Bi2 Sr2 Ca1 Cu2 O8+x /Ag wires have been produced by using the ‘‘powder in tube’’ method and an appropriate annealing procedure. At 77 K these wires yield critical current densities up to 1.2×103 A/cm2 in zero magnetic field. Using ac susceptibility measurements it is shown that flux creep is limiting the critical current density at temperatures above ∼20 K. At lower temperatures flux creep becomes insignificant. At 4.2 K critical current densities up to 5.5×104 A/cm2 in zero magnetic field and up to 1.5×104 A/cm2 in a field of 26 T could be achieved, rendering possible the use of this material for very high field application.

High‐field critical current densities in Bi2Sr2Ca1Cu2O8+x/Ag wires

Several physical effects allow free floatation of solid and even liquid matter. Materials may be levitated by a jet of gas, by intense sound waves, or by beams of laser light. In addition, conductors levitate in strong radio-frequency fields, charged particles in alternating electric fields, and magnets above superconductors or vice versa. Although levitation by means of ferromagnets is unstable, supper-conductors may be suspended both above and below a magnet as a result of flux pinning. Levitation is used for containerless processing and investigation of materials, for frictionless bearings and high-speed ground transportation, for spectroscopy of single atoms and microparticles, and for demonstrating superconductivity in the new oxide superconductors.

https://science.sciencemag.org/content/sci/243/4889/349.full.pdf

Levitation in Physics

Relation of critical current irreversibility to trapped flux and microstructure in polycrystalline YBa2Cu3O7

A magnetic field harmonically varying in time (to probe the sample) and a lock-in technique (to register the sample response sensed by a pick-up coil) are widely used for characterizing superconductors. Measuring the temperature dependence of the complex AC susceptibility is the most common procedure of this type. This paper reviews these techniques, introducing in addition the complex AC susceptibility, the so-called `wide-band AC susceptibility'. The latter quantity refers to the magnetic flux and often offers an easier meeting between theory and experiment. Starting from models for linear flux diffusion, reversible screening, volume and surface flux pinning and the intermediate regime in a type II superconductor, the expressions for the complex AC susceptibility in different cases are presented and compared with those derived for the wide-band AC susceptibility. Derivation of the basic physical properties of high- superconducting materials from the AC data (resistivity, critical temperatures and fields, London and Campbell penetration depths, critical current density, granularity and content of superconducting phase, irreversibility line, pinning potential) is then thoroughly discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/10/8/001/pdf

Characterization of high-temperature superconductors by AC susceptibility measurements

Model for flux creep in high Tc superconductors

A sintered bulk sample of YBa2Cu3O7 has been investigated resistively and inductively with respect to its resistivity ϱ(B, T), upper critical fieldBc2(T), and current carrying capabilityjc(B, T). The inductive measurements were repeated after powdering the sample to a maximum particle size of 60 μm. Results show that the intrinsic-intragrain properties as approximately revealed by the powder measurements, are severely masked in bulk measurements by the intergrain weak-link structure. As an example, the current densityjc(≃0 T, 4.2 K) in the powdered YBa2Cu3O7 has a value of 4·106 A/cm2, which is about three orders of magnitude higher than in the bulk material. The high values of the intragrainjc are probably due to pinning at twin boundaries. — Besidesjc the normal state resistivity just aboveTc, ρn, is influenced by the interconnecting structure, both properties being directly related: a smaller ρn leads to a higherjc in bulk samples. — The upper critical field shows a pronounced anisotropyBc2|/Bc2⊥ of about 20 to 30, whereBc2|=330 T is the extrapolated upper critical field at zero temperature for the field orientation parallel to the (Cu−O)-planes. — For the time being, theBc2 anisotropy and the weak-link structure of sintered bulk samples are the main obstacles for high field applications.

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

Homogenized ${\mathrm{V}}_{3}$Si crystals were irradiated with fast neutrons at conditions which result in a proportionality between the induced defect density and the neutron fluence. The critical current density is corrected for differences in the fluence dependence of the upper critical field and of the Ginzburg-Landau parameter. The resulting rise of the volume pinning force is investigated as a function of the density of the weak interacting defects. A second power law which points to a collective interaction is observed at low defect densities and at low fields. With increasing defect density and increasing field, this quadratic dependence vanishes abruptly and an almost linear relation dominates. The exponent of this first power law decreases continuously with further increase of field or defect density. These observations are compared with the collective pinning theory.

Experimental study on the collective theory of flux pinning in a high-field superconductor

The transitions from the ordered Bragg glass into the disordered vortex states at low and high fields are investigated by magnetization measurements varying the angle between magnetic field and crystal lattice orientations in a distinct manner. A single crystalline sphere is used to obtain an uniform demagnetization and not to introduce vortex disorder from sample edges. No angular dependence was observed when the pinning energy dominates. However, if the elastic energy of the vortex lattice is superior, the formation of the Bragg glass depends strongly on the relative orientation of both lattices. If the sphere is rotated around the [100] direction of the crystal with the field being perpendicular to it, a fourfold symmetry, both of the transition fields and of the strength of the vortex metastability, is found. The recent observation by Yethiraj et al. [Phys. Rev. Lett. 82, 5112 (1999)], that the vortex lattice reflects the symmetry of the crystal lattice also in ${\mathrm{V}}_{3}\mathrm{Si}$ is in good agreement with our investigations. Comparative measurements made on the sphere and on a crystal with edges revealed the influence of disorder on the Bragg glass induced by the sample shape.

Correlation of the vortex order-disorder transition with the symmetry of the crystal lattice in V 3 Si

Fast neutron irradiation of YBa/sub 2/Cu/sub 3/O/sub 7/ leads to a decrease of T/sub c/, the variation of the ratio T/sub c/( Phi t)/T/sub c/(O) being comparable to that observed in conventional superconducting compounds. Superconductivity vanishes above a fluence of 2*10/sup 19/ cm/sup 2/. The phase-locking temperature of the intergrain junctions decreases faster with fluence than T/sub c/. Lower AC losses due to a complete destruction of weak links and lower intergrain critical-current densities with increasing fluence reflect in addition a larger radiation sensitivity of the intergrain junction properties. The intragrain critical-current density j/sub cG/ increases with increasing fluence, but its dependence on B and Phi t is not at all in accordance with increasing pinning by radiation-induced defects and decreasing T/sub c/. The intragrain current drops remarkably at low fields, passing a minimum and a subsequent maximum beyond which the common decrease of j/sub cG/ with increasing field is observed. The low-field behavior, together with the much lower increase of j/sub cG/ with fluence than above the maximum, points to a junction character of the current in this field range. >

H. Küpfer

Papers

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

Experimental study on the collective theory of flux pinning in a high-field superconductor

Correlation of the vortex order-disorder transition with the symmetry of the crystal lattice in V 3 Si

Influence of fast neutron irradiation on inter- and intragrain properties of ceramic YBa/sub 2/Cu/sub 3/O/sub 7/

Radiation induced changes in critical superconducting properties of V3Si