C.A.M. dos Santos

On the transport properties in granular or weakly coupled superconductors

Abstract This work reports details about magnetic field-tuned broadening in the superconducting transition of polycrystalline samples of the Y 1− x Pr x Ba 2 Cu 3 O 7− δ system. It is observed that magneto-resistance measurements exhibit three distinct regimes which are discussed within the framework of the penetration of vortices inside normal state region and superconducting islands. The resistive behavior of granular or weakly coupled superconductors are summarized in four magnetic field ranges which are related to the intragranular and intergranular critical fields.

Inhomogeneous superconductivity in Bi2Sr2Ca1-xPrxCu2O8+δ

Abstract We have studied Pr doping effect on the superconducting transition temperature onset T C ( x ) in Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O 8+ δ superconducting polycrystalline samples by means of both resistance R ( T , x ) and magnetization M ( T , x ) measurements. It is found that while the resistively determined T CR ( x ) rapidly decreases with x for x >0.4, the T CM ≈85 K obtained from magnetization measurements remains unchanged. We argue that the suppression of T CR ( x ) is related to the reduction of the sample superconducting volume fraction which ultimately leads to disconnected superconducting clusters. Resistive behavior measured at different applied currents in a sample close to percolation threshold supports this discussion.

Two-fluid model for transport properties of granular superconductors

A two-fluid model is proposed to describe the transport properties of granular superconductors. Using the resistively shunted junction model and some aspects of the two-level system theory, a statistical model is developed which takes into account the ratio between the number of normal and superconducting electrons carrying the applied current. The theoretical model reveals excellent agreement when compared to transport properties of four high-Tc superconductors. The results suggest that the two-fluid model is independent of the sample composition, critical temperature and whether the superconducting compound is electron or hole-doped.

Influence of Pr content on the kinetics of Bi2Sr2CaCu2O8+δ phase formation

Abstract Reaction kinetics of high critical temperature superconductors (HTS) has been less studied than their superconducting properties. Usually, doped samples are prepared by solid-state reaction method from oxides and carbonate powders in the same sintering conditions as applied for non-doped samples. This fact has caused publication of different results in the literature about the solubility limit in HTS. Thus, in order to obtain more information about the influence of Pr on the Ca site of the Bi2Sr2CaCu2O8+δ (Bi2212) phase, we have studied in this work the kinetics of phase formation in the Bi2212+Pr system. To investigate the phases formed during sintering at 820°C, several samples of different Pr content were analyzed by X-ray powder diffractometry after various sintering durations (6–1500 h). We observed that increasing Pr content, the sintering time necessary to prepare single phase samples increases significantly at same temperature.

Applying the resistively shunted junction model to polycrystalline superconducting materials

Abstract In order to apply the resistively shunted junction model (RSJM) to high critical-temperature superconductors, we have measured magneto-transport properties in polycrystalline samples of the Y 1− x Pr x Ba 2 Cu 3 O 7− δ system close to the superconductor–insulator transition. Samples were prepared using a solid-state reaction method by mixing high purity Y 2 O 3 , Pr 6 O 11 , BaCO 3 , and CuO powders in the 0⩽ x ⩽0.80 range and characterized by X-ray powder diffractometry, which showed the presence of single phase material in the entire Pr content range. Results of current–voltage ( I – V ) curves measured in several applied magnetic fields showed a magnetic field independent linear regime which was discussed within the context of the RSJM. Finally, the different results of the I–V curves, magneto-resistance measurements and resistance as a function of temperature were analyzed in the same point of view.

57Fe Mössbauer studies on semiconducting and superconducting La-Ba-Cu-O

Abstract La 2− x Ba x CuO 4 samples substituted with 57 Fe (less than 1%) were studied for 0≦ x ≦0.35. Despite the fact that there is only one Cu site which can be occupied by the iron-substituent, the 57 Fe Mossbauer spectra at room temperature show three quadrupole doublets whose intensities were followed in different steps of sample preparation. They are characteristic of Fe 3+ , and their Mossbauer parameters depend on the Ba content. One of the iron species may be due to defects since it disappears with further heat treatment, while the others are present even in the undoped La 2 CuO 4 . Magnetic ordering has been found at 4.2 K for samples with 0.5% Fe even in the superconducting samples with x =0.15.

Possible electrical-current-driven superconductor-insulator transition

Abstract In this work we have studied the effect of applied electrical current on the resistive behavior of single phase Y 1−X Pr X Ba 2 Cu 3 O 7−Y [Y(Pr)123] polycrystalline samples with x close to the critical Pr concentration x c ≈ 0.57 above which the superconductivity vanishes. The obtained results suggest the occurrence of a superconducting-insulator transition (SIT) driven by the electrical current. In particular, a crossing of current-voltage (I–V) isotherms at magnetic-field-dependent current I C (H) is found, and the scaling behavior of nonlinear resistance R(I,T) = V(T)/I similar to that predicted in the scaling theory of SIT is obtained. We speculate that this possible current-induced SIT can be considered as the dynamical counterpart of the magnetic-field-tuned SIT.

Superconducting YBa2Cu3O7−δ films on SrTiO3 by electrodeposition process

Abstract An ac electrodeposition (ED) process to obtain good quality YBa 2 Cu 3 O 7−δ (YBCO) high-temperature superconductor (HTS) films was developed. These films were deposited on SrTiO 3 (100) single crystal substrates previously coated with a thin silver layer (800 A) that was deposited by chemical reduction. A small amount of Ag (0.7% in weight) added to the electrolyte bath was shown to improve the quality of the deposited films. YBCO films after an appropriate heat treatment present single phase material with an onset T c ∼ 90 K, Δ T c ∼ 2 K, homogeneous grain morphology and strong biaxial texture. Transport measurements show a metallic behavior in the normal state and a self-field critical current densities ∼ 10 5 A/cm 2 at 4.2 K and ∼ 10 4 A/cm 2 at 77K.