Grain Boundaries In YBa2Cu3O7 Research Articles

Influence of variable Ca-doping on the critical current density of low-angle grain boundaries in YBa2Cu3O7−d

The rapid decrease in the critical current density Jc with grain boundary (GB) misorientation angle θ strongly limits the current-carrying capability of yttrium–barium–copper-oxide (YBCO), and the residual low-angle GB distribution is the most important current-limiting mechanism in biaxially textured YBCO coated conductors. To deepen the understanding of the Ca doping in low-angle GBs in YBCO, transport characteristics of low-angle GBs in YBa2Cu3O7−d bicrystals with different Ca contents x (Y1−xCaxBa2Cu3O7−d) were examined to verify the influence of the carrier density at the GB due to the Ca doping level. Y1−xCaxBa2Cu3O7−d (x = 0, 0.10, 0.15, and 0.30) epitaxial films were deposited by pulsed laser deposition on single crystal SrTiO3 (STO) substrates and 7° and 9° symmetric [001]-tilt bicrystal STO substrates. We found that 15% Ca doping was the most effective at increasing the Jc behavior across the 7° GB in intermediate fields, completely eliminating the GB dissipation signature in the V–I characteristics. For the 9° GB, 10% Ca doping yielded the highest depairing current Jd across the GB, calculated from the flux flow resistivity, even though a significant GB dissipation signature remained in the V–I curves.

First-Principles Calculations of Electronic States and Self-Doping Effects at a 45° Grain Boundary in the High TemperatureYBa2Cu3O7Superconductor

The charge redistribution at grain boundaries determines the applicability of high-T_{c} superconductors in electronic devices because the transport across the grains can be hindered considerably. We investigate the local charge transfer and the modification of the electronic states in the vicinity of the grain-grain interface by ab initio calculations for a (normal-state) 45 degrees -tilted [001] grain boundary in YBa2Cu3O7. Our results explain the suppressed interface transport and the influence of grain boundary doping in a quantitative manner, in accordance with the experimental situation. The charge redistribution is found to be strongly inhomogeneous, which has a substantial effect on transport properties since it gives rise to a self-doping of 0.10+/-0.02 holes per Cu atom.

Enhanced current flow through meandering grain boundaries in YBa2Cu3O7−δ films

In YBa2Cu3O7−δ (YBCO) coated conductors grown by metal organic deposition, in-plane meandering of grain boundaries (GBs) has been linked to higher critical current density. The authors investigate this link in individual GBs using transport measurements and scanning Hall probe microscopy with current reconstruction. They observe current-induced flux entry into a coated conductor, then model its behavior by imaging YBCO films with single, straight GBs tilted at various angles to the applied current. They find a strong dependence of critical current on angle, sufficient to explain the enhancement observed for meandering GBs.

The normal-state resistivity of grain boundaries in YBa2Cu3O7−δ

Using an optimized bridge geometry, we have been able to make accurate measurements of the properties of YBa2Cu3O7−δ grain boundaries above Tc. The results show a strong dependence of the change of resistance with temperature on grain boundary angle. Analysis of our results in the context of band bending at the boundary allows us to estimate the height of the potential barrier present at the grain boundary interface.

XPS studies of YBa2Cu3O7/Ag high‐TC superconductor

The chemical nature of grain boundaries in YBa2Cu3O7−δ (YBC) ceramic and YBC/Ag random composites was investigated using x-ray photoemission spectroscopy carried out on in situ fractured specimens. A change in the Ba core emission level indicated a cleaning of grain boundaries when silver was present. For low silver content (5 wt.%), an increase of the formal Cu3+/Cu2+ ratio was observed and interpreted as silver incorporation in the YBC lattice. For higher silver content (20 wt.%), Ag–Cu intermetallic species were observed, resulting from reaction of the excess silver present at grain boundaries with the YBC. These chemical modifications were consistent with resistivity measurements. Copyright © 2000 John Wiley & Sons, Ltd.

