Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3

Since the first days of high-Tc superconductivity, the materials science and the physics of grain boundaries in superconducting compounds have developed into fascinating fields of research. Unique electronic properties, different from those of the grain boundaries in conventional metallic superconductors, have made grain boundaries formed by high-Tc cuprates important tools for basic science. They are moreover a key issue for electronic and large-scale applications of high-Tc superconductivity. The aim of this review is to give a summary of this broad and dynamic field. Starting with an introduction to grain boundaries and a discussion of the techniques established to prepare them individually and in a well-defined manner, the authors present their structure and transport properties. These provide the basis for a survey of the theoretical models developed to describe grain-boundary behavior. Following these discussions, the enormous impact of grain boundaries on fundamental studies is reviewed, as well as high-power and electronic device applications.

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Grain boundaries in high- T c superconductors

The growth and characterization of functional oxide thin films that are ferroelectric, magnetic, or both at the same time are reviewed. The evolution of synthesis techniques and how advances in in situ characterization have enabled significant acceleration in improvements to these materials are described. Methods for enhancing the properties of functional materials or creating entirely new functionality at interfaces are covered, including strain engineering and layering control at the atomic-layer level. Emerging applications of these functional oxides such as achieving electrical control of ferromagnetism and the future of these complex functional oxides is discussed.

/pdf/advances-in-the-growth-and-characterization-of-magnetic-1r6yol3qt6.pdf

Advances in the growth and characterization of magnetic, ferroelectric, and multiferroic oxide thin films

The properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400017489

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in‐plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu‐O‐Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7−x grains on (001) MgO substrates

For crystalline advanced materials, such as the high transition temperature oxide superconductors, the growth of defect‐free crystals is often the most sought after desideratum because it opens the doors to fundamental studies and the development of practical applications. We report the observation of YBa2Cu3O7−x(123) thin films having unprecedented structural perfection, at temperatures near 700 °C on [001] LaAlO3. The film’s c axis is in the surface plane, unlike films grown at higher temperatures. This orientation has important advantages for device applications and fundamental studies. The Tc,0 is only 70 K, presumably due to oxygen deficiency caused by thermal stresses; if so, it should be possible to raise the Tc,0 by relieving these stresses.

Structural perfection of Y‐Ba‐Cu‐O thin films controlled by the growth mechanism

We have examined the atomic structure of growth interfaces in thin films of Y–Ba–Cu–O grown on [001] perovskite or cubic substrates. At substrate heater temperatures in the range of 780–820 °C c-axis oriented growth is observed on these substrates. On SrTiO3, the first layer appears to be either a BaO or a CuO2 plane while on LaAlO3 the first layer appears to be a CuO chain layer. The mismatch on the a-b plane is accommodated by the formation of interface dislocations. Defects on the substrate surface propagate as defects in the film. These defects are primarily translational boundaries and in some cases second phases. At lower substrate heater temperatures, i.e., 650–700 °C, a, b-axis growth dominates. Defects and steps on the substrate surface are more detrimental in the growth of a, b-axis oriented films, since they tend to favor the nucleation of c-axis oriented domains. This is ascribed to the ledge mechanism of c-axis film growth, for which the surface steps are good nucleation sites.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400014126

The atomic structure of growth interfaces in Y-Ba-Cu-O thin films

We have observed enhanced Kosterlitz-Thouless transitions in multilayers composed of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ with a thickness of two and four unit cells, separated by ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ sufficient to isolate them from one another. The parameter ${\mathrm{\ensuremath{\tau}}}_{\mathrm{KT}}$=(${\mathit{T}}_{\mathit{c}0}$-${\mathit{T}}_{\mathrm{KT}}$)/${\mathit{T}}_{\mathit{c}0}$ is up to an order of magnitude larger than in single crystals or in films, showing that two-dimensional behavior is approached more closely in the multilayers. The fact that the enhancement of two-dimensionality is accompanied by a lowering of ${\mathit{T}}_{\mathit{c}0}$ demonstrates the importance of interlayer coupling.

Enhanced Kosterlitz-Thouless transition in YBa2Cu3O7-x/PrBa2Cu3O7-x multilayers as a measure of two-dimensionality

Fundamental issues in preparing high-quality high-T/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films are addressed. The techniques of inverted cylindrical magneton sputtering and pulsed laser deposition are chosen as successful examples to illustrate how the key problems can be solved. The fabrication of YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-x/ superlattices where superconductivity in a single unit cell layer of YBa/sub 2/Cu/sub 3/O/sub 7-x/ was observed demonstrates the state of the art of thin-film deposition of high-T/sub c/ materials. Systematic variations of the deposition parameters result in changes of superconducting and structural properties of the films that correlate with their microwave and infrared characteristics.

T. Venkatesan

Papers

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7−x grains on (001) MgO substrates

Structural perfection of Y‐Ba‐Cu‐O thin films controlled by the growth mechanism

The atomic structure of growth interfaces in Y-Ba-Cu-O thin films

Enhanced Kosterlitz-Thouless transition in YBa2Cu3O7-x/PrBa2Cu3O7-x multilayers as a measure of two-dimensionality

Preparation of thin film high temperature superconductors