REBa2Cu3O7-x (REBCO, RE: rare earth) coated conductors (CCs) suffer from great critical current Ic differences between different manufacturers, Ic variations within individual manufacturers, and often significant lengthwise fluctuations. The understanding of such variations is complicated by the lack of a direct correlation between Ic and the critical current density Jc. In fact, although Jc is the fundamental property determined by the local vortex pinning landscape, Ic is often limited by variable current blocking mechanisms. An important practical complexity is commercial practices of performing Jc and Ic evaluations based on Ic at 77 K and self-field (sf), where connectivity variation dominates over vortex pinning variations. However, at higher fields and lower temperatures, vortex pinning becomes more complex and highly variable, making predictions of Ic and Jc at arbitrary temperature T, magnetic field H, and field orientation theta, quite uncertain. To address some aspects of this problem, we conducted detailed spool-to-spool performance characterization on recently manufactured REBCO CCs. Despite Jc(77 K, sf) varies by only similar to 11%, Jc(77 K, 1 T) of its minimum and maximum (for H//ab-plane) show variations of similar to 21% and similar to 32%, respectively. This emphasizes the shortcoming in using Jc(77 K, sf) as parameter for evaluating even the low field performance. An even more remarkable spool-to-spool Jc variation of similar to 68% was observed at 20 K and 15 T for H//ab-plane. To identify the origin of such lack of reproducibility we performed microstructural characterizations, which revealed, within the REBCO layer, large variation in the density of copper oxide (CuOx) particles ranging from 0.1 to 2 mu m in size. We believe that they play a decisive role in reducing the effective cross-section of the REBCO layer by not simply blocking current themselves, but also by nucleating off-axis REBCO grains, whose misoriented grain boundaries adversely impact REBCO grain-to-grain connectivity. The REBCO growth associated with high density CuOx particles also leads to the more disordered spatial arrangement of BaHfO3 precipitate arrays, which, when self-aligning along the ab-planes, generate stronger pinning enhancing Jc(H//ab) at all temperatures. In this way, we established that variations of both connectivity and vortex pinning are thus directly coupled. Our results also explain why the so-called 'lift-factor', typically defined by the ratio between Ic(T, H) and Ic(77 K, sf), frequently turns out to be unreliable.

The emergent antiferromagnetic insulating phase of SrIr1-xSnxO3 has functional spintronic properties but is critically sensitive to the substitution concentration x, which is difficult to precisely tune in thin films grown by conventional pulsed laser deposition (PLD). We demonstrate the efficient composition control of epitaxial SrIr1-xSnxO3 thin films on SrTiO3(001) using a dual-beam PLD technique, co-ablating SrIrO3 and SrSnO3 targets. By controlling the relative beam intensity, we achieve wide-range tuning of x (from similar to 0.15 to similar to 0.45), which is estimated from using the out-of-plane lattice parameter c. This substitution control is confirmed by a systematic evolution of the magnetic and transport properties, including a monotonic increase in resistivity and a dome-like evolution of the Neel temperature and remnant magnetization. This work establishes dual-beam PLD as an efficient method for substitution control in iridates with the lattice parameter as a reliable indicator of the property evolution.

We systematically investigated the growth of Bi2Ru2O7 thin films on a Y-stabilized ZrO2(111) substrate using pulsed laser deposition by mapping the influence of growth temperature and oxygen partial pressure on phase stability, lattice parameters, and cation ratio. The results show that the epitaxial stabilization requires a minimum growth temperature, which is rather insensitive to the pressure. Meanwhile, the Bi:Ru ratio decreases when increasing growth temperature or decreasing pressure. By constructing the temperature-pressure phase diagram, an optimal growth window within the epitaxial phase was established. On the other hand, the electrical resistivity remains at a similar level within the epitaxial phase with only subtle changes to the temperature dependence, indicative of the robustness of the conductivity against composition variation. Our study provides a foundation for future investigations on thin films and heterostructures that utilize Bi2Ru2O7.