The Intrinsic Formation Of Localized Electronic States At [001] Tilt Grain Boundaries In YBa2Cu3O7 And Their Effect On Jc

Abstract Grain boundaries have long been known to have a deleterious effect on the superconducting critical current that can be carried by YBa2Cu3O7-δ. Recent theoretical analyses have proposed that the origin of this behavior may be band bending, which results in the depletion of charge carriers at the grain boundaries. For this to occur in these p-type superconductors there must, by definition, be a high density of localized donor states in the boundary plane. Here we describe a structural feature intrinsic to all [001] tilt boundaries that may be the origin of these localized states. Direct atomic-resolution images of asymmetric [001] tilt grain boundary structures have been obtained using the Z-contrast imaging technique. The grain boundaries are observed to be composed of distinct structural units, as shown in figure 1. Within these structural units, a 2×1 reconstruction of the CuO columns is seen to occur. This reconstruction, which is caused by the constraint imposed on the structure by the sizes of the component atoms, leads to effective oxygen vacancies in the grain boundary plane (figure 2). Further oxygen annealing of the boundary cannot fill these vacancies as there is no space in the structure for more oxygen atoms.

Transport and structural properties of the top and bottom grain boundaries in YBa2Cu3O7−δ step-edge Josephson junctions

We present a method to study separately the electrical transport properties of the grain boundaries (GBs) formed at the top and at the bottom edges of YBa2Cu3O7−δ(YBCO) step-edge Josephson junctions. The step-edge junctions were fabricated on (100) LaAlO3 steps using tilted Ar ion milling to define the electrodes and the microbridges. Due to the shadowing effect of the step, a continuous YBCO stripe remains along and at the bottom of the step on both sides of a microbridge. We found that the top GB is responsible for the weak link behavior of our step-edge junctions. The transport properties were correlated with the different microstructural properties of the two GBs formed at the edges of the step.

Microstructure of artificial 45° [001] tilt grain boundaries in YBCO films grown on (001) MgO

It is well known that high-angle grain boundaries in YBa2Cu3O7−x (YBCO) show weak-link effects and behave as Josephson junctions. This kind of grain boundary junction (GBJ) has potential applications in magnetic field measurement and electronic devices. This work studies the microstructure of artificially made GBJs in YBCO films on (001) MgO and the mechanism of the boundary formation, with the goal to improve the GBJ quality and obtain a better understanding of the junctions’ transport properties.Ion-sputter-induced epitaxy is used to form YBCO films with isolated 45° [001] tilt grain boundaries. Prior to YBCO film growth the (001) MgO substrate is selectively sputtered by a low energy Ar ion beam. A portion of the substrate remains non-sputtered by protecting the surface with a patterned photoresist mask. After removing the mask, a YBCO film is grown on the substrate using pulsed organometallic molecular beam epitaxy (POMBE). Under suitable conditions single crystal YBCO c-axis films can be reproducibly obtained in both the sputtered and non-sputtered regions. The orientation between the films and the substrate has been examined by both x-ray diffraction and electron diffraction. The in-plane orientation relation is [110]YBCO//[100]MgO on the non-sputtered epitaxially polished MgO and [100]YBCO//[100]MgO on the sputtered MgO. Thus, 45° tilt boundaries are formed in the film at the boundary between the sputtered and non-sputtered substrate regions.

Anomalous dependence of the critical current of 45° grain boundaries in YBa2Cu3O7−x on an applied magnetic field

Grain boundaries grown by the biepitaxial technique do not show Fraunhofer-type critical current Ic vs. magnetic field Ha dependences, which are a hallmark of standard Josephson junctions. To clarify the reason for this unusual behavior, we have fabricated asymmetric 45° grain boundaries using the bicrystal technique and analyzed their Ic (Ha) characteristics. These characteristics crisply show a number of remarkable features, which suggest that due to the particular orientation of these junctions, their Ic (Ha) dependence is intrinsically non-Fraunhofer. Conventional models, relying on standard tunneling and a superconducting order parameter with s-wave symmetry, do not account for the Ic (Ha) characteristics observed.

Electrical transport properties of [001] tilt bicrystal grain boundaries in YBa2Cu3O7

The zero-field electrical transport properties of 24° [001] tilt bicrystal grain boundaries in YBa2Cu3O7 were found to be in excellent agreement with the Ambegaokar–Halperin model over an extended range of currents and voltages. This model gives a firm basis for characterizing and comparing boundaries, and provides two independent measures of the critical current, which were proportional to (1−T/Tc)2 close to the transition temperature Tc.

