X.P. Zhang

Global and local critical current density in superconducting SmFeAsO1−xFx measured by two methods

Abstract The critical current densities of polycrystalline bulk SmFeAsO 1− x F x prepared by the powder-in-tube (PIT) method and by a conventional solid-state reaction were investigated using the remnant magnetic moment method and Campbell’s method. Two types of shielding current, corresponding to global and local critical current densities J c were observed using both measurement methods. The global and local J c were on the order of 10 7 xa0A/m 2 and 10 10 xa0A/m 2 at 5xa0K, respectively. The local J c decreased slightly with increasing magnetic field. The global J c was independent of the preparation method, while the local J c was larger for samples prepared by PIT than for those prepared by solid-state reaction.

Critical current characteristics and history dependence in superconducting SmFeAsOF bulk

The superconducting SmFeAsO1−xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbells) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1−xFx bulk.

Comparative study of different chemical doping effect on the Jc-B properties of MgB2 tapes

The effect of chemical doping on the critical current density of MgB2 tapes is studied. As impurities, we tried silicides, carbon and organic compound. Most of them lead to an obviously improvement of critical current density (Jc), and weaken its field dependence. For the silicides addition, the reaction products, such as Mg2Si play an important role in the enhancement of flux pinning ability. For the doping of carbon containing impurities, carbon substitution to boron contributes significantly to the enhancement of flux pinning ability and upper critical field, and eventually improves the Jc-B performance of MgB2 tapes. By modifying starting materials preparation process and optimizing carbon source, further improvement of in-filed Jc is expected.

Anomalous resistive oscillations in mesoscopic Al superconducting wires

Abstract Oscillations in R–T curves of mesoscopic Al wires near superconducting transition temperature were observed under weak AC currents. We attribute this observation to the non-equilibrium superconductivities near the N–S interfaces and the movement of these interfaces.

The interaction of phase slip centers in mesoscopic aluminum line

Abstract The I – V characteristics of mesoscopic aluminum wire have been studied. We found step structures in the I–V characteristics, which we believe are due to the formation of phase slip centers. With decreasing temperature, the step structure is replaced by peak structure. This anomalous behavior was analyzed in terms of the interaction between the phase slip centers.

The anomaly of current–voltage characteristics of a mesoscopic aluminum line

The I - V characteristics of mesoscopic aluminum wire have been studied. Step structures are found near superconducting transition temperature and the number of the steps is different at different temperature. At lower temperatures, the steps gradually turn into strong peaks. These anomalous features can be suppressed by a magnetic field. We propose an interpretation based on the nonequilibrium superconductivity due to the formation of phase slip centers (PSCs). The PSCs are excited one by one with increasing of current and the interaction between them will affect the charge imbalance length, leading to the anomalous peaks in the I - V characteristics near the critical current