT. Moreover, the SOT efficiency is increased from 3.05 to 5.21, which may be attributed to the Cu intercalation being beneficial to improve the interface between Py and ZrSe3. Theoretical calculation has shown that the Cu spacer can change the conductivity of ZrSe3 from semiconductor to conductor, thereby decreasing the Schottky barrier and increasing the transmission efficiency of the spin current. Furthermore, magneto-optical Kerr effect (MOKE) microscopy is employed to verify the current-driven magnetization switching in these structures. In comparison to the ZrSe3/Py bilayer, the critical current density of ZrSe3/Cu/Py is reduced when inserting 1 nm Cu, demonstrating the higher SOT efficiency and lower power consumption in ZrSe3/Cu/Py structures.

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.

Correlating the Microstructure and Current Density of the Li/Garnet Interface

Solid-state lithium batteries hold great promise for next-generation energy storage systems. However, the formation of lithium filaments within the solid electrolyte remains a critical challenge. In this study, we investigate the crucial role of morphology in determining the resistance of garnet-type electrolytes to lithium filaments. By proposing a new test method, namely, cyclic linear sweep voltammetry, we can effectively evaluate the electrolyte resistance against lithium filaments. Our findings reveal a strong correlation between the microscopic morphology of the solid electrolyte and its resistance to lithium filaments. Samples with reduced pores and multiple grain boundaries demonstrate remarkable performance, achieving a critical current density of up to 3.2 mA cm-2 and excellent long-term cycling stability. Kelvin probe force microscopy and finite element method simulation results shed light on the impact of grain boundaries and electrolyte pores on lithium-ion transport and filament propagation. To inhibit lithium penetration, minimizing pores and achieving a uniform morphology with small grains and plenty of grain boundaries are essential.

Superconducting In Situ/Post In Situ MgB2 Joints

The superconducting joints of superconducting in situ MgB2 wires have been of great interest since the first MgB2 wires were manufactured. The necessity of joining fully reacted wires in applications such as NMR brings complexity to the methodology of connecting already reacted wires sintered under optimised conditions via a mixture of Mg + 2B and subsequential second heat treatment to establish fully superconducting MgB2 joints. Some of the data in the literature resolved such a procedure by applying high cold pressure and sintering at a low temperature. A topical review publication did not address in depth the question of whether cold sintering is a potential solution, suggesting that hot pressing is the way forward. In this paper, we discuss the potential joint interfacial requirements, suggesting a thermo-mechanical procedure to successfully form a superconductive connection of two in situ reacted wires in the presence of Mg + 2B flux. The critical current at 25 K of the researched junction achieved 50% Ic for an individual in situ wire.

Robust and Intimate Interface Enabled by Silicon Carbide as an Additive to Anodes for Lithium Metal Solid-State Batteries

Garnet-type solid-state electrolytes are among the most reassuring candidates for the development of solid-state lithium metal batteries (SSLMB) because of their wide electrochemical stability window and chemical feasibility with lithium. However, issues such as poor physical contact with Li metal tend to limit their practical applications. These problems were addressed using β-SiC as an additive to the Li anode, resulting in improved wettability over Li6.75 La3 Zr1.75 Ta0.25 O12 (LLZTO) and establishing an improved interfacial contact. At the Li-SiC|LLZTO interface, intimacy was induced by a lithiophilic Li4 SiO4 phase, whereas robustness was attained through the hard SiC phase. The optimized Li-SiC|LLZTO|Li-SiC symmetric cell displayed a low interfacial impedance of 10 Ω cm2 and superior cycling stability at varying current densities up to 5800 h. Moreover, the modified interface could achieve a high critical current density of 4.6 mA cm-2 at room temperature and cycling stability of 1000 h at 3.5 mA cm-2 . The use of mechanically superior materials such as SiC as additives for the preparation of a composite anode may serve as a new strategy for robust garnet-based SSLMB.

Titanium-Oxygen Clusters Brazing Li with Li6.5La3Zr1.5Ta0.5O12 for High-Performance All-Solid-State Li Batteries

Garnet-based solid-state electrolytes are considered crucial candidates for solid-state Li batteries due to their high Li+ conductivity and nonflammability; however, poor interfacial contact with the Li anode and growth of Li dendrites limit their application. Herein, a high-activity titanium-oxygen cluster is used as a brazing filler to braze the Li6.5La3Zr1.5Ta0.5O12 (LLZTO) with an Li anode into the whole unit. The brazing layer leads to a significantly lower interfacial impedance of 8.32 Ω cm2. Furthermore, the brazing layer is an isotropic amorphous ion-electron hybrid conductive layer, which significantly promotes Li+ transport and regulates the distribution of the electric field, therefore inhibiting the growth of Li dendrites. The cell exhibits an ultrahigh critical current density of 2.3 mA cm-2 and stable cycling of over 4000 h at 0.5 mA cm-2 (25 °C).

