Grain Boundaries In high-Tc Superconductors Research Articles

Critical Current of Low-Angle Grain Boundaries in High-Tc Superconductors

Dependence of the critical current on the misorientation angle in high-temperature superconductor (001) tilt bicrystal is theoretically examined. We suppose that in the case of relatively small values of the bicrystal misorientation angle θ (θ ≤ 10÷15 0 ) the critical current is determined by depinning of vortices, which are locked by edge dislocations aligned along the bicrystal grain boundary. Dependence of the depinning critical current on the misorientation angle is calculated for this case and it reveals a good agreement with experimental data obtained on Y-Ba-Cu-O bicrystals with (001) tilt low-angle grain boundaries.

Properties of grain boundaries in high-Tc superconductors – Notes on a recent presentation

The purpose of this article is to discuss a view concerning key datasets of the properties of grain boundaries in high-Tc superconductors that was recently expressed in Ref. [R. Gross, Physica C 432 (2005) 105]. The reference also criticizes our previous research. Using examples, I disprove this criticism.

Enhanced current transport at grain boundaries in high-Tc superconductors

Large-scale applications of high-transition-temperature (high-T(c)) superconductors, such as their use in superconducting cables, are impeded by the fact that polycrystalline materials (the only practical option) support significantly lower current densities than single crystals. The superconducting critical current density (J(c)) across a grain boundary drops exponentially if the misorientation angle exceeds 2 degrees -7 degrees. Grain texturing reduces the average misorientation angle, but problems persist. Adding impurities (such as Ca in YBa2Cu3O7-delta; YBCO) leads to increased J(c) (refs 9, 10), which is generally attributed to excess holes introduced by Ca2+ substituting for Y3+ (ref. 11). However, a comprehensive physical model for the role of grain boundaries and Ca doping has remained elusive. Here we report calculations, imaging and spectroscopy at the atomic scale that demonstrate that in poly-crystalline YBCO, highly strained grain-boundary regions contain excess O vacancies, which reduce the local hole concentration. The Ca impurities indeed substitute for Y, but in grain-boundary regions under compression and tension they also replace Ba and Cu, relieving strain and suppressing O-vacancy formation. Our results demonstrate that the ionic radii are more important than their electronic valences for enhancing J(c).

Ca-doping-induced enhancement of the critical currents of coated conductors grown by ion-beam-assisted deposition

One of the most promising technologies for the fabrication of high-Tc cables is the ion-beam-assisted deposition (IBAD) technique. The performance of the superconductors fabricated by IBAD, and the fabrication costs, are to a great extent determined by the critical current densities of the superconductors’ grain boundaries. Since, in bicrystalline samples, overdoping has been found to improve the transport properties of grain boundaries in high-Tc superconductors, we have explored whether overdoping also enhances the critical currents of IBAD samples. The measurements show that, depending on the critical current density of the superconducting film, Jc (77 K) is increased by factors up to 2.2, also in applied magnetic fields of several tesla.

Grain boundary critical currents - a new perspective

More than a decade of research on grain boundaries in high-Tc superconductors has brought extensive knowledge about their often peculiar electronic properties. However, a comprehensive understanding of the physical principles underlying these properties is still in development. The universal character of the grain boundary transport properties for many different high-Tc compounds suggests that these are to a large extent controlled by generic aspects of the cuprate materials, such as the grain boundary microstructure, the predominant dx2-y2 symmetry of the order parameter and the possibility of bending of the electronic band structure, leading to the formation of space-charge layers. The latter offers a new perspective to improve grain boundary interfaces, e.g. by appropriately doping the material. This is demonstrated by the example of Ca doping of YBa2Cu3O7-, a procedure with which the grain boundary Jc has been strongly enhanced.

Investigating the atomic scale structure and chemistry of grain boundaries in high-Tc superconductors

The short superconducting coherence length in high-Tc materials makes them extremely susceptible to the deleterious effect of atomic scale defects. Perhaps the most important of these defects for large-scale technological applications, are grain boundaries. Here we describe an atomic resolution investigation of structural and chemical changes that occur at grain boundaries in high-Tc materials using scanning transmission electron microscopy (STEM). STEM is ideally suited to this analysis, as atomic resolution Z-contrast images and electron energy loss spectra (EELS) can be acquired simultaneously. This permits a direct correlation between the structural images and the local electronic structure information in the spectrum. From this detailed experimental characterization of the grain boundaries, simple theoretical models can be derived that allow the structure-property relationships in high-Tc superconductors to be inferred. Results obtained from YBa2Cu3O7−δ and (Bi/Pb)2Sr2Ca2Cu3O10 show that there is a charge depletion zone formed at grain boundaries. This charge depletion zone can act as a tunnel barrier to the flow of superconducting charge carriers and appears to increase in width with increasing misorientation angle. The magnitude of the critical current across grain boundaries in high-Tc materials predicted from these models is in excellent agreement with the widely reported electrical transport results.

Investigating the Relationship Between Atomic Structure and Carrier Depletion at [001] tilt Grain Boundaries in Ybco

AbstractRecent theoretical analyses [1] have suggested that the origin of the reduced Jc at grain boundaries in high-Tc superconductors may be band bending, which results in the depletion of mobile charge carriers at the 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 report a “structural unit” analysis of [0011 tilt grain boundaries in YBa2Cu 3O7-δ (YBCO) that indicates there may be a simple structural origin for such localized donor states.

Investigation of the Local Superconducting Properties at Grain Boundaries in High-Tc Superconductors

Abstract Developing an atomic scale study of the structure-property relationships of grain boundaries in high-Tc superconductors is essential to understand their current dissipation mechanism and for incorporating these materials into viable devices. Thin YBa2Cu3O7-δ films have been deposited by pulsed laser deposition (PLD) on SrTiO3 symmetric bicrystals. Transport measurements in a magnetic field have been conducted across the grain boundaries through a wide bridge. The data obtained are consistent with microstructural observation in a VG Microscopes HB603 U and a VG HB501 UX dedicated STEM. Of particular interest in the study of high-Tc materials is the use of EELS, which can highlight the presence of non-superconducting regions through interpretation of the onset positions and finestructure (ELNES) of characteristic core-edge features. The V(I) curves recorded across a 24° boundary for several magnetic fields (fig. 1 - left) show an onset critical current density followed by a linear differential ohmic behavior which gives a negative intercept.

Nontunnelling character of the transport process through grain boundaries in high-Tc superconductors

The nonlinear current-voltage characteristics of high Tc superconductors are analysed in order to determine which kind of transport process is dominant across their intrinsic interfaces. The CVC of ceramic samples as well as of samples with silver coated grains are discussed. It is shown that the CVC of grain boundaries are consistent with metallic conductance, instead of tunnelling conductance as the charge transport process in high temperature superconductors.

Grain boundaries in high-Tc superconductors

One crucial property for most superconductor applications is the materials critical current density. Measurements on single crystals of YBa2Cu3O7, show critical currents to be in excess of 105 A/cm2 at 77K. However, polycrystalline samples show values of 103 A/cm2 or less at 77K. The critical current, unlike other superconductivity properties, is not a property of a specific composition but of a specific sample. Defects, introduced during the material processing, exert a pinning force which makes it possible for Type-II superconductors to carry current without losses. At the same time these defects represent an interruption in the structural and chemical order which has been shown to be necessary for superconductivity in the YBa2Cu3Ox phase, and so they may also act as a weak link in the conduction path. The most direct measurements of the role of individual grain boundaries on the critical current density are those of Chaudhari et al. who examined patterned YBa2Cu3O7 films grown on SrTiO3 substrates.