High-temperature Superconductors Research Articles

Research on magnetic field distribution characteristics of 2G-HTS dynamo in superconducting wireless power supply applications

Abstract The high-temperature superconducting (HTS) dynamo injects direct current (DC) into the winging of superconducting machines through non-electrical contact, solving issues such as thermal leakage in traditional current leads and current decay due to flux motion, joint resistance and AC losses. However, it has been observed that the DC output voltage decreases with an increasing air gap between the rotor magnet and HTS stator. To increase the output of the HTS dynamo at a fixed gap, this study employs an efficient numerical model based on the equivalent current method to investigate the magnetic field distribution of the magnets with different structural parameters. The relationship between the magnetic field distribution of the rotor magnet and the open-circuit voltage of the stator is established and extensively validated against simulation modeling and experimental data. Experimental results indicate that the rotor’s magnetic field distribution and the stator’s magnetic field penetration influence the open circuit voltage of the HTS dynamo. Specifically, when the distance between adjacent magnets is large, the magnetic field penetration occurs only on both sides of the stator, causing circuit voltage to increase initially and then decrease with the magnet distance decreases. A reverse point opposite to the magnetic field direction on both sides is generated at the center of the stator when the distance decreases further, which increases the average induced current density, and suppresses the downward trend. By optimizing the magnetic field distribution of the rotor magnets on the stator, the DC output power of the dynamo can be effectively improved. This model and the results contained in this article provide a comprehensive theoretical basis for researchers to compare and optimize their own modeling and experiment of the HTS dynamo.

Signature of Superconductivity in Pressurized Trilayer-nickelate Pr4Ni3O10-δ

Abstract The discovery of high-temperature superconductivity in La3Ni2O7 and La4Ni3O10 under pressure has drawn extensive attention. Herein, we report systematic investigations on the structure, magnetism, and electrical resistance evolutions of Pr4Ni3O10-δ polycrystalline samples under various pressures. Pr4Ni3O10-δ exhibits density wave transitions on Ni and Pr sublattices at about 157.6 K and 4.3 K, respectively, and the density wave can be progressively suppressed by pressure. A structural transformation from the monoclinic P21/a space group to the tetragonal I4/mmm occurs at around 20 GPa. An apparent drop in resistance with evident magnetic field dependence is observed as pressure above 20 GPa, indicating the emergence of superconductivity. The discovery of superconductivity in Pr4Ni3O10-δ broadens the family of nickelate superconductors. Pr4Ni3O10-δ provides a new platform for investigating the mechanisms of superconductivity in the Ruddlesden–Popper phases of nickelates.

Hydrogen anion unit and high-temperature superconductivity in ternary hydride Mg3TiH12 under moderate pressures

Compressed ternary superhydrides have been the most prominent candidates for high-temperature superconductors, which usually exhibit high-temperature superconductivity at relatively low pressures owing to the synergistic effect among elements. In the current work, a series of unprecedented thermodynamically stable ternary Mg−Ti−H hydrides are identified at the pressure range of 0−200GPa. The electron-phonon coupling calculations suggest that four H-rich phases, namely, Pmn21-MgTiH6, R3m-Mg3TiH12, Pmmn-MgTiH6, and Immm-Mg2TiH9, are potential high-temperature superconductors. In particular, R3m-Mg3TiH12 possesses the highest superconducting transition temperature of 71.185K at megabar pressure, which stems from the high-frequency H-derived phonon modes and the high contribution of the hydrogen at the Fermi level. The electronic properties researches reveal that Pmn21-MgTiH6, R3m-Mg3TiH12, Pmmn-MgTiH6, and Immm-Mg2TiH9 are all metallic and the hydrogens in these structures mainly exist in the form of the H− entity because a large amount of charge transfers from metal atom to H atom.

