F. de la Cruz

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.

Two dimensional collective flux pinning in melt spun superconducting amorphous Zr70Cu30

Abstract We have measured the critical currents in melt spun amorphous Zr70Cu30 and their evolution with heat treatment. We found that the pinning forces may be interpreted in terms of the Larkin-Ovchinnikov two dimensional collective pinning theory.

Electrical resistivity of amorphous Zr70Cu30 and the Kondo like model

Abstract Electrical resistivity measurements of amorphous Zr70Cu30 as a function of the concentrations of two level systems show that a Kondo like theory cannot explain the observed temperature dependence.

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 Laue5f8Srue5f8Cuue5f8O system in a wide temperature range show that the previous statement is only an approximation valid for some values of the Sr concentration.

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.

Imaging the structure of the interface between symmetries interconnected by a discontinuous transition

We have been able to observe with single particle resolution the interface between two structural symmetries that cannot be interconnected by a continuous transition. By means of an engineered 2D potential that pins the extremity of vortex strings a square symmetry was imposed at the surface of a 3D vortex solid. Using the Bitter decoration technique and on account of the continuous vortex symmetry, we visualize how the induced structure transforms along the vortex direction before changing into the expected hexagonal structure at a finite distance from the surface.

Critical current and topology of the supercooled vortex state in NbSe2

We study the behavior of the critical current, I c (H,T), of pure and Fe-doped NbSe 2 crystals in the denominated disordered vortex region, limited by the critical field H c 2 (T) and the field H p (T) at which the peak effect in I c (H,T) is detected. The critical current follows an individual pinning response as demonstrated by its field-independent universal function of the superfluid density. Transport measurements combined with Bitter decorations show no evidence of the existence of an amorphous phase in the high-temperature region.

Size dependence of the superconducting critical temperature and fields of Nb/Al multilayers

The critical temperatureTcof Nb/Al multilayers decreases as the total sample thicknessdTis decreased while the thickness of each Nb and Al layer is kept constant. To understand this behavior, models based on the proximity effect and on weak two-dimensional (2D) localization are employed. The latter uses a characteristic length, the thermal diffusion length, in relation todTto obtain 2D behavior and leads to a reasonable explanation ofTc(dT). It is also found that the slope atTc(dT) of the critical magnetic field perpendicular to the layers is independent ofdTwhen the Nb and Al layer thicknesses are kept constant. The angular dependence of the critical field is also measured.

Electrical resistance of epitaxial crystalline films of (SN)x

Abstract The electrical resistance of oriented (SN)x-films has been measured from 300 to 0.07 K. The results indicate a finite intrinsic conductivity in the direction perpendicular to the chains, supporting that (SN)x is a strong anisotropic conductor rather than a quasi-one-dimensional system.