The behavior of grain boundaries in the Fe-based superconductors

The Fe-based superconductors (FBS) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries in the cuprates are a cautionary example. In this review we first consider the positive role that grain boundary properties play in the metallic, low temperature superconductors and then review the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires. Based on this evidence, we conclude that grain boundaries in FBS are weak linked in a qualitatively similar way to grain boundaries in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for grain boundary modification as a better understanding of their pairing state and grain boundary properties are developed.Comment: To appear in Reports on Progress in Physic

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(RE)BCO; RE = rare earth] bulk superconductors to trap magnetic field is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter 25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.This is the final published version, distributed under a Creative Commons Attribution License. This can also be found on the publisher's website at: http://iopscience.iop.org/0953-2048/27/8/08200

Composite stacks for reliable > 17 T trapped fields in bulk superconductor magnets

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ~ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-{\delta} bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families

A trapped field of 14.3 T in Y-Ba-Cu-O bulk superconductors fabricated by buffer-assisted seeded infiltration and growth

The two-step Top Seeded Infiltration and Growth (TSIG) melt process has emerged as a successful and reliable technique for the fabrication of single grain (RE)Ba2Cu3O7- (where RE is a rare-earth element or yttrium) bulk high temperature superconductors with engineered microstructures that exhibit improved superconducting properties. In this study, the performance of these materials in large applied magnetic fields has been investigated by field cooling single grain samples in a magnetic field of 18 T. YBa2Cu3O7- samples processed without added Ag by the TSIG technique, in the two-sample stack configuration, trapped a magnetic field of 14.3 T at 28 K after field cooling from 100 K and subsequent removal of the applied field. This result is particularly significant in that, previously, only single grain (RE)Ba2Cu3O7- bulk superconductors containing Ag have been reported to be able to tolerate the large stresses on the samples inherent in the magnetisation process at large fields. The samples prepared in the present study were pre-stressed using a reinforcing stainless-steel ring, although, otherwise, they did not contain any additives, dopants or resin impregnation. The ability of samples processed by TSIG to withstand large tensile forces without Ag-addition is attributed to the reduced incidence of intrinsic cracks/pores in the single grain microstructure

Bayesian Inference Underlies the Contraction Bias in Delayed Comparison Tasks

Delayed comparison tasks are widely used in the study of working memory and perception in psychology and neuroscience. It has long been known, however, that decisions in these tasks are biased. When the two stimuli in a delayed comparison trial are small in magnitude, subjects tend to report that the first stimulus is larger than the second stimulus. In contrast, subjects tend to report that the second stimulus is larger than the first when the stimuli are relatively large. Here we study the computational principles underlying this bias, also known as the contraction bias. We propose that the contraction bias results from a Bayesian computation in which a noisy representation of a magnitude is combined with a-priori information about the distribution of magnitudes to optimize performance. We test our hypothesis on choice behavior in a visual delayed comparison experiment by studying the effect of (i) changing the prior distribution and (ii) changing the uncertainty in the memorized stimulus. We show that choice behavior in both manipulations is consistent with the performance of an observer who uses a Bayesian inference in order to improve performance. Moreover, our results suggest that the contraction bias arises during memory retrieval/decision making and not during memory encoding. These results support the notion that the contraction bias illusion can be understood as resulting from optimality considerations

Association of a beta-2 adrenoceptor (ADRB2) gene variant with a blunted in vivo lipolysis and fat oxidation

Background and aims:Obesity is associated with a blunted beta-adrenoceptor-mediated lipolysis and fat oxidation. We investigated whether polymorphisms in codon 16, 27 and 164 of the beta (2)-adrenoceptor gene (ADRB2) and exon 10 of the G protein beta (3)-subunit gene (GNB3) are associated with alterations in in vivo lipolysis and fat oxidation.Design and methods:Sixty-five male and 43 female overweight and obese subjects (body mass index (BMI) range: 26.1-48.4 kg/m(2)) were included. Energy expenditure (EE), respiratory quotient (RQ), circulating free fatty acid (FFA) and glycerol levels were determined after stepwise infusion of increasing doses of the non-selective beta-agonist isoprenaline (ISO).Results:In women, the Arg16 allele of the ADRB2 gene was associated with a blunted increase in circulating FFA, glycerol and a decreased fat oxidation during ISO stimulation. In men, the Arg16 allele was significantly associated with a blunted increase in FFA but not in glycerol or fat oxidation.Conclusion:These results suggest that genetic variation in the ADRB2 gene is associated with disturbances in in vivo beta-adrenoceptor-mediated lipolysis and fat oxidation during beta-adrenergic stimulation in overweight and obese subjects; these effects are influenced by gene-gender interactions.International Journal of Obesity advance online publication, 28 November 2006; doi:10.1038/sj.ijo.0803499

Does administration of non-steroidal anti-inflammatory drug determine morphological changes in adrenal cortex: ultrastructural studies

Rofecoxib (Vioxx© made by Merck Sharp & Dohme, the USA) is a non-steroidal anti-inflammatory drug which belongs to the group of selective inhibitors of cyclooxygenasis-2, i.e., coxibs. Rofecoxib was first registered in the USA, in May 1999. Since then the drug was received by millions of patients. Drugs of this group were expected to exhibit increased therapeutic action. Additionally, there were expectations concerning possibilities of their application, at least as auxiliary drugs, in neoplastic therpy due to intensifying of apoptosis. In connection with the withdrawal of Vioxx© (rofecoxib) from pharmaceutical market, attempts were made to conduct electron-microscopic evaluation of cortical part of the adrenal gland in preparations obtained from animals under influence of the drug. Every morning animals from the experimental group (15 rats) received rofecoxib (suspension in physiological saline)—non-steroidal anti-inflammatory drug (Vioxx©, Merck Sharp and Dohme, the USA), through an intragastric tube in the dose of 1.25 mg during 8 weeks. In the evaluated material, there was found a greater number of secretory vacuoles and large, containing cholesterol and other lipids as well as generated glucocorticoids, lipid drops in cytoplasm containing prominent endoplasmic reticulum. There were also found cells with cytoplasm of smaller density—especially in apical and basal parts of cells. Mitochondria occasionally demonstrated features of delicate swelling. The observed changes, which occurred on cellular level with application of large doses of the drug, result from mobilization of adaptation mechanisms of the organism