The determination of how the phase of the superconducting order parameter in a superconductor varies with the spatial direction, which can be done only through Josephson-effect-based phase-sensitive measurements, is crucial for the establishment of the precise pairing symmetry of the superconductor. So far, such measurements have been done on high-Tc cuprate superconductors but only in a couple of directions for Sr2RuO4 because of the difficulty in preparing Josephson junctions between Sr2RuO4 and an s-wave superconductor with a chosen mutual orientation. Another long-standing issue in Sr2RuO4, which was shown previously to feature a spontaneously broken time-reversal symmetry by muon spin rotation and other measurements, is that the expected presence of chiral surface currents, domains, and domain walls is yet to be explicitly shown experimentally. To address these issues, we have long sought the preparation of high-quality Josephson junctions between Sr2RuO4 and a conventional s-wave with a controllable orientation relative to the symmetry axes in Sr2RuO4. We report in this article the successful fabrication of ramp Josephson junctions of Al/Ti/Sr2RuO4 on thin single crystals of Sr2RuO4 obtained by mechanical exfoliation. These junctions were found to show high-quality quantum oscillations consistent with a single-domain Josephson coupling. The normal junction resistance was found to depend extremely sensitively on the supercurrent flowing in the Sr2RuO4 crystal on which the Josephson junction was made. This finding was used in the present work to provide an estimate of the size of the chiral surface current, which is shown to agree with its upper bound established previously.

The REBCO coated conductor has the potential to be widely used in ultrahigh field magnets. It is well known, however, that it is not mechanically strong against delamination in the direction normal to its surface due to its intrinsic layered structure. Therefore, conductor delamination is one of the major design challenges for REBCO magnet coils. As a part of the development of the 40 T all-superconducting magnet at the National High Magnetic Field Laboratory, USA (NHMFL), a dry-wound resistive-insulation-nested-coils (RINC) was designed to reach 25.8 T. It used surface-treated stainless-steel tape as a co-wind to control the turn-to-turn contact resistance, and was fabricated and tested in a liquid helium bath. During the test, two of the double pancake modules exhibited resistive transitions at a current significantly lower than the designed value. The postmortem inspection of the REBCO conductor of these modules by reel-to-reel magnetization at 77 K found sections of very low critical current. Further investigations of one section by chemical etching, visual inspection, and electron microscopy revealed that conductor of this section was delaminated. We present the detailed findings of these postmortem characterizations. The implication of this type of delamination for future magnet designs will be discussed.

The emergent antiferromagnetic insulating phase of SrIr1−xSnxO3 has functional spintronic properties but is critically sensitive to the substitution concentration x, which is difficult to precisely tune in thin films grown by conventional pulsed laser deposition (PLD). We demonstrate the efficient composition control of epitaxial SrIr1−xSnxO3 thin films on SrTiO3(001) using a dual-beam PLD technique, co-ablating SrIrO3 and SrSnO3 targets. By controlling the relative beam intensity, we achieve wide-range tuning of x (from ∼0.15 to ∼0.45), which is estimated from using the out-of-plane lattice parameter c. This substitution control is confirmed by a systematic evolution of the magnetic and transport properties, including a monotonic increase in resistivity and a dome-like evolution of the Néel temperature and remnant magnetization. This work establishes dual-beam PLD as an efficient method for substitution control in iridates with the lattice parameter as a reliable indicator of the property evolution.

Comprehensive understanding is needed in order for the commercial pulsed laser deposited REBa2Cu3O7-delta (RE: rare earth) coated conductors to be effectively used in the magnet and cable designs. Here we demonstrate the superconducting property characterization and microstructural analysis to show how different or similar the 3 commercial REBCO coated conductors are at present. Our work shows that the ab-plane of REBCO tilts from the coated conductor tape plane, leading to the offset of critical current density peak from the tape plane. In particular, the offset angle varies from less than 0.5 degrees for one coated conductor to close to 5 degrees for another. RE2O3 nanoparticles are dominant pins at 20 K and 15 T. The weak pinning introduced by RE2O3 particles and cation disorders contribute to critical current density at 20 K as well.

