W. Schauer

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.

Magnetic properties of the high-Tc superconductor La1.85Sr0.15CuO4

For the high-Tc superconductor La1.85Sr0.15CuO4, we present results for the low-field magnetization and the lower and upper critical fieldsBc1(T) andBc2(T). ForBc1(0) we find a value of about 20 mT. Extrapolating our resistive data for the transition midpoint taken in fields up to 12 T, we deduce a value of about 50 T forBc2(0). TheBc2 values obtained by inductive measurements are significantly lower. We explain this by a largeBc2 anisotropy due to the layered structure of these compounds. Losses in the Meissner state observed in magnetization and susceptibility measurements disappear after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample.

Contactless measurement of voltage-current characteristics of high-Tc thin film superconductors

Abstract A contactless inductive method is proposed and tested to determine E(j) and jc(B) characteristics of ring shaped samples patterned from YBa2Cu3O7−x thin films. The method is based on monitoring the magnetic field Bs generated by a current in the high-Tc ring, which is induced by an external magnetic field sweep dBe/dt. The sample field Bs is detected by a small area Hall sensor (≈50 μm × 50 μm). The capability of the method is demonstrated for two 1 μ thick ring samples. Films were prepared by cylindrical magnetron sputtering and patterned photolithographically (mean ring diameter ≅ 3.5 mm, ring width ≅0.5 mm). Magnetic field profiles along the ring diameter and hysteresis loops Bs(Be) are interpreted using Beans model. A sensitivity limit of E≈-5 × 10−5 μV/cm allows us to adopt E-criteria in jc (B) measurements four orders of magnitude lower than in resistive experiments. An exponential E(j) relation is observed in an electric field interval of three decades.

Critical Field and Current of the High-Tc Superconductor YBa2Cu3O7

We report on resistive and inductive measurements of the upper critical field Bc2 and the critical current jc of YBa2Cu3O7. The strong field dependence of the resistively and inductively measured superconductive transition is explained by a strong anisotropy of Bc2. After grinding the sample the critical current density exceeds 106 A/cm2 in the grain, whereas in the sintered bulk sample this value is three orders of magnitude smaller.

Magnetic Properties of the High-Tc Superconductors La1.85Sr0.15CuO4 and YBa2Cu3O7

For the high-Tc superconductors La1.85Sr0.15CuO4 and YBa2Cu3O7 we present results for the low-field magnetization and the lower and upper critical fields Bc1(T) and Bc2(T). Losses in the Meissner state observed in magnetization and susceptibility measurements disappeared after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample. The Bc2(T) obtained by inductive measurements indicates a large anisotropy due to the layered structure of these compounds.

Angular dependence of the irreversibility temperature for Y- and Nd-BaCuO

Abstract For c- and a-axis YBaCuO films and a NdBaCuO single crystal the irreversibility temperature Tirr(B,Θ) has been determined from the third harmonic of the AC-susceptibility in dependence on field (B) and field orientation (Θ=∠(B, sample normal n)). The sample geometry and the intrinsic anisotropy give rise to the angular dependence of Tirr. The approximately elliptical lines of constant temperature in a plot B||n versus B⊥n, deduced from the experiments, are in agreement with a combination of the isotropic Ginzburg–Landau (GL) theory for thin films and the anisotropic GL theory. Anisotropic pinning probably due to twin lamellae in the NdBaCuO single crystal is seen as a bulge in the elliptical contour at Θ=0° (B||c -axis||n).

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.