L. Civale

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.

The granular nature of bulk superconductivity at 40K in La1.8Sr0.2CuO4

Abstract We have measured the electrical resistivity, magnetic flux expulsion and critical fields of the new high temperature superconductor La 1.8 Sr 0.2 CuO 4 after different heat treatments. Our data show that the complete flux expulsion in these samples is consistent with the presence of shielding currents induced in multiple connected structures. The magnetization measurements indicate that the lowest field where flux penetrates into the sample should not be taken to be the bulk Hcl. It is shown that the absolute value of the electrical resistivity and its temperature dependence are not correlated with the superconducting critical field and temperature. We discuss the behaviour of the superconducting parameters and their relation with the electrical resistivity.

The nature of the magnetic flux behavior of the superconductor LaSrcuO

Abstract Precise magnetization measurements in La1.85Sr.15CuO4 from 1.5K to 40K and from 1 mOe to 300 Oe show that the magnetic flux penetration can be understood in terms of a simple critical state model, applied to a multiple connected non homogeneous superconductor. The magnetization data together with the results from vibrating reed experiments show that the magnetic flux penetration at the lowest fields is not associated with superconducting vortices. The vortices penetrate at a field Hcl characteristic of the regions where the order parameter has its maximum value.

Critical temperature and density of states in amorphous Zr70Cu30

Abstract By means of thermal heat treatment of amorphous Zr 70 Cu 30 it is shown that the induced decrease in critical temperature is followed by a reduction in the electronic density of states of the same magnitude as that obtained by changing the Cu concentration. This result indicates that the density of states is the fundamental microscopic parameter determining the superconducting behavior.

Meissner fraction in insulating samples of oxide superconductors

Abstract The tendency towards superconducting granularity with sample degradation is a common feature of oxide superconductors. We present magnetic, transport and calorimetic results in LiTi 2 O 4 which show that the topological tendency to segregate the superconducting state into islands surrounded by thin insulating layers is related to the structure of the material rather than to a mechanism associated to high T c . We emphasize that insulating samples with small Meissner fraction can have a superconducting volume fraction close to unity.

Logarithmic to nonlogarithmic flux creep transition and magnetic flux hardening in BiSrCaCuO superconducting ceramics

Abstract A sharp temperature and field dependent transition from a logarithmic to a non-logarithmic flux creep behavior has been found in BiSrCaCuO superconductors. The transition is not followed by any noticeable change in the magnetization. However, vibrating reed measurements show that it coincides with a strong hardening of the magnetic structure. The transition occurs at fields and temperatures well below the reversibility line. The possible relation to the recently proposed flux lattice melting is discussed.

PERCOLATION AND GLASS BEHAVIOR IN CERAMIC SUPERCONDUCTORS: OXYGEN EFFECTS

It is shown that small variations of oxygen content in La1.8Sr0.2CuO4−δ can be used to control the reversible and irreversible magnetization temperature dependence of the superconducting state. A grain bond model is appropriate to describe the superconducting response of the ceramics. The study of the temperature dependence of the magnetization and the resistive transition temperature, in a slightly deoxygenated sample, show that the reversible superconducting state is strongly correlated to superconducting percolation. Deoxygenation has been shown to be an excellent technique to study the time evolution of metastable states induced by currents in the bonds and or in the grains. It is shown that superconducting grains are not correlated to ceramic grains.

Surface normal regions in superconducting Zr70Cu30 induced by thermal relaxation

Abstract Penetration depth measurements show that thermal heat treatment in amorphous Zr 70 Cu 30 induces a normal region 5000 A thick at the surface of ultrarapid quenched ribbons. Upper critical field measurements indicate that the new induced phase is a normal one while the rest of the sample remains as a homogeneous superconducting phase.

Superconducting Phase Diagram of a High Tc Ceramic

In this work we focus our attention on the magnetic response of the superconducting state of high Tc ceramic materials. The superconducting granularity of La-Sr-Cu-O ceramics is controlled by small changes of oxygen concentration. The experimental results show evidence of a hierarchy of weak links connecting superconducting islands.