R. Meier-Hirmer

Intragrain junctions in YBa2Cu3O7−x ceramics and single crystals

Abstract Intragrain critical current densities of polycrystalline bulk samples and single crystals were investigated by inductive flux profile measurements. In addition to the well known granularity in a polycrystal, granularity within a single crystal was detected. In analogy to the polycrystalline samples increasing values of j c with decreasing size of the powder particles ground from single crystal gives strong evidence for the existence of intragrain junctions. The investigation of the field dependence of j c reveals as a common feature in ceramics and single crystals a maximum at fields far below the upper critical field. This observation and the behaviour of j c in the field region below this maximum are not in accordance with a critical current as expected from a pinning interaction, but rather reveal junction character. This is supported by data of fast neutron irradiation showing a lowering of the junction current, whereas above the critical current maximum j c increases by radiation induced pinning centres. The fields at which the maxima appear are in rough quantitative agreement with the irreversibility line from the glassy state or flux creep model. Once the phase coherence of the intragrain junctions is destroyed by the neutron damage, j c shows the expected continuous decrease with increasing field.

Fast neutron irradiation of YBa2Cu3O7

The influence of fast neutron irradiation on the superconducting transition temperature,Tc, the transition width, upper critical field,Bc2, and critical current density,jc, has been investigated in YBa2Cu3O7 up to a fluence of 1019 cm−2 (E>1 MeV). TheTc degradation with fluence is slightly less than in PbMo6S8, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainjc and the superconducting volume fraction.

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

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.

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highTc oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl2Ca2Ba2Cu3O10 specimen shows a connective nature similar to YBa2Cu3O7 with low intergrain currents (100 A/cm2 at 100K,B=0 T) and high intragrain ones (4.5·105 A/cm2 at 100K,B=0 T). The investigation of the Bi1Ca1Sr1Cu2Ox specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerTc phase.

Irreversible behaviour of oriented grained YBa2Cu3Ox. Part 1: transport and shielding currents

Abstract The critical current of oriented grained YBa 2 Cu 3 O x bulk material was investigated by resistive transport and inductive flux profile measurements. The applied magnetic field was directed parallel and perpendicular to the a–b plane. No features of granularity could be detected if the induced shielding current directed along the c-axis passed grain boundaries. The comparison of shielding and transport currents further ruled out the existence of intragrain junctions. The observation of a temperature scaling law for the volume pinning force allows comparison of the pinning strengths in both geometries. The volume pinning force is found to be about one order of magnitude larger if the magnetic field is parallel to the twin planes. Its field dependence resembles grain boundary pinning due to electron scattering, as observed in Nb 3 Sn. In the perpendicular geometry pinning may be explained by a collective interaction due to intrinsic or background pinning. The flux bundle volume, which may be identified with the correlation volume of the collective model, was experimentally determined.

Investigation of collective creep in YBaCuO

Abstract Time dependent decay of the magnetic moment M(t) and magnetization measurements were used to study flux creep in melt textured YBaCuO samples. From both measurements voltage current characteristics were reconstructed between 10 −1 and 10 −7 μV/cm up to the region of the irreversibility field. The exponent μ of the power law dependence of the pinning barrier was obtained from M(t) using the time constant of the initial stage of relaxation and from E(j) analysis. The current dependence of μ shows a maximum, negative values on both sides as well as the field dependence are not in agreement with theoretical predictions.

Flux creep and fishtail in melt-textured and single-crystalline YBa2Cu3O7

Time-dependent decay of the magnetic moment and magnetization measurements were used to study pinning and flux creep in two samples with and without aj(B) maximum in nonzero field (fishtail effect). From both measurements theE(j) relation was reconstructed forB∥c and the characteristic current exponentμ was obtained. At highj and lowB μ values are between 1 and 4; with increasingB μ passes a maximum and approaches negative values. At lowj μ is below 0.5 and not dependent onB orj. This behavior, which is qualitatively the same in both samples, is compared with proposed phase diagrams of the vortex lattice. Largeμ values are correlated with the plateau of the normalized creep rateS λ0.025; both observations indicate low relaxation and are found in that lowB region for whichj(B) has its minimum. This observation rejects a dominating influence of relaxation on the fishtail effect.

Strong coupling effects in the temperature dependence of the flux-flow resistance nearHc2

A refined experimental technique is applied to measure the flux-flow resistance of the strong coupling alloys Pb0.9In0.1, Pb0.8In0.2 and Pb0.7In0.3 and to get the deviations from weak coupling theories. In order to test a theory by Imai devised for arbitrary coupling strengths static quantities entering the flux-flow resistance formulas are calculated using extrapolated tunnel spectra.

AC Susceptibility and Inductive Critical Current Measurements in Polycrystalline YBa2Cu307

Polycrystalline sintered specimens of YBa2Cu307 with densities up to 95% and resistivities as low as 150 μΩcm at 100 K were investigated by means of ac susceptibility, resistivity and inductive critical current measurements. The tremendous increase of the transition width in an applied field observed by ac susceptibility measurements is attributed to the anisotropy of the upper critical field of the intragrain material and to the intergrain weak links which become superconducting at different fields and temperatures. The screening and loss behavior of the inter- and intragrain current system determines the ac susceptibility from which weak link properties are obtained by comparison of various specimens prepared following different procedures.