Defining optimal thickness for maximal self-fieldJcin YBCO/CeO2multilayers grown on buffered metal

The effect of multilayering YBa2Cu3O6+x(YBCO) thin films with sequentially deposited CeO2layers between YBCO layers grown on buffered metallic template is investigated to optimize the self-field critical current densityJc(0). We have obtained that the improvement inJc(0)clearly depends on the YBCO layer thickness and temperature, where at high temperatureJc(0)can be increased even 50% when compared with the single layer YBCO films. Based on our experimental results and theoretical approach to the growth mechanism during multilayer deposition, we have defined a critical thickness for the YBCO layer, where the maximal self-fieldJc(0)is strongly related to the competing issues between the uniform and nonuniform strain relaxation and the formation of dislocations and other defects during the film growth. Our results can be directly utilized in the future coated conductor technology, when maximizing the overall in-fieldJc(B)by combining both the optimal crystalline quality and flux pinning properties typically in bilayer film structures.

High-Pressure Synthesis and the Enhancement of the Superconducting Properties of FeSe0.5Te0.5

A series of FeSe0.5Te0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >104 A/cm2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor.

Superconducting State Properties of CuBa2Ca3Cu4O10+δ

The superconducting state properties of the CuBa₂Ca₃Cu₄O_10+δ (Cu-1234) system, with a transition temperature as high as 117.5 K, were investigated. The ac magnetic susceptibility measurements confirmed a very sharp transition to the superconducting state. The upper critical field, H_c2, as high as 91 T, and the irreversibility field, H_irr, as high as 21 T at 77 K, were determined using dc SQUID magnetization measurements. The intragrain critical current density, j_c, estimated from a magnetic hysteresis loop, is as high as 5 × 10⁹ A/m² in a self-generated magnetic field at 77 K. However, the intergrain critical current density in the studied material is smaller by four orders of magnitude due to very weak intergrain connections.

The Performance of the Two-Seeded GdBCO Superconductor Bulk with the Buffer by the Modified TSMG Method

The multiseeding technique is a method to grow large-sized REBa₂Cu₃O_7-δ (REBCO, where RE is a rare earth element) high temperature superconducting bulks. However, due to the existence of grain boundaries between seed crystals, the superconducting properties of bulks are not always better than those of single grain bulks. In order to improve the superconducting properties caused by grain boundaries, we introduced buffer layers with a diameter of 6 mm in the growth of GdBCO bulks. Using the modified top-seeded melt texture growth method (TSMG), that is, YBa₂Cu₃O_7-δ (Y123) as the liquid phase source, two GdBCO superconducting bulks with buffer layers with a diameter of 25 mm and a thickness of 12 mm were successfully prepared. The seed crystal arrangement of two GdBCO bulks with a distance of 12 mm were (100/100) and (110/110), respectively. The trapped field of the GdBCO superconductor bulks exhibited two peaks. The maximum peaks of superconductor bulk S_A (100/100) were 0.30 T and 0.23 T, and the maximum peaks of superconductor bulk S_B (110/110) were 0.35 T and 0.29 T. The critical transition temperature remained between 94 K and 96 K, with superior superconducting properties. The maximum J_{C, self-field} of S_A appeared in specimen b5, which was 4.5 × 10⁴ A/cm². Compared with S_A, the J_C value of S_B had obvious advantages in a low magnetic field, medium magnetic field and high magnetic field. The maximum J_{C, self-field} value appeared in specimen b2, which was 4.65 × 10⁴ A/cm². At the same time, it showed an obvious second peak effect, which was attributed to Gd/Ba substitution. Liquid phase source Y123 increased the concentration of the Gd solute dissolved from Gd211 particles, reduced the size of Gd211 particles and optimized J_C. For S_A and S_B under the joint action of the buffer and the Y123 liquid source, except for the contribution of Gd211 particles to be the magnetic flux pinning center with the improvement of J_C, the pores also played a positive role in improving the local J_C. More residual melts and impurity phases were observed in S_A than in S_B, which had a negative impact on the superconducting properties. Thus, S_B exhibited a better trapped field and J_C.