Cu matrix composites, because of their high mechanical strength, are often used as conductors in high-performance electrical applications. These composites are manufactured through thermomechanical processing, which introduces a high density of particles that act as obstacles to dislocation motion. Increasing the density of these particles enhances the mechanical strength of the conductors, which we tested under static loading. Under cyclic loading, especially pulsed electrical mechanical loading, conductors may soften, harden, or even fail. Failure is likely to occur whenever the applied stress exceeds the flow stress of the conductors. Understanding and predicting the performance of conductors under cyclic loading can help researchers estimate the lifespan of any apparatus made from these conductors. The performance of conductors depends on whether the strengthening particles are characterized by ionic interatomic bonding or metallic bonding. During fabrication, we observed both the accumulation of dislocations and the dissolution of particles (which added more solute atoms to the matrix). Because both dislocations and solute atoms tend to migrate at room temperature or higher, the complexity of microstructure changes increases in composites under cyclic loading. To minimize such complexity, we designed our test to determine fatigue properties at 77 K. We subjected the conductors to cyclic fatigue tests using a load-controlled mode (the mode most commonly used in applications). This work sheds light on the correlation between tensile properties and fatigue properties in our composite conductors. We found that the correlation varied, depending on whether the conductors had been strengthened by ionic bond or metallic bond particles.

The large anomalous Hall conductivity (AHC) of the Fe3(Ge, Ga)Te2 compounds has attracted considerable attention. Here, we expose the intrinsic nature of the AHC in Fe3GaTe2 crystals characterized by high conductivities, which show disorder-independent AHC with a pronounced value sigma xc y approximate to 420 Q-1cm-1. In the low-conductivity regime, we observe the scaling relation Qxy oc axx1.6, which crosses over to Qxy sigma xc y as axx increases. Disorder in low-conductivity crystals is confirmed by the broadening of a first-order transition between ferromagnetism and the ferrimagnetic ground state. Through density functional theory (DFT) calculations, we reveal that the dominant sources of Berry curvature are located a few hundred meV below the Fermi energy around the P point. Therefore, Fe3GaTe2 clearly exposes the disorder-induced crossover among distinct AHC regimes, previously inferred from measurements on different ferromagnets located on either side of the crossover region.

The spontaneous breaking of time-reversal symmetry (TRS), one of the hallmarks of unconventional superconductivity, has been observed in the superconducting state of Sr2RuO4 by muon spin rotation in several independent studies. However, the chiral edge current expected in such a superconductor has not yet been established experimentally. In addition, the angle dependence of the phase of the superconducting order parameter (OP) in Sr2RuO4, which would enable determination of the full symmetry properties of the OP, has been determined only for a couple of angles. Both issues can be addressed by preparing high-quality Josephson junctions between Sr2RuO4 and a conventional s-wave superconductor with varying orientations relative to the crystal axes. Here we report the successful fabrication of ramp Josephson junctions of Al/Ti/ Sr2RuO4 on thin single crystals of Sr2RuO4 obtained by mechanical exfoliation. These junctions exhibit high-quality quantum oscillations as a function of magnetic field. Moreover, the junction resistance was found to be extremely sensitive to the current flowing in the Sr2RuO4 crystal, a feature that was used in this work to show that the chiral edge current. The approach to the chiral edge current detection, which was not used previously, was verified by a control experiment.

The layered compound Fe3GaTe2 is attracting attention due to its high Curie temperature, low dimensionality, and the presence of topological spin textures above room temperature, making Fe3GaTe2 a good candidate for applications in spintronics. Here, we show, through transmission electron microscopy (TEM) techniques, that Fe3GaTe2 single crystals break local inversion symmetry while maintaining global inversion symmetry according to X-ray diffraction. Coupled to the observation of Néel skyrmions via Lorentz-TEM, our structural analysis provides a convincing explanation for their presence in centrosymmetric materials. Magnetization measurements as a function of the temperature display a sharp first-order thermodynamic phase-transition leading to a reduction in the magnetic moment. This implies that the ground state of Fe3GaTe2 is globally ferrimagnetic and not a glassy magnetic state composed of ferrimagnetic, and ferromagnetic domains as previously claimed. Neutron diffraction studies indicate that the ferromagnetic-to-ferrimagnetic transition upon reducing the external magnetic field might be associated with a change in the magnetic configuration/coupling between Fe1 and Fe2 moments. We observe a clear correlation between the hysteresis observed in both the skyrmion density and the magnetization of Fe3GaTe2. This indicates that its topological spin textures are affected by the development of ferrimagnetism upon cooling. Observation, via magnetic force microscopy, of magnetic bubbles at the magnetic phase boundary suggests skyrmions stabilized by the competition among magnetic phases and distinct exchange interactions. Our study provides an explanation for the observation of Néel skyrmions in centrosymmetric systems while exposing a correlation between the distinct magnetic phases of Fe3GaTe2 and its topological spin textures.

