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

Preface. 1: ELECTRICALLY SMALL ANTENNAS. 1.1 Introduction. 1.2 Fundamental Limitations. 1.3 Electrically Small Antennas: Canonical Types. 1.4 Clever Physics, but Bad Numbers. 1.5 Pathological Antennas. 1.6 ESA Summary. References. Index. 2: SUPERDIRECTIVE ANTENNAS. 2.1 History and Motivation. 2.2 Maximum Directivity. 2.3 Constrained Superdirectivity. 2.4 Bandwidth, Efficiency, and Tolerances. 2.5 Miscellaneous Superdirectivity. 2.6 Matching Circuit Loss Magnification. 2.7 Non-Foster Matching Circuits. 2.8 SD Antenna Summary. References. Index. 3: SUPERCONDUCTING ANTENNAS. 3.1 Introduction. 3.2 Superconductivity Concepts for Antenna Engineers. 3.3 Dipole, Loop, and Patch Antennas. 3.4 Phasers and Delay Lines. 3.5 SC Antenna Summary. References. Subject Index.

Electrically small, superdirective, and superconducting antennas

A novel geometry, the geometry of Space-Filling Curves (SFC) is defined in the present invention and it is used to shape a part of an antenna. By means of this novel technique, the size of the antenna can be reduced with respect to prior art, or alternatively, given a fixed size the antenna can operate at a lower frequency with respect to a conventional antenna of the same size.

Space-filling miniature antennas

The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.

Magnetization and relaxation curves of fast relaxing high-Tc superconductors

TOPICAL REVIEW: High-? transistors

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ have been grown by electron beam evaporation of the metals in the presence of atomic oxygen, with small systematic variations in cation composition. Films grown close to the stoichiometric composition or with excess yttrium are smooth while those with excess barium are substantially rougher. The effect of these differences in cation composition on both microwave and dc properties is dominated by the associated large changes in film morphology. This determines the oxygenation state, and hence the electrical properties, of the superconductor both in the as-grown state and after annealing in a furnace or with atomic oxygen. For all films a strong correlation is found between penetration depth, resistivity and c-lattice parameter which all decrease with increasing oxygen content. In our standard growth regime and at higher oxygen contents the films are overdoped and T/sub c/ falls with increasing oxygen content. >

Effect of composition and oxygen content on the microwave properties of evaporated Y-Ba-Cu-O thin films

Measurements of the complex microwave surface impedance of YBa 2Cu3O7 ~YBCO! thin films in a coplanar resonator geometry are reported for a number of films as a function of temperature and field at 8 and 16 GHz. Values are derived for the temperature and field dependence of the Labusch pinning parameter kp and for the temperature dependence of h, which are consistent with an independent flux-line model with only weak interflux-line interactions. The observation of losses in the mixed state scaling as ;v and of an effective pinning potentialU;6kBT are shown to be consistent with a glassy-state model, with flux lines thermally activated between local pinning sites separated by energy barriers with a wide range of values extending down to meV energies. The induced curvature of flux lines between point pinning centers is shown to be unimportant in determining the observed microwave properties. @S0163-1829 ~98!00605-5#

Field, temperature, and frequency dependence of the surface impedance of YBa 2 Cu 3 O 7 thin films

D.C. SQUID based gradiometers and magnetometers have been fabricated using thin films of YBa/sub 2/Cu/sub 3/O/sub 7/ and PrBa/sub 2/Cu/sub 3/O/sub 7/ on MgO substrates. Flip-chip magnetometers with 1 cm and 6.25 cm/sup 2/ substrates have been constructed. Our device on the larger substrate with a 430 mm/sup 2/ pickup loop had a sensitivity of (42/spl plusmn/3) fT//spl radic/Hz above 3 Hz and (75/spl plusmn/7) fT//spl radic/Hz at 1 Hz. Numerical simulations of d.c. SQUIDs at 77 K have been conducted over an inductance range from 2 to 1000 pH and a critical current range from 3.2 to 322 /spl mu/A. The results emphasise the need to reduce SQUID inductances in order to obtain lower flux noise. Three layer gradiometric SQUIDs with c-axis microbridge junctions and low inductance have been fabricated. First order gradiometric flux transformers were also made with twin 20 turn spiral SQUID input coils and a pair of larger area balanced sensing coils. These were mounted with the gradiometric SQUIDs in a flip chip configuration to complete the highly symmetrical gradiometers. Measurements at 79 K on one gradiometer gave a gradient sensitivity of (49/spl plusmn/3) pT/m//spl radic/Hz at 300 Hz and (103/spl plusmn/7) pT/m//spl radic/Hz at 1 Hz. >

Low noise HTS gradiometers and magnetometers constructed from YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-y/ thin films

Non-linear microwave surface impedance of patterned YBa2Cu3O7 thin films

We have measured the microwave properties of high quality YBCO thin films using a copper cavity (87 GHz) and a dielectric resonator (19 GHz) for unpatterned films as well as a coplanar resonator (5, 8, 10, 15 and 16 GHz) for patterned films. For the patterned films the surface resistance (R/sub S/) and non-linear effects were studied as a function of temperature below T/sub C/. For practical applications, the assessment of non-linear effects in the surface resistance are of great importance. The best films showed R/sub S/ approximately constant up to microwave peak current densities of the order of the dc critical current density. The measurements were performed on samples fabricated by a number of different techniques. >

R.G. Humphreys

Papers

Effect of composition and oxygen content on the microwave properties of evaporated Y-Ba-Cu-O thin films

Field, temperature, and frequency dependence of the surface impedance of YBa 2 Cu 3 O 7 thin films

Low noise HTS gradiometers and magnetometers constructed from YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-y/ thin films

Non-linear microwave surface impedance of patterned YBa2Cu3O7 thin films

Microwave properties of YBCO thin films