Biepitaxial Josephson junctions with high critical current density based on YBa2Cu3O7−δ films on silicon on sapphire

High-current-density Josephson junctions have been produced in high-temperature superconducting YBa2Cu3O7−δ films deposited on silicon on sapphire and using biepitaxial grain boundaries. The technique is estimated to be useful in integrating superconducting and semiconducting components. A multilayer system of epitaxially grown films was used to form the junctions. A double buffer layer CeO2/ZrO2(9.5% Y2O3) prevented interactions between YBa2Cu3O7−δ and Si during the high-temperature deposition and promoted formation microcrack-free films with a critical current density of 2×106 A/cm2 at 77 K. A MgO seed layer, with (001)MgO∥(001)CeO2 orientation, was used to induce a 45° crystallographic grain boundary in YBa2Cu3O7−δ at its edge. An additional epitaxial buffer double layer of YBa2Cu3O7−δ and SrTiO3 on top of the seed layer promoted the formation of a grain boundary of better crystallinity and stoichiometry. It improved the critical current of the junction about tenfold and resulted in characteristic IcRn products of 150 μV at 77 K in microbridges crossing the grain boundary. Microwave-induced steps were detected at 77 K up to voltages corresponding to the characteristic IcRn value. Peak-to-peak responses to superconducting quantum interference devices reached values of 7 μV at 77 K.

Grain-boundary constraint and oxygen deficiency in YBa2Cu3O7−δ: Application of the coincidence site lattice model to a non-cubic system

Abstract Local lattice parameters and oxygen (hole) densities in the vicinity of large-angle grain boundaries in bulk YBa2Cu3O7−δ were measured simultaneously using convergent-beay electron diffraction and electron-energy-loss spectroscopy with a less than 20 A field emission probe respectively. The oxygen (hole) contents were quantified by Gaussian fitting to the observed oxygen K-edge absorption spectra and comparing them with results for a set of standard samples with different oxygen contents. The local lattice parameters were determined by matching the simulated high-order Laue zone patterns with experimental patterns by a least-squares algorithm. We found that large-angle grain boundaries in YBa2Cu3O7−δ often exhibit volume expansion. The increase of the c-lattice parameter near the boundary core can be attributed to lattice distortion and/or to an oxygen deficiency. The lattice distortion extends less than 20 A, while the deficiency can extend more than 100 A. Comparison of the c/a ratio of the lat...

Atomic structure of grain boundaries in YBa2Cu3O7-? as observed by Z-contrast imaging

Determination of the atomic structure of grain boundaries is the key to fundamental understanding of the critical current density in polycrystalline superconductors. High-resolution images with incoherent characteristics, obtained using a high-angle annular detector on an atomic resolution scanning transmission electron microscope, are used to study the atomic arrangements of these technologically important boundaries. The incoherent Z-contrast images do not experience contrast reversals with defocus or sample thickness and display no Fresnel Fringe effects at boundaries. Observed rigid shifts of atomic columns at grain boundaries are independent of sample thickness and objective lens defocus. These characteristics allow unambiguous and intuitive interpretations of the generated images. We find the atomic structures at grain boundaries in YBa2Cu3O7-δ are strongly influenced by the strong tendency of this compound to exist only as complete unit cells terminated at {001} and {100} planes. The weak-link behavior associated with high-angle grain boundaries may follow from this structure in which there is no clear connection between the {100} facets of adjacent grains. Symmetric grain boundaries where adjacent grains share a common boundary plane have also been observed in YBa2Cu3O7-δ. In these boundaries partial structural coupling of the grains is maintained. There is evidence that these two boundary forms produce junctions with very different superconducting properties.

A geometrical rationalization of the special properties of the 14° [001] grain boundary in YBa2Cu3O7−δ

14° [001] tilt grain boundaries in YBa2Cu3O7−δ have been found to exhibit anomalously good superconducting critical currents, associated with magnetic flux pinning, although no special interfacial structure has heretofore been associated with this misorientation. We present an analysis of the geometry of the grain boundary in question and show that it can correspond to a special interface, within the definition of constrained lattice coincidence, which may lead to new understanding of the superconducting behavior of grain boundaries. Our analysis shows that the tetragonal-to-orthorhombic phase transformation may have important effects upon grain boundary structure and properties, indicating new approaches to producing high-current superconductors.