FPGA-Stabilized Magnetic Levitation of the APEX-LD High-Temperature Superconducting Coil

FPGA-Stabilized Magnetic Levitation of the APEX-LD High-Temperature Superconducting Coil

Superconducting DC busbar with low resistive joints for all-electric aircraft propulsion system

High-temperature superconductors (HTS) can carry high currents with almost zero loss when transmitting direct current (DC). Their compact size and lower weight make them suitable for the application of all-electric aircraft. However, the current carrying capability of a single HTS tape is limited to a few hundred amps; therefore, for high-current applications, multiple HTS tapes need to be connected in parallel. The flat geometry of HTS tape and its critical current (IC) dependence on strain complicate grouping them in parallel. Furthermore, the length of HTS tape is limited by its crystal structure, necessitating low-resistance joints for extended applications. A superconducting busbar design for high-current applications is developed and tested to address these challenges. The superconducting busbar is designed in a way that it helps to reduce the effect of the self-field on critical current and also ride through the fault events. Yttrium barium copper oxide (YBCO) tapes are used to develop the busbar prototype, tested against DC currents in a liquid nitrogen environment. Joint optimization is carried out to determine the required length for efficiently joining HTS tapes. Two busbar prototypes are developed with 180° and 90° joints to join 5 HTS tapes and tested in self-field. A joint resistance of 100 nΩ is measured at self-field for the 180° joint busbar, and 800 nΩ is measured for the 90° joint busbar. Both busbar prototypes are subjected to power cycling and thermal cycling to assess joint performance in self-field and any degradation of the joint electrical parameters during testing.

Density-wave-like gap evolution in La3Ni2O7 under high pressure revealed by ultrafast optical spectroscopy

Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La3Ni2O7. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La3Ni2O7 single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidences of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La3Ni2O7.

Gigantic-oxidative atomic-layer-by-layer epitaxy for artificially designed complex oxides

Abstract In designing material functionalities for transition metal oxides, lattice structure and d-orbital occupancy are key determinants. However, the modulation of these two factors is inherently limited by the need to balance thermodynamic stability, growth kinetics, and stoichiometry precision, particularly for metastable phases. We introduce a methodology, namely the gigantic-oxidative atomic-layer-by-layer epitaxy (GOALL-Epitaxy), enhancing oxidation power 3-4 orders of magnitude beyond conventional pulsed laser deposition (PLD) and oxide molecular beam epitaxy (OMBE), while ensuring atomic-layer-by-layer growth of designed complex structures. Thermodynamic stability is markedly augmented with stronger oxidation at elevated temperatures, whereas growth kinetics is sustained by laser ablation at lower temperatures. We demonstrate the accurate growth of complex nickelates and cuprates, especially an artificially designed structure with alternating single and double NiO2 layers possessing distinct nominal d-orbital occupancy, as a parent of high-temperature superconductor. The GOALL-Epitaxy enables material discovery within the vastly broadened growth parameter space.

ITER and TRT—Technological Platforms for Controlled Thermonuclear Fusion

To solve the main problems of designing a thermonuclear tokamak reactor, such as the experimental demonstration of quasi-stationary thermonuclear burning, generation of non-inductive quasi-stationary current; development of plasma technologies and materials of the first wall and divertor, the International Thermonuclear Experimental Reactor ITER is being designed, projects of DEMO demonstration reactors are being developed, and a Tokamak with Reactor Technologies TRT is being developed in Russia. The main components of the ITER (superconducting electromagnetic system (EMS) made of Nb3Sn and NbTi, the first wall of W coated with a low-Z material, systems for additional plasma heating, experimental modules of a breeder blanket, plasma control systems, etc.) and TRT (EMS of high-temperature superconductors, first wall options of W with B4C coating, TiB2–AlN composite and liquid metal lithium, additional heating and quasi-stationary non-inductive current drive systems, innovative divertor, experimental breeder and hybrid blanket modules, reactor-compatible diagnostics and remote plasma control systems, etc.) technology platforms are presented. The technological platforms of the ITER being under construction and the TRT being designed contain an almost complete, according to modern understanding, set of technologies for the future thermonuclear reactor.

The Electron–Phonon Interaction in Non-Stoichiometric Bi2Sr2CaCu2O8+δ Superconductor Obtained from the Diffuse Elastic Scattering of Helium Atoms

Previously, helium atom scattering (HAS) has been shown to probe the electron–phonon interaction at conducting crystal surfaces via the temperature dependence of the specular peak intensity. This method is now extended to non-stoichiometric superconductors. The electron–phonon interaction, as expressed by the mass-enhancement factor λ, is derived from the temperature dependence of the diffuse elastic scattering intensity, which specifically depends on the non-stoichiometric component responsible for superconductivity. The measured value of the mass-enhancement factor for Bi2Sr2CaCu2O8+δ at the optimal doping δ = 0.16 is λ = 0.55 ± 0.08 is in good agreement with values of λ recently estimated with other methods. This also confirms the relevant role of electron–phonon interaction in high-temperature non-stoichiometric cuprate superconductors.

