H.  Rietschel

High-Tc Superconductivity: Present Status of Fundamental Research and Applications

Magnetic flux can penetrate a type-II superconductor in the form of flux-lines or Abrikosov vortices,each of which carry a quantum of flux and arrange in a more or less regular triangular lattice. Underthe action of an electric current, these flux lines move and dissipate energy unless they are pinned bymaterial inhomogeneities. In conventional superconductors, depinning occurs when a critical currentdensity Jc is exceeded. In high-Tc superconductors (HTSC), thermally activated depinning causes a finiteresistivity t9 even at current densities J &#8810; Jc. At sufficiently large temperature T, linear (ohmic) resistivityis observed down to J &#8594; 0. This indicates that the flux lines are in a "liquid state" with no shear stiffnessand with small depinning energy. At lower T, &#961;(J) is highly nonlinear, since the pinning energy increaseswith decreasing J. In highly anisotropic Bi- and Tl-based HTSC, thermal depinning occurs at ratherlow T, since short vortex segments ("pancake vortices" in the CuO layers) can depin individually withvery small activation energy.

Ernst Helmut Brandt

Pinning in High-Tc Superconductors

N. E. Alekseevskii

S. F. Kim

Relaxation of the Magnetic Moment of Bi2CaSr2Cu2O8+x Single Crystals in the Millisecond Region

Analysis of a comprehensive set of experimental results has proven the substantial effect of the precisepreparation conditions upon the structure and superconducting parameters of Ti-Ca-Ba-Cu-O samples.The techniques required for the preparation of specimens of single phase and long-term stability,together with methods for improvement of the characteristic parameters have been determined. Comparingresults derived from samples prepared by different methods has allowed the quantitative differencesof the characteristic parameters to be evaluated.

I.  Kirschner

S.  Lepp&#228;vuori

A.  Uusim&#228;ki

Relationship Among Preparation, Structure and Superconductivity inTi-Ca-Ba-Cu-O Compounds

S. Mészáros

K. Vad

Some Problems With HTSC in Low-Frequency Electric and Magnetic Measurements

57Fe, 119Sn, and 151Eu Mössbauer spectroscopy was used to study the chemical structure, phonon mode
changes, and low-temperature phase transformation around the Tc as well as suppression of superconductivity
in high-temperature superconductors and related compounds. Anomalous temperature-dependent
changes in the total 57Fe spectral area fraction and in the Mössbauer line shift were simultaneously
found around the Tc in a EuBa2(Cu0.9957Fe0.01)3O7-d superconductor. These anomalous changes
were attributed to phonon softening and low-temperature phase transformation occurring around
the superconducting transition. Significant differences were observed between the 57Fe Mössbauer
spectra of superconducting EuBa2(Cu0.9957Fe0.01)3O7-d and the isostructural non-superconducting
PrBa2(Cu0.9957Fe0.01)3O7-d. The differences were interpreted in connection with the suppression of superconductivity
(by hole filling or hybridization of Pr, Cu and O states) in the Pr-containing compound.
The unusually high isomer shift value observed in the Pr-containing material can give evidence for the
charge transfer mechanism between the Cu(1) chains and the Cu(2) planes and for its role in the
suppression of superconductivity.

E. Kuzmann

Z. Homonnay

S. Nagy

M. Gál

A. Vértes

High-Temperature Superconductors and Related Compounds Studied by
Mössbauer Spectroscopy

The significance of powder and paste characteristics was studied in order to improve the morphologyand structure of superconducting (Bi,Pb)-Sr-Ca-Cu-O screen printed thick films. Powder with nominalcomposition of Bi1.75Pb0.4Sr1.9Ca2.1Cu3.2Oy was prepared via the oxalate route and decomposed at 430 to795&#8451;. The decomposed powders were characterized by X-ray diffraction, transmission electron microscopyand specific surface area measurements. These studies showed a variation of specific surfacearea from 16.5 to 0.9 m2/g and a variation of the average particle size from 100 nm to 800 nm in thestudied temperature interval. The phase structure showed three distinct temperature areas; between430 to 620, 620 to 715, and at 795&#8451;.Thick-film pastes were made by the addition of an organic vehicle at 500, 620, 650 and 795&#8451; todecompose annealed oxalate synthesized powders and, for comparison, the vehicle was also added tosintered mixed-oxide/carbonate-based powder in weight ratios from 0.42 to 0.54:1. Films were screenprinted on single crystal MgO (100) substrates and melt annealed at 890 to 895&#8451; for 3 min andsubsequently, for prolonged diffusion, annealed at 852&#8451;. After firing, the films were mainly composedof the (001) textured (Bi,Pb)2Sr2Ca2Cu3Oy phase. The reference films, made from mixed oxide/carbonate powder, resembled the films based on oxalate powders decomposed at 795&#8451;. Films based on oxalatepowders, decomposed at lower temperatures, were smoother and were able to carry noticeably highercurrents than films based on powders decomposed at higher temperatures.