(001) grain boundaries and critical currents

Magnetization measurements and high resolution electron microscopy (HREM) have been carried out on the series Y1-xCaxBa2-xLaxCu3O7- delta to determine the correlation between critical current and (001) grain boundaries. Substituting Ca for Y reduces the fraction of (001) grain boundaries and increases the fraction of grain boundaries other than (001). The critical current increases linearly with the fraction of clean grain boundaries other than the (001) boundaries. It is concluded that the (001) grain boundaries in YBa2Cu3O7 act as weak links. It is argued that the atomic structure of the interface is responsible. In addition it is observed that the intragrain critical current decreases with increase of x.

The Morphology of Grain Boundary Interactions with Twins in YBa2Cu3O7−δ

AbstractVarious morphologies are observed where twins meet grain boundaries in YBa2Cu3O7−δ. Twins may be “correlated” at the boundary (i.e. twin boundaries from one grain may meet a twin boundary from the other grain in a quadruple junction) and the twins may be narrowed or “constricted” at the boundary. These effects are determined by the interfacial energy. We estimate the energy of the various interfaces by determining the dislocation arrays they contain, using the constrained coincidence site lattice (CCSL) model and Bollmann's O2-lattice formalism. Our approach indicates that there are significant changes in the energy of the interfaces and is thus able to explain the variety of observed morphologies.

The 45° grain boundaries in YBa2Cu3O7−δ

High-angle grain boundaries in YBa2Cu3O7−δthin films are of technological interest because of the weak coupling observed between the grains; however, not all high-angle grain boundaries show this weak-link behavior. The microstructure of both these boundaries is not understood, nor is the reason for the differing electrical transport properties. High-angle grain boundaries in YBa2Cu3O7−δthin films on MgO, where the angular misorientation between the grains is ∼45°, have been examined using high-resolution electron microscopy. The results show that the boundary structure can appear quite different even when the angular misorientation between the two grains is the same. The stability of the grain boundaries under the electron irradiation in the electron microscope was found to be a function of the accelerating voltage—400 kV leads to rapid disordering of the boundary region.

Weak links and dc SQUIDS on artificial nonsymmetric grain boundaries in YBa2Cu3O7−δ

We have studied weak links and dc–SQUIDs made from pulsed laser deposited YBa2Cu3O7−δ thin films on Y–ZrO2 bicrystal substrates. The transport properties of the weak links were studied as a function of the misorientation angle (θ) between the two halves of the bicrystal and an exponential dependence of the weak link critical current density was observed for angles up to 40° at 77 K. Josephson effects with clear microwave and magnetic field responses were observed. An optimum dc–SQUID performance at 77 K was obtained for θ=32°. At this temperature, we achieved a periodic magnetic field response with a modulation depth of 12 μV.

Atomic structure and local electrical behaviour of grain boundaries in YBa2Cu3O7−x

Abstract The influence of the grain boundaries types and of their distribution on the electrical behaviour of YBCO superconductors has been analyzed. Three kinds of materials have been considered: polycrystals, textured materials and bicrystals. The relationships between the structure and the local electrical behaviour have been investigated by combining a local electrical probe method with transmission electron microscopy.

Special grain boundaries in YBa2Cu3O7− x thin films

Grain boundaries described by rotations about the c[001] axis are discussed in terms of a near-CSL model for orthorhombic materials. Grain boundaries in an oriented YBa 2Cu 3O 7- x (YBCO) thin film grown on an (001) ZrO 2 substrate by electron-beam coevaporation of the metals have characterized by transmission electron microscopy. Bright-field and dark-field images and selected-area diffraction patterns recorded from near-Σ=1,5,13, 17, and 29 [001] tilt boundaries and near-Σ=3 and 13 [100] tilt boundaries are presented. It is also shown that low-angle grain boundaries with misoreintations near 7° [001] can be introduced at triple junctions containing two high-angle near-CSL boundaries. A table including coincidence, near-coincidence, and “low-index’ facet planes expected for near-CSL misorientations is given and some preliminary observations of the faceting behaviour of such grain boundaries are made.