R. Decca

La1.80 Sr0.20 CuO4−δ a clean limit superconductor

Abstract The electrical resistivity of the superconductor La 1.80 Sr 0.20 Cu0 4−δ has been measured in a wide range of temperatures as a function of oxygen and vacuum heat treatments. The resistivity changes reversibly orders of magnitude with oxygen concentration. There is no sign of saturation at high temperatures, even for samples where the resistivity is increased by heat treatment in vacuum. Using the experimental data and general arguments it is concluded that the ceramic superconductors are high κ materials in the clean limit.

Anisotropic energy dissipation, flux flow and topological pinning in ceramic superconductors

The dissipation in the high Tc ceramic superconductor La1.85Sr0.15CuO4- delta is interpreted in terms of interacting magnetic flux systems: vortices in the ceramic grains and fluxons between them. In zero field cooling experiments the dissipation shows a sin2 Theta dependence, where Theta is the angle between current and field. This result and the existence of an angle independent critical current are analyzed in the framework of a granular system where the dissipation is due to two contributions, one induced by movement of magnetic flux and the other associated to dissipation in the junctions.

Topological pinning and flux flow in ceramic superconductors

Abstract The transport current dissipation in the high Tc superconducting ceramic La1.85Sr0.15CuO4−δ shows a sin2 Θ voltage dependence where Θ is the angle between the transport current and the magnetic field. This result and the experimental evidence of an angle independent critical current cannot be explained by the conventional description of flux motion in homogeneous type II superconductors. The results are analyzed in the framework of a granular system where pinning is induced by the topological distribution of current flowing through a multiply connected medium. The dissipation in the superconducting state is found to be due to two additive contributions, one induced by the movement of magnetic flux and the other associated to dissipation in the Josephson junction.

Non linear temperature dependence of the resistivity of LaSrCuO: Effects of Sr content

Abstract The linear temperature dependence of the electrical resistivity has been taken to be one of the characteristic features of the ceramic superconductors. Measurements of the electrical resistance of the LaSrCuO system in a wide temperature range show that the previous statement is only an approximation valid for some values of the Sr concentration.

Double He gas circuit cryostat for 4-300 K temperature range

Abstract A simple and versatile cryostat for experiments in the 4–300 K range is described. The simultaneous use of cold and warm He gas heat exchangers allows a fast temperature sweep and a temperature stabilization better than 20 mK over the whole temperature range. The sample holder can be introduced by a top loading system or by direct access by removing a vacuum jacket.

Linear field dependence of flux flow resistance in high temperature superconductors

Abstract The V-I characteristics curves of polycrystalline La1.8Sr0.2CuO4−δ show that the dissipation in the superconducting grain boundaries is quenched at high fields and temperatures. This new superconducting state is explained taking into account that the magnetization is the relevant parameter determining the quantum interference in the weak links. The only dissipation left in the sample after quenching that in the links is quantitatively understood within the vortex flow models.

Intergrain barrier contribution to the electrical resistance of ceramics in the normal state

Although the granular nature of superconductivity in ceramics is well established, the origin of the intergrain barriers and their influence on the transport properties of the normal state are not well understood. Resistance measurements below Tc as a function of temperature and field, allow the determination of the barrier contribution to the electrical resistance in the normal state. The experimental results indicate that the granular picture is appropriate for the superconducting as well as for the normal state.

Long-range-order correlations in isotropic systems: HT phase diagram of polycrystalline high Tc superconductors

Abstract We report on the effect of thermal fluctuations in the superconducting state of polycrystalline La 1.8 Sr 0.2 CuO 7-δ in the presence of an applied magnetic field. The intrinsic isotropy of the system allowed us to study the EJ characteristics with J either parallel or perpendicular to H. The scaling behavior of the EJ curves are consistent with predictions of critical phenomena theories. However, the long-range-order correlation loss takes place in two stages in the high field region. When increasing temperature, we first observe the long-range-order loss in the direction transversal to the field. At higher temperatures the correlation parallel to the field also disappears. In the weak field region thermal fluctuations cause the transition to occur in a single line.