Comparison of Commercial REBCO Tapes Through Flux Pinning Energy

This work presents a comparison of different commercial tapes belonging to the second-generation High-Temperature Superconductors (2G HTS) produced by SuNAM Co., Ltd., SuperOx, and Shanghai Superconductors Technology Co., Ltd. (SST) companies. The aim is to investigate pinning mechanisms responsible for best performances, looking at the anisotropy of the irreversibility field and of the flux pinning energy. The irreversibility line states the upper limit of current-carrying capacity, whereas the flux pinning energy explores the ability of material defects to act as weak collectively or strong single vortex pinning centers. All investigated samples have artificial pinning centers (APCs) included in the superconducting matrix: BHO-doped EuBCO for SST, Y2O3 in YBCO for SuperOx, and Gd2O3 particles trapped in GdBCO for SuNAM. Resistive transition curves were measured in high magnetic fields up to 16 T for magnetic field orientations parallel and perpendicular to the tape surface. We found that the anistropy of SST tape shows an overall independence both on temperature and magnetic field, while the other two samples show a more complex behavior. This leads to the conclusion that properly engineered APC optimization in coated conductors can further reduce anisotropy of superconducting properties.

The synchronous measurement of mechanical and magnetic characteristics of superconducting materials in extreme environments

Thermomagnetic instability typically occurs before the quench of high temperature superconductors. Magnetic perturbation and thermal agitation are known to be the main driving force of thermomagnetic instability. However, the interaction between mechanical deformation and the thermomagnetic instability is still unclear. To investigate the phenomenon and inner principle of the thermomagnetic instability induced by mechanical deformation, an experimental method is proposed for the in-situ, real-time, and synchronous test of mechanical deformation and magnetic flux based on digital imaging correlation (DIC) and magneto-optical imaging (MOI). Under the current-carrying and external magnetic field conditions, uniaxial tensile tests of YBa2Cu3O7-δ (YBCO) coated conductors (CCs) are carried out, in which the evolution and distribution magnetic flux induced by strain are studied. At the same time, chemical etching is adopted to explore the damage caused by mechanical deformation in YBCO layer. It is found through the experiment that the mechanical deformation can induce thermomagnetic instability, and threshold of strain for inducing flux motions is obtained. Meanwhile, magnetic flux avalanche occurs in front end of the flux penetration area in the case of current carrying. In addition, plenty of various-sizes transverse cracks are discovered in the superconducting layer whose distribution area basically coincides with the flux penetration area. The experiment results reveal the intrinsic correlation between the mechanical deformation and thermomagnetic instability of high temperature superconducting wires, which provides a direct experimental approach for the study of unpredicted quench behaviors of superconducting magnets.

Dynamic charge order from strong correlations in the cuprates

Charge order has been a central focus in the study of cuprate high-temperature superconductors due to its intriguing yet not fully understood connection to superconductivity. Recent advances in resonant inelastic x-ray scattering (RIXS) in the soft x-ray regime have enabled the first momentum-resolved studies of dynamic charge order correlations in the cuprates. This progress has opened a window for a more nuanced investigation into the mechanisms behind the formation of charge order (CO) correlations. This review provides an overview of RIXS-based measurements of dynamic CO correlations in various cuprate materials. It specifically focuses on electron-doped cuprates and Bi-based hole-doped cuprates, where the CO-related RIXS signals may reveal signatures of the effective Coulomb interactions. This aims to explore a connection between two central phenomena in the cuprates: strong Coulomb correlations and CO-forming tendencies. Finally, we discuss current open questions and potential directions for future RIXS studies as the technique continues to improve and mature, along with other probes of dynamic correlations that would provide a more comprehensive picture.

Synergistic Effects of BaTiO3 and MFe2O4 (M = Mn, Ni, Cu, Zn, and Co) Nanoparticles as Artificial Pinning Centers on the Performance of YBa2Cu3Oy Superconductor.