The Influence of Preparation Temperature on the Different Facets of Bulk MgB2 Superconductors

Two MgB₂ samples were prepared using the spark plasma sintering (SPS) technique at different temperatures-950 °C (S1) and 975 °C (S2)-for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB₂ samples. We analyzed the superconducting properties of the PeF and PaF of two MgB₂ samples prepared at different temperatures from the curves of the critical temperature (T_C), the curves of critical current density (J_C), the microstructures of MgB₂ samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, T_c,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher J_C compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h₀ and K_n of the PeF were lower than those of the PaF, except for K_n of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (J_C) was 503 kA/cm² self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the J_C of MgB₂. However, in high field, S2-PeF had the highest J_C value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher J_C.

Cometal Addition Effect on Superconducting Properties and Granular Behaviours of Polycrystalline FeSe0.5Te0.5

The enhanced performance of superconducting FeSe_0.5Te_0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe_0.5Te_0.5 (FeSe_0.5Te_0.5 + xPb + ySn; x = y = 0-0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (T_c^onset) of pure FeSe_0.5Te_0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce T_c^onset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe_0.5Te_0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, J_c, and the pinning force, F_p, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.

Critical Current Density in d-Wave Hubbard Superconductors

In this work, the Generalized Hubbard Model on a square lattice is applied to evaluate the electrical current density of high critical temperature d-wave superconductors with a set of Hamiltonian parameters allowing them to reach critical temperatures close to 100 K. The appropriate set of Hamiltonian parameters permits us to apply our model to real materials, finding a good quantitative fit with important macroscopic superconducting properties such as the critical superconducting temperature (Tc) and the critical current density (Jc). We propose that much as in a dispersive medium, in which the velocity of electrons can be estimated by the gradient of the dispersion relation ∇ε(k), the electron velocity is proportional to ∇E(k) in the superconducting state (where E(k)=(ε(k)-μ)2+Δ2(k) is the dispersion relation of the quasiparticles, and k is the electron wave vector). This considers the change of ε(k) with respect to the chemical potential (μ) and the formation of pairs that gives rise to an excitation energy gap Δ(k) in the electron density of states across the Fermi level. When ε(k)=μ at the Fermi surface (FS), only the term for the energy gap remains, whose magnitude reflects the strength of the pairing interaction. Under these conditions, we have found that the d-wave symmetry of the pairing interaction leads to a maximum critical current density in the vicinity of the antinodal k-space direction (π,0) of approximately 1.407236×108 A/cm², with a much greater current density along the nodal direction (π2,π2) of 2.214702×109 A/cm². These results allow for the establishment of a maximum limit for the critical current density that could be attained by a d-wave superconductor.

Grain Boundary Engineering in Ta-Doped Garnet-Type Electrolyte for Lithium Dendrite Suppression

Solid-state lithium batteries (SSLBs) based on Ta-doped Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) suffer from lithium dendrite growth, which hinders their practical application. Herein, first principles simulations indicate that the Ta element prefers to segregate along grain boundaries in the form of Ta₂O₅ precipitates due to a high energy difference induced by Ta doping. Grain boundary engineering is employed to regulate the distribution of the Ta element and enhance the density of LLZTO by introducing the La₂O₃ additive. The sufficient La₂O₃ additive reacts with the Ta₂O₅ precipitates, while the residual La₂O₃ nanoparticles fill up void defects, promoting the homogeneous distribution of the Ta element and improving the relative density to ∼98%. Critical current density of the symmetric Li battery reaches 2.12 mA·cm^-2 at room temperature with the solid-state electrolyte (LLZTO + 5 wt % La₂O₃), which increases by 41% compared to pure LLZTO. SSLBs with the LiFePO₄ cathode achieve a stable cycling performance with a discharge capacity of 138.6 mA·h·g^-1 after 400 cycles at 0.2 C. This work provides theoretical insights into the distribution of Ta-doped LLZTO and inhibits lithium dendrite growth through grain boundary engineering.