J.  Hagberg

The Properties of Screen Printed (Bi,Pb)-Sr-Ca-Cu-O Thick Films Based OnDecomposed Oxalate Powders

Thermoelectric power (TEP) of YBa2Cu3O7-X (123) and YBa2Cu4O8 (124) samples are measured as a function of pressure. The TEP of an as-sintered 123 polycrystalline sample shows a broad maximum around 1.25 GPa whereas that of the cold-pressed sample decreases steadily. The ab-plane TEP of 123
single crystal decreases monotonically and the c-axis TEP exhibits a maximum around 1.5 GPa. Disappearance
of the broad maximum in the polycrystalline sample upon cold-pressing is interpreted in
terms of the crystallite orientation occurring under pressure. TEP of the 124 polycrystalline sample
decreases sharply up to 4.5 GPa and at a lower rate with further increase in pressure, a trend similar
to that is observed in the variation of Tc as a function of pressure.

Sheela K. Ramasesha

Thermoelectric Power of YBa2Cu3O7-X and YBa2Cu4O8 Under Pressure

High Tc superconducting YBa2Cu3O7-d thin films and YBa2Cu37-&#948;/PrBa2Cu3O7-&#948; superlattices with various periods and layer thicknesses were deposited by laser ablation on SrTiO3 (100) single crystal substratesusing an XeCl excimer laser (&#955; = 308 nm). The structure of the films was studied by SEM/EDS andX-ray diffraction. The films and superlattices deposited in an in situ process were epitaxial with the c-axisperpendicular to the substrate surface and the a- and b-axes parallel with the SrTiO3 [100] or [010]directions. The electrical properties were studied by measuring the current-voltage characteristics,resistivity and critical current. For the in situ deposited YBa2Cu3O7-d films the zero resistivity temperature was 87 to 90 K and the critical current at 77 K was above 2 &#215; 106 A/cm2. For the films deposited atroom temperature and post annealed under in situ deposition conditions, the critical temperature wasthe same, but the critical current was 0.5 &#215; 106 A/cm2.

J.  Levoska

High-Tc Superconducting YBa2Cu3O7-&#948; Filmsand YBa2Cu3O7-&#948;/PrBa2Cu3O7-&#948; Superlattices Deposited by Laser Ablation

We present typical results of experiments carried out in (YBaCuO)1-xAgx tubular samples, for different values of the silver concentration x, having in mind the possible use of this material as a magneticshield. We measured the times governing the dynamics of magnetic flux penetration, on a microsecondtime-scale, and tried to correlate them with d.c. measurements of the resistivity and the critical transportcurrent density, using a simple theoretical model based on the flux-flow r&#233;gime.

H.  Castro

E.  Holgu&#237;n

J. F. Loude

L.  Rinderer

Magnetic Shielding With High-Tc (YBa2Cu3O7-&#948;)1-xAgx Superconducting Tubes

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Seppo Leppävuori

Istvan Kirschner

High-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mtext>T</mml:mtext><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math> Superconductivity

Active and Passive Electronic Components

High-Tc Superconductivity

High-Tc Superconductivity

Articles

High-T_c Superconductivity: Present Status of Fundamental Research and Applications

Pinning in High-T_c Superconductors

Relaxation of the Magnetic Moment of Bi₂CaSr₂Cu₂O_8+x Single Crystals in the Millisecond Region

Relationship Among Preparation, Structure and Superconductivity in Ti-Ca-Ba-Cu-O Compounds

Some Problems With HTSC in Low-Frequency Electric and Magnetic Measurements

High-Temperature Superconductors and Related Compounds Studied by Mössbauer Spectroscopy

The Properties of Screen Printed (Bi,Pb)-Sr-Ca-Cu-O Thick Films Based On Decomposed Oxalate Powders

Thermoelectric Power of YBa₂Cu₃O_7-X and YBa₂Cu₄O₈ Under Pressure

High-T_c Superconducting YBa₂Cu₃O_7-δ Films and YBa₂Cu₃O_7-δ/PrBa₂Cu₃O_7-δ Superlattices Deposited by Laser Ablation

Magnetic Shielding With High-T_c (YBa₂Cu₃O_7-δ)_1-xAg_x Superconducting Tubes