Large-scale superconductor applications necessitate a superconducting matrix with pinning sites (PSs) that immobilize vortices at elevated temperatures and magnetic fields. While previous works focused on the single addition of nanoparticles, the simultaneous inclusion of different nanoparticles into a superconducting matrix can be an effective way to achieve an improved flux pinning capacity. The purpose of this study is to explore the influence of mixed-nanoparticle pinning, with the co-addition of non-magnetic (BaTiO3; BT) and various types of magnetic spinel ferrite (MFe2O4, abbreviated as MFO, where M = Mn, Co, Cu, Zn, and Ni) nanoparticles, on the superconductivity and flux pinning performances of the high-temperature superconductor YBa2Cu3Oy (YBCO). An analysis of X-Ray diffraction (XRD) data of BT-MFe2O4-co-added YBCO samples showed the formation of an orthorhombic structure with Pmmm symmetry. According to electrical resistivity measurements, the emergence of the superconducting state below Tcoffset (zero-resistivity temperature) was proven for all samples. The highest Tcoffset value was recorded for the Y-BT-MnFO sample, while the minimum value was obtained for the Y-BT-ZnFO sample. Direct current (DC) magnetization results showed good magnetic flux pinning performance for all the co-added samples compared to the pristine sample but with some discrepancies. At 77 K, the values of the self-critical current density (self-Jcm) and maximum pinning force (Fpmax) for the Y-BT-MnFO sample were found to be eight times higher and seventeen times greater than those for the pristine sample, respectively. The results acquired suggested that mixing the BT phase with an appropriate type of spinel ferrite nanoparticles can be a practical solution to the problem of degradation of the critical current density of the YBCO material.

Performance evaluation of a racetrack high temperature superconductor winding in an HTS levitation platform

Abstract High temperature superconducting (HTS) is a key technology for next generation high-speed railway systems, which guarantees high current and highly efficient traction power supplies. HTS devices can reduce system weight and thus improve loads of vehicles. In high-speed maglev trains, HTS windings wound by second-generation HTS tapes play an important role in suspension and traction. However, in complex ferromagnetic environments, the performance of windings is notably influenced by the varying magnetic fields. In this paper, a new maglev platform is designed and comprised of a winding of six racetrack HTS coils, an E-shaped iron core and a magnetic guide rail. The air core structure of the winding is modeled and tested and the electromagnetic characteristics are analyzed for three dimensional finite element analysis of the platform. An A formulation is used to calculate magnetic vectors A and infer magnetic flux densities and current densities for the structures. After the winding is installed on the iron core and the magnetic guide rail is suspended above the iron core, the air gap between the iron core and the rail is varied from 0 mm to 20 mm in models and experiments. The magnetic fields, critical currents and losses of the winding in the cases of the air core and the iron core conditions are used for analysis. The comparison between the air core and the iron core conditions confirms the reliability of the proposed models. This study provides a means for the analysis of complex superconducting maglev systems.

Minimal lateral damage fabrication of high-temperature superconducting nanowires via focused helium ion beam irradiation

Abstract Low-temperature superconducting nanowire single-photon detectors have become a key infrared photon counting technology in communication and astronomy applications. However, the constrained physical space of devices demands high-performance superconducting detectors capable of operation at higher temperatures. To date, high-temperature superconductor nanowires still face seriously uneven lateral damage in the ion etching process during fabrication. In this work, we report a promising fabrication method for high-temperature superconducting YBa2Cu3O7−x (YBCO) nanowires, using a focused helium ion beam to minimize the lateral damage of the cut. Based on simulations, we designed tangent circles and adjacent isosceles triangles to replace lines in cutting nanowires to reduce the superimposed damage by He+ ions. The lateral damage of a single helium ion cut has been reduced with a decrease in superimposed damage width from 58.8 nm to 29.7 nm. This work provides a platform for boosting YBCO nanowires to achieve single photon detection.

Semi-analytical modeling AC loss of a flat stack of Y-Ba-Cu-O tapes

We propose semi-analytical models to compute alternating current (AC) power loss in a stack of N high-temperature superconductor YBa2Cu3O7−x (or Y–Ba–Cu–O) tapes subjected to a time-varying magnetic field perpendicular to the tapes with zero transport current. The models take into account screening of the interior superconducting tapes of the stack from the external magnetic field. We validate the results by experiments carried out at temperature T=77.2K under an applied magnetic field with the amplitude of its induction Bext=0.57T and frequencies up to 110 Hz. As follows from our models, the AC loss per tape in stacks of N tapes decreases with N in agreement with experiments. The approach is extended to compute the AC loss for lower temperatures, larger magnetic fields strengths, and for frequencies up to several kHz. These studies are important for understanding and predicting the AC loss for contemporary motors and generators.