Grain Boundary Engineering Enabled High-Performance Garnet-Type Electrolyte for Lithium Dendrite Free Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) are attracting increasing attentions as one of the promising next-generation technologies due to their high-safety and high-energy density. Their practical application, however, is hindered by lithium dendrite growth and propagation in solid-state electrolytes (SSEs). Herein, an in situ grain boundary modification strategy relying on the reaction between Li₂ TiO₃ (LTO) and Ta-substituted garnet-type electrolyte (LLZT) is developed, which forms LaTiO₃ along with lesser amounts of LTO/Li₂ ZrO₃ at the grain boundaries (GBs). The second phases of LTO/Li₂ ZrO₃ inhibit abnormal grain growth. The presence of LaTiO₃ at the GBs reduces electronic conductivity and improves mechanical strength, which can hinder dendrite formation and block lithium dendrite penetration through the LLZT. Moreover, the adjacent grains by LaTiO₃ build a continuous Li⁺ transport path, providing a homogeneous Li⁺ flux throughout the whole LLZT-4LTO. As a result, symmetric cells of Li | LLZT-4LTO | Li shows a high critical current density of 1.8 mA cm^-2 and a long cycling stability up to 2000 h at 0.3 mA cm^-2 . Moreover, the high-voltage full cells demonstrate remarkable cycling stability and rate performance. It is believed that this novel grain boundary modification strategy can shed light on the constructing of high-performance SSEs for practical SSLMBs.

Pinning Potential of the Self-Assembled Artificial Pinning Centers in Nanostructured YBa2Cu3O7-x Superconducting Films

For high-field power applications of high-temperature superconductors, it became obvious in recent years that nano-engineered artificial pinning centers are needed for increasing the critical current and pinning potential. As opposed to the artificial pinning centers obtained by irradiation with various particles, which is a quite expensive approach, we have studied superconducting samples having self-assembled defects, created during the sample fabrication, that act as effective pinning centers. We introduced a simple, straight-forward method of estimating the frequency-dependent critical current density by using frequency-dependent AC susceptibility measurements, in fixed temperatures and DC magnetic fields, from the positions of the maxima in the dependence of the out-of-phase susceptibility on the amplitude of AC excitation magnetic field. The results are compatible with a model that stipulates a logarithmic dependence of the pinning potential on the probing current. A mathematical derivation allowed us to estimate from the experimental data the pinning potentials in various samples, and in various DC magnetic fields. The resulted values indicate large pinning potentials, leading to very small probability of magnetic flux escaping the pinning wells, hence, leading to very high critical currents in high magnetic fields.

Excellent Li/Garnet Interface Wettability Achieved by Porous Hard Carbon Layer for Solid State Li Metal Battery

Garnet-type Li_6.4 La₃ Zr_1.4 Ta_0.6 O₁₂ (LLZTO) electrolyte is considered as a promising solid electrolyte because of its relatively high ionic conductivity and excellent electrochemical stability. The surface contamination layer and poor Li/LLZTO interface contact cause large interfacial resistance and quick Li dendrite growth. In this paper, a porous hard carbon layer is introduced by the carbonization of a mixed layer of phenolic resin and polyvinyl butyral on the LLZTO surface to improve Li/garnet interfacial wettability. The multi-level pore structure of the hard carbon interlayer provides capillary force and large specific surface area, which, together with the chemical activity of the carbon material with Li, promote the molten Li infiltration with garnet electrolyte. The Li/LLZTO interface delivers a low interfacial resistance of 4.7 Ω∙cm² at 40 °C and a higher critical current density, which can achieve stable Li⁺ conduction for over 800 h under current densities of 0.1 and 0.2 mA∙cm^-2 . The solid-state battery coupled with Li and LiFePO₄ exhibits excellent rate and cycling performance, demonstrating the application feasibility of the hard carbon interlayer for a solid state Li metal battery.

Efficient Anion Fluoride-Doping Strategy to Enhance the Performance in Garnet-Type Solid Electrolyte Li7La3Zr2O12

Garnet-type solid-state electrolyte Li₇La₃Zr₂O₁₂ (LLZO) is expected to realize the next generation of high-energy-density lithium-ion batteries. However, the severe dendrite penetration at the pores and grain boundaries inside the solid electrolyte hinders the practical application of LLZO. Here, it is reported that the desirable quality and dense garnet Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20 can be obtained by fluoride anion doping, which can effectively facilitate grain nucleation and refine the grain; thereby, the ionic conductivity increased to 7.45 × 10^-4 at 30 °C and the relative density reached to 95.4%. At the same time, we introduced a transition layer to build the Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t electrolyte in order to supply a stable contact; as a result, the interface resistance of Li|Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t decreases to 12.8 Ω cm². The Li|Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t|Li symmetric cell achieved a critical current density of 1.0 mA cm^-2 at 25 °C, which could run stably for 1000 h without a short circuit at 0.3 mA cm^-2 and 25 °C. Moreover, the Li|LiFePO₄ battery exhibited a high Coulombic efficiency (>99.5%), an excellent rate capability, and a great capacity retention (123.7 mA h g^-1, ≈80%) over 500 cycles at 0.3C and 25 °C. The Li|LiNi_0.8Co_0.1Mn_0.1O₂ cell operated well at 0.2C and 25 °C and delivered a high initial discharge capacity of 151.4 mA h g^-1 with a good capacity retention (70%) after 195 cycles. This work demonstrates that the anion doping in LLZO is an effective method to prepare a dense garnet ceramic for the high-performance lithium batteries.