REBCO coated conductor has very promising applications as the conductor for high field magnets for particle accelerators and nuclear fusion reactors. One of its main failure modes is the delamination of its multilayer structure. Despite the decade-long efforts by the superconducting magnet community, the microscopic understanding of the delamination and the methods of mitigation are still lacking. This work presents the transmission electron microscopy (TEM) investigation of the microstructures of the Superpower Inc. REBCO coated conductor to correlate with its delamination strength. We looked into the microscopic cause of the buffer layer delamination of low delamination strength tape. We found that the low delamination strength tape primarily delaminated at MgO/Y2O3 interface, and it was due to nano-voids formed at the Y2O3 top surface adjacent to the IBAD MgO layer. Higher density nano-voids would reduce the MgO and Y2O3 contact area resulting in a weaker bonded interface. The dislocations with extra planes inserted at the REBCO/LaMnO3 interface also resulted in a weaker REBCO/ LaMnO3 interface.

While the spin-ice state of bulk pyrochlores such as Dy_{2}Ti_{2}O_{7} and Ho_{2}Ti_{2}O_{7} has been extensively studied in the past several decades due to its unique degenerate ground state and emergent monopole excitation, whether it survives in the thin-film form remains a mystery. The limited volume of the thin-film sample makes it challenging to study the intrinsic magnetic properties. Here, we synthesized 18-nm-thick Dy_{2}Ti_{2}O_{7} thin film on yttria-stabilized zirconia with 9.5  mol% Y_{2}O_{3} substrate and capped it by a thin conductive Bi_{2}Ir_{2}O_{7} layer and performed the proximitized magnetoresistance measurements. Our Letter found that the ice-rule-breaking phase transition survives but with a modified effective nearest-neighbor interaction (J_{eff}=1.054  K) and distorted Ising spin axes (ε=+0.051) compared to the bulk crystal. The results are supported by the simultaneously measured capacitive torque magnetometry. Our Letter demonstrates that proximitized transport is an effective tool for thin films of insulating frustrated magnets.

Ferromagnetism is an essential ingredient for anomalous Hall effect. SrRuO3 is a representative ferromagnetic oxide that exhibits anomalous Hall effect even down to the monolayer confinement limit. Paramagnetic metal SrIrO3, on the other hand, becomes an antiferromagnetic insulator when confined to a monolayer. Here we show that, by forming a confined bilayer structure of SrRuO3 and SrIrO3 monolayers with SrTiO3 spacers, the anomalous Hall effect is significantly enhanced while the ferromagnetism as well as the perpendicular magnetic anisotropy are suppressed. These effects originate from interfacial Ru-Ir hybridization that modifies the electronic structure in the vicinity of the Fermi level. Our work demonstrates that confined bilayer heterostructure is an useful design for exploring the synergistic combination of interfacial coupling and quantum confinement of complex oxides.