Flux jumps and mechanical effects in high-temperature superconductors with multi-field coupling model

Mechanical deformation during the magnetization process of high-temperature superconductors can lead to the deterioration in their critical current. A multi-field coupling model involving four physical fields is established to investigate the role of mechanical strain in the flux jump of the superconducting bulk. The electrical, magnetic, thermal, and mechanical coupling behavior is presented and analyzed under different applied fields. The simulation results of the coupled model are compared with experimental data, which showing an agreement. The numerical results indicate that in the superconductors, mechanical deformation significantly affects the electrical, magnetic, and thermal properties owing to a reduction in the critical current density under mechanical strain. The moment and location of magnetic flux jump will change as the mechanical strain is considered. Interestingly, the occurrence of magnetic flux jumps during field-cooling magnetization leads to a decrease in stress in bulk superconductor, suggesting that the fracture may not be caused by flux jumps, which is contrary to common understanding. Moreover, the temperature changes uniformly at different locations within the superconducting bulk. In the extended study, the thermo-magnetic and mechanical properties of the bulk material under ZFCM are presented using the coupled model. The effect of mechanics on the flux jump differs between FCM and ZFCM, which is attributed to the nonlinear changes in the electric field.

Superconductivity in the pressurized nickelate La3Ni2O7 in the vicinity of a BEC–BCS crossover

Ever since the discovery of high-temperature superconductivity in cuprates, gaining microscopic insights into the nature of pairing in strongly correlated systems has remained one of the greatest challenges in modern condensed matter physics. Following recent experiments reporting superconductivity in the bilayer nickelate La3Ni2O7 (LNO) with remarkably high critical temperatures of Tc = 80 K, it has been argued that the low-energy physics of LNO can be described by the strongly correlated, mixed-dimensional bilayer t–J model. Here we investigate this bilayer system and utilize density matrix renormalization group techniques to establish a thorough understanding of the model and the magnetically induced pairing through comparison to the perturbative limit of dominating inter-layer spin couplings. In particular, this allows us to explain appearing finite-size effects, firmly establishing the existence of long-range superconducting order in the thermodynamic limit. By analyzing binding energies, we predict a BEC–BCS crossover as a function of the Hamiltonian parameters. We find large binding energies of the order of the inter-layer coupling that suggest strikingly high critical temperatures of the Berezinskii–Kosterlitz–Thouless transition, raising the question of whether (mixD) bilayer superconductors possibly facilitate critical temperatures above room temperature.

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

An analytical model for predicting the magnetization loss in HTS sector-shaped conductors for fusion

Abstract Within the framework of magnetic confinement fusion, several projects worldwide are demonstrating the possibility of integrating high-temperature superconductors (HTS) in the coil systems. HTS-based technologies are highly attractive for practical applications because they can extend the operating margins of fusion coils in terms of higher temperatures, transport currents and magnetic fields. Based on the results achieved with the twisted-stacked tape cable, we have designed a novel low-loss HTS sector cable-in-conduit conductor, with a target of 60 kA at 4.5 k, 18 T, which is presently of interest for the DEMO Central Solenoid coil. In HTS cables, the AC losses can represent a significant limiting factor, therefore they must be taken into consideration both in the design phase and in the assessment of the overall magnet thermal budget. In this work, to assess the loss behavior and to optimize the cable design, we have explored different aspect ratios and arrangements of the stacked tapes within the cable layout. The magnetization losses are calculated with a 2D finite-element model based on the T-A formulation and analytical approximations based on the Brandt-Halse critical state model. Specifically, we have developed an analytical formulation that allows for the calculation of the instantaneous power losses in HTS stacked cables with a limited number of tapes per stack, achieving sufficient accuracy at high fields. The analytical model enables a sufficiently accurate assessment of the heat deposited on the conductor during those particular instants of a plasma scenario where the variation of the field is very high, such as during the critical initial discharge period of the plasma initiation.