Improving Superconducting Performance of Fe(Se, Te) with In Situ Formed Grain-Boundary Strengthening and Flux Pinning Centers

It is well known that the existence of interstitial Fe is a great obstacle to enhancing the superconducting properties of the Fe(Se, Te) system. In this work, a silver and oxygen codoping effect toward enhancement of the superconductivity and flux pinning in Fe(Se, Te) bulks is reported. The oxygen ions from SeO₂ can induce the precipitation of interstitial Fe as Fe₂O₃, thus simultaneously optimizing the superconducting properties of Fe(Se, Te) and forming extra flux pinning centers, while the existence of Ag can enhance the intergrain connections of the polycrystalline material by improving the electron transport at grain boundaries. Compared with the undoped sample, the critical current density, the upper critical field, and the thermally activated flux flow activation energy are greatly enhanced by 4.7, 1.7, and 1.5 times, respectively. The novel synthesis technique and optimized properties of this work can pave the way for the development of high-performance Fe(Se, Te) superconducting wires or tapes.

Controllable Domain Walls in Two-Dimensional Ferromagnetic Material Fe3GeTe2 Based on the Spin-Transfer Torque Effect

Recently, two-dimensional magnetic material has attracted attention worldwide due to its potential application in magnetic memory devices. The previous concept of domain walls driven by current pulses is a disordered motion. Further investigation of the mechanism is urgently lacking. Here, Fe₃GeTe₂, a typical high-Curie temperature (T_C) two-dimensional magnetic material, is chosen to explore the magnetic domain dynamics by in situ Lorentz transmission electron microscopy experiments. It has been found that the stripe domain could be driven, compressed, and expanded by the pulses with a critical current density. Revealed by micromagnetic simulations, all the domain walls cannot move synchronously due to the competition between demagnetization energy and spin-transfer torque effect. In consideration of the reflection of high-frequency pulses, the disordered motion could be well explained together. The multiple stable states of the magnetic structure due to the weak exchange interaction in a two-dimensional magnet provides complex dynamic processes. Based on plenty of experiments, a cluster of domain walls could be more steady and move more synchronously under the drive of pulse current. The complication of domain wall motions presents a challenge in race track memory devices and two-dimensional magnetic material will be a better choice for application research.

Synthesis of Dense MgB2 Superconductor via In Situ and Ex Situ Spark Plasma Sintering Method

In this study, high-density magnesium diboride (MgB₂) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (J_c-B) in MgB₂ bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and J_c-B using two methods, ex situ (sintering MgB₂ synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB₄ and MgO were found to be effective in improving the J_c-B characteristics. In the ex situ method, the degradation of MgB₂ due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the J_c-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and J_c-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (f_P) analysis revealed the possibility of refined MgO inclusions and MgB₄ phase as new pinning centers, which greatly contributed to the J_c-B properties. The contributions of the sintering conditions on f_P for both synthesis methods were analyzed.

The Defect Chemistry of Carbon Frameworks for Regulating the Lithium Nucleation and Growth Behaviors in Lithium Metal Anodes

Carbon materials have been widely considered as the frameworks in lithium (Li) metal anodes due to their lightweight, high electrical conductivity, and large specific surface area. Various heteroatom-doping strategies have been developed to enhance the lithiophilicity of carbon frameworks, thus rendering a uniform Li nucleation in working Li metal batteries. The corresponding lithiophilicity chemistry of doping sites has been comprehensively probed. However, various defects are inevitably introduced into carbon materials during synthesis and their critical role in regulating Li nucleation and growth behaviors is less understood. In this contribution, the defect chemistry of carbon materials in Li metal anodes is investigated through first-principles calculations. The binding energy towards a Li atom and the critical current density are two key descriptors to reveal the defect chemistry of carbon materials. Consequently, a diagram of designing carbon frameworks with both high lithiophilicity and a large critical current density is built, from which the Stone-Wales defect is predicted to possess the best performance for delivering a uniform Li deposition. This work uncovers the defect chemistry of carbon frameworks and affords fruitful insights into defect engineering for achieving dendrite-free Li metal anodes.