Polycrystalline Mn5SiC was synthesized by using a high-temperature solid-state method. Mn5SiC adopts a polar space group (Cmc21) with six crystallographic Mn sites confirmed by X-ray and neutron diffraction, transmission electron microscopy, and second harmonic generation experiments. The complex crystal structure features edge-sharing trigonal prisms and icosahedra, as well as face/edge-sharing pentagonal prisms. Magnetic measurements indicate ferrimagnetic ordering with a transition temperature of 284 K. The ferrimagnetic structure (magnetic space group Cm'c'21) was further identified by powder neutron diffraction, where collinear Mn spins align along the crystallographic c-axis. The refined magnetic moment for each crystallographic Mn site at 4 K is 1.8(2), -2.42(9), -1.72(8), 0.51(6), 0.50(4), and 1.7(2) μB. Density functional theory calculations confirm both the metallic behavior and the ferrimagnetic structure observed experimentally and further provide insight into the observed Mn moment dependence across crystallographic sites. The resistivity and specific heat measurements and density functional theory calculations reveal a substantially large Kadowaki-Woods ratio of 5 × 10-5 μΩ·cm/(mJ/mol)2 and a many-body renormalization factor of 5.5, indicating the unusual heavy Fermion behavior in such an itinerant magnetic metal.

Remote epitaxy, in which an epitaxial relation is established between a film and a substrate through remote interactions, enables the development of high-quality single crystalline epilayers and their transfer to and integration with other technologically crucial substates1,2. It is commonly believed that in remote epitaxy, the distance within which the remote interaction can play a leading part in the epitaxial process is less than 1 nm, as the atomically resolved fluctuating electric potential decays very rapidly to a negligible value after a few atomic distances3. Here we show that it is possible to achieve remote epitaxy when the epilayer-substrate distance is as large as 2-7 nm. We experimentally demonstrate long-distance remote epitaxy of CsPbBr3 film on an NaCl substrate, KCl film on a KCl substrate and ZnO microrods on GaN, and show that a dislocation in the GaN substrate exists immediately below every remotely epitaxial ZnO microrod. These findings indicate that remote epitaxy could be designed and engineered by means of harnessing defect-mediated long-distance remote interactions.

In REBCO current leads, it is important to minimize the thermal conduction while maintain stable electrical conduction. Therefore, thermal transport property of REBCO tapes need to be characterized. We measured thermal conductivity of REBCO tapes in the longitudinal direction at 4.2 - 200 K. Samples with Ag, Ag-3at%Au and Cu stabilizers of various thicknesses were measured. The residual-resistance-ratio (RRR) of these stabilizers were also measured and correlated with thermal conductivity. For samples with 10 µm or thicker Cu stabilizer (50 µm substrate), thermal conductivity is dominated by the Cu contribution. The sample with Ag-3at%Au stabilizer has significantly lower thermal conductivity than that with Ag stabilizer. It is concluded that REBCO with Ag-3at%Au stabilizer is promising for current lead applications.

Halide perovskites is a new class of semiconductors with exceptional optoelectronic properties. Among many advantages offered by halide perovskites, the bandgap energy can be tuned in a much broader range than what was possible in conventional semiconductors. This was commonly achieved in previous research by mixing different species of halides into solid solutions. The tuned bandgap using this method, however, often underwent an energy shift under optical or electrical stimuli due to halide segregation. In this work, we discovered an alternative approach to achieve super-bandgap electroluminescence from CsPbBr3. The peak energy of the light emission can be 0.7 eV higher than the reported bandgap energy. Evidence pointed to the radiative recombination at the perovskite-PEDOT:PSS interface being responsible for the unexpected blueshift of electroluminescence. We speculated that perovskite nanocrystals were formed therein and produced higher-energy photons due to quantum confinement. The results suggested an alternative strategy to manipulate and stabilize the color of electroluminescence and achieve particularly blue emission in perovskite-based LEDs.

We synthesized four Fe(67_X)Cr31CoXSi2 (X = 9, 14, 19, and 24 at.%) medium-entropy alloys and investigated the effect of Co on the connection between magnetic properties and spinodal structure. When the value of X was increased from 9 to 14 at.%, both the volume fraction of alpha 1 phase and the composition difference between alpha 1 and alpha 2 phases increased, leading to an increase in coercivity. This increase also resulted in an increase in the size of alpha 1 phase, leading to a decrease in remanence. When the value of X was increased beyond 14 at.%, both coercivity and remanence were significantly enhanced, even though the volume fraction of the alpha 1 phase remained unchanged. We attribute this enhancement to (1) intensification of the composition fluctuation in the spinodal structure, (2) increase in lattice misfits, and (3) refinement of the alpha 1 phase. Our results will be valuable in future magnet design.