Effect of Heat Treatments under High Isostatic Pressure on the Transport Critical Current Density at 4.2 K and 20 K in Doped and Undoped MgB2 Wires

Annealing undoped MgB₂ wires under high isostatic pressure (HIP) increases transport critical current density (J_tc) by 10% at 4.2 K in range magnetic fields from 4 T to 12 T and significantly increases J_tc by 25% in range magnetic fields from 2 T to 4 T and does not increase J_tc above 4 T at 20 K. Further research shows that a large amount of 10% SiC admixture and thermal treatment under a high isostatic pressure of 1 GPa significantly increases the J_tc by 40% at 4.2 K in magnetic fields above 6 T and reduces J_tc by one order at 20 K in MgB₂ wires. Additionally, our research showed that heat treatment under high isostatic pressure is more evident in wires with smaller diameters, as it greatly increases the density of MgB₂ material and the number of connections between grains compared to MgB₂ wires with larger diameters, but only during the Mg solid-state reaction. In addition, our study indicates that smaller wire diameters and high isostatic pressure do not lead to a higher density of MgB₂ material and more connections between grains during the liquid-state Mg reaction.

Influence of Amorphous Boron Grain Size, High Isostatic Pressure, Annealing Temperature, and Filling Density of Unreacted Material on Structure, Critical Parameters, N-Value, and Engineering Critical Current Density in Mgb2 Wires

Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (B_irr), critical temperature (T_c), upper magnetic field (B_c2), engineered critical current density (J_ec), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on B_irr, T_c, B_c2, J_ec, and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest B_irr, T_c, B_c2, J_ec, and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB₂.

Enhanced Performance of Li6.4La3Zr1.4Ta0.6O12 Solid Electrolyte by the Regulation of Grain and Grain Boundary Phases

The application of Li-ion conducting garnet electrolytes is challenged by their large interfacial resistance with the metallic lithium anode and the relative small critical current density at which the lithium dendrites short-circuit the battery. Both of these challenges are closely related to the morphology and the structure of the garnet membranes. Here, we prepared four polycrystalline garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) pellets with different particle sizes (nano/micro) and grain boundary additive (with/without Al₂O₃) to investigate the influence of grain size, the composition of the grain boundary, and the mechanical strength of the pellet on the total Li-ion conduction of the pellet, Li/garnet interfacial transfer, and lithium dendrite growth in all-solid-state Li-metal cells. The results showed that the garnet pellets prepared with nanoparticles and LiAlO₂-related grain boundary phase had decreased total Li-ion conductivity because of the increased resistance of the grain boundary; however, these pellets showed higher mechanical strength and improved capability to suppress lithium dendrite growth at high current densities. By controlling the grain size and optimizing the grain boundary with Al₂O₃ sintering additive, the hot-pressing sintered LLZTO solid electrolytes can reach up to 1.01 × 10^-3 S cm^-1 in Li⁺ conductivity and 0.29 eV in activation energy. LLZTO with nanosized grain and LiAlO₂-modified grain boundary showed the highest critical current density, which is 0.6 mA cm^-2 at room temperature and 1.7 mA cm^-2 at 60 °C. This study offers a useful guideline for preparing a high-performance LLZTO solid electrolyte.

Improved Ionic Conductivity and Li Dendrite Suppression Capability toward Li7P3S11-Based Solid Electrolytes Triggered by Nb and O Cosubstitution

It is still a big challenge to simultaneously enhance the ionic conductivity, dendrite suppression capability, and interfacial compatibility of sulfide solid electrolytes. In this work, a novel Li₇P_2.88Nb_0.12S_10.7O_0.3 solid electrolyte is prepared via Nb and O cosubstitution of glass-ceramic Li₇P₃S₁₁. This sulfide-based electrolyte possesses a high ionic conductivity (3.59 mS cm^-1) at 298 K, improved critical current density (1.16 mA cm^-2), and excellent interfacial compatibility between the sulfide electrolyte and Li₂S active material. The improved electrochemical stability of the sulfide solid electrolyte against metallic lithium is attributed to the formation of Nb and Li₂O at the interface, which can induce uniform Li deposition and prevent further side reaction. The all-solid-state Li/Li₂S batteries based on this electrolyte exhibit remarkably enhanced cycling stability and rate performance.

Densified Li6PS5Cl Nanorods with High Ionic Conductivity and Improved Critical Current Density for All-Solid-State Lithium Batteries

Solid electrolytes are receiving great interest owing to their good mechanical properties and high lithium-ion transference number, which could potentially suppress lithium dendrites. However, lithium dendrites can still penetrate solid electrolytes even at low current densities. In this work, a flat-surface Li₆PS₅Cl nanorod pellet with high density is achieved, which exhibits an ionic conductivity as high as 6.11 mS cm^-1 at 25 °C. The flat surface of the pellet is beneficial for the homogeneous lithium deposition, and the dense pellet microstructure can suppress the growth of lithium dendrites along the grain boundaries, leading to a significantly improved critical current density of 1.05 mA cm^-2 at 25 °C. The resultant dense Li₆PS₅Cl pellet is further employed in a LiCoO₂/Li₆PS₅Cl/Li all-solid-state lithium battery, showing an initial discharge capacity of 115.3 mAh g^-1 at 1C (0.35 mA cm^-2, 25 °C) with a capacity retention of 80.3% after 100 cycles.

Carbon-Coating Layers on Boron Generated High Critical Current Density in MgB2 Superconductor

Boron particles with a homogeneous carbon-coating layer were employed as the precursor to fabricate MgB₂ superconductors to generate artificial two-dimensional (2D) flux-pinning centers. Systematic microstructure investigation reveals that the carbon layers are well-distributed in the MgB₂ matrix without agglomeration. The thickness of the carbon layers is smaller than the MgB₂ coherent length, which makes them transparent to supercurrent. The critical current density is increased because of the strong flux-pinning effects of the 2D carbon layers in the superconductor as highly efficient flux-pinning centers and the increased irreversibility field due to the carbon-doping effects.

High Performance Bi-2212 Round Wires Made with Recent Powders

Multifilamentary Bi₂Sr₂CaCu₂O_x (Bi-2212) wire made by the powder-in-tube technique is the only high temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J _C ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and a very promising conductor for very high field NMR and accelerator magnets. We studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J _C (4.2 K, 15 T) = 6640 A/mm² and J _C (4.2 K, 30 T) = 4670 A/mm², which correspond to engineering critical current densities J _E (4.2 K, 15 T) = 1320 A/mm² and J _E (4.2 K, 30 T) = 930 A/mm².

Extrinsic Two-Dimensional Flux Pinning Centers in MgB2 Superconductors Induced by Graphene-Coated Boron

Extrinsic two-dimensional flux pinning centers, via graphene-encapsulated boron powder as precursors, have been introduced into MgB₂ superconductors by means of in situ and diffusion sintering methods. Uniform graphene encapsulation of the boron powders was achieved by the hydrothermal method with highly dispersed graphene oxide as the precursor. The graphene coating layers induce remaining graphene layers and other defects acting as flux pinning centers in the matrix as well as improved connectivity in between grains. The increased critical current density ( J_c) is attributed to the enhanced flux pinning force and improved connectivity. Two-dimensional flux pinning centers provided by thin graphene layers and grain boundaries in MgB₂ possess high flux pinning efficiency without suppressing the connectivity of the MgB₂ superconductor.

Effects of filament size on critical current density in overpressure processed Bi-2212 round wire

Bi₂Sr₂CaCu₂O_x (Bi-2212) conductor is the only high temperature superconductor manufactured as a round wire and is a very promising conductor for very high field applications. One of the key design parameters of Bi-2212 wire is its filament size, which has been previously reported to affect the critical current density (J_c ) and ac losses. Work with 1 bar heat treatment showed that the optimal filament diameter was about 15 μm but it was not well understood at that time that gas bubbles were the main current limiting mechanism. Here we investigated a recent Bi-2212 wire with a 121×18 filament architecture with varying wire diameter (1.0 to 1.5 mm) using 50 bar overpressure processing. This wire is part of a 1.2 km piece length of 1.0 mm diameter made by Oxford Superconducting Technology. We found that J_c is independent of the filament size in the range from 9 to 14 μm, although the n value increased with increasing filament size. A new record J_c (4.2 K, 15 T) of 4200 A/mm² and J_E (4.2 K, 15 T) of 830 A/mm² were achieved.

Architecture and Transport Properties of Multifilamentary MgB2 Strands for MRI and Low AC Loss Applications

Standard in-situ type MgB2 strands manufactured by Hyper Tech Inc have 19 - 36 subelements, a monel outer sheath, and a Cu interfilamentary matrix. Typical transport Jc s of the strands are 2×105 A/cm2 with n-values of 20 - 30 at 4.2 K and 5 T. This work introduces two new MgB2 conductor designs. First, a new class of MgB2 strand is designed for magnetic resonance imaging applications. This type has a higher Cu content designed to enhance protection of a magnet wound with it, and a larger diameter to increase the critical current. Second, a new class of low AC loss MgB2 strand with high filament count and a high resistance matrix is discussed. Transport properties at 4.2 K and fields up to 10 T are reported. Optical techniques are used to study the macro- and micro-structures of these MgB2 strands.

Flux avalanche in a superconducting film with non-uniform critical current density

The flux avalanche in type-II superconducting thin film is numerically simulated in this paper. We mainly consider the effect of non-uniform critical current density on the thermomagnetic stability. The nonlinear electromagnetic constitutive relation of the superconductor is adopted. Then, Maxwell's equations and heat diffusion equation are numerically solved by the fast Fourier transform technique. We find that the non-uniform critical current density can remarkably affect the behaviour of the flux avalanche. The external magnetic field ramp rate and the environmental temperature have been taken into account. The results are compared with a film with uniform critical current density. The flux avalanche first appears at the interface where the critical current density is discontinuous. Under the same environmental temperature or magnetic field, the flux avalanche occurs more easily for the film with the non-uniform critical current density. The avalanche structure is a finger-like pattern rather than a dendritic structure at low environmental temperatures.

Internally oxidized Nb₃Sn strands with fine grain size and high critical current density

Nb3Sn strands fabricated using Nb-Zr alloy can be internally oxidized, provided that oxygen is properly supplied via an oxide powder. This allows the formation of fine intragranular and intergranular ZrO2 particles in a Nb3Sn matrix. These particles can refine the grain size by a factor of three and thereby greatly enhance the Nb3Sn critical current density.

Photochromism-induced amplification of critical current density in superconducting boron-doped diamond with an azobenzene molecular layer

A key issue in molecular electronics is the control of electronic states by optical stimuli, which enables fast and high-density data storage and temporal-spatial control over molecular processes. In this article, we report preparation of a photoswitchable superconductor using a heavily boron-doped diamond (BDD) with a photochromic azobenzene (AZ) molecular layer. BDDs electrode properties allow for electrochemical immobilization, followed by copper(I)-catalyzed alkyne-azide cycloaddition (a "click" reaction). Superconducting properties were examined with magnetic and electrical transport measurements, such as field-dependent isothermal magnetization, temperature-dependent resistance, and the low-temperature voltage-current response. These measurements revealed reversible amplification of the critical current density by 55% upon photoisomerization. This effect is explained as the reversible photoisomerization of AZ inducing an inhomogeneous electron distribution along the BDD surface that renormalizes the surface pinning contribution to the critical current.

Iron chalcogenide superconductors at high magnetic fields

Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.

'Beautiful' unconventional synthesis and processing technologies of superconductors and some other materials

Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of 'beautiful' technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches.

Form-function relations in cone-tipped stimulating microelectrodes

Metal microelectrodes are widely used in neuroscience research, and could potentially replace macroelectrodes in various neuro-stimulation applications where their small size, specificity, and their ability to also measure unit activity are desirable. The design of stimulating microelectrodes for specific applications requires knowledge on how tip geometry affects function, but several fundamental aspects of this relationship are not yet well understood. This study uses a combined experimental and physical finite elements simulation approach to formulate three new relationships between the geometrical and electrical properties of stimulating cone-tipped tungsten microelectrodes: (1) The empirical relationship between microelectrode 1-kHz impedance and the exposed tip surface area is best approximated by an inverse square-root function (as expected for a cone-tipped resistive interface). (2) Tip angle plays a major role in determining current distribution along the tip, and as a consequence crucially affects the charge injection capacity of a microelectrode. (3) The critical current for the onset of corrosion is independent of tip surface area in sharp microelectrodes.

On the influence of magnetic field processing on the texture, phase assemblage and properties of low aspect ratio Bi2Sr2CaCu2O x /AgMg wire

Bi₂Sr₂CaCu₂O _x /AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth.