M. Dhalle

Ceramic superconductors synthesized by sol-gel methods

A sol-gel method is described based on the use of organic complexing agents in liquid solution, such as citric acid, ethylene-diamine-tetra-acetic acid (EDTA) and others. It is shown how the procedures can be designed on a scientific basis, using pM′-pH diagrams for the respective cation and complexing agent. This technique yields powders and sintered materials with superior properties to the dry mix and calcine method. An important advantage of the method is its versatility due to the fact that stable complexes can be formed for a wide range of elements. Precise control of stoichiometry is possible, however the residual carbon level is more difficult to control.

Quantum interference and confinement phenomena in mesoscopic superconducting systems

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic AI samples of different topology: lines, open and filled squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j, has been achieved. Sharp integer and rational matching peaks in the j,(H) curve are observed. The possibility of the “quantum design” of the superconducting critical parameters (H,(T) and jc(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

High temperature superconductivity in the compound (BiCaSr)CuxOy

Abstract An onset of superconductivity at 90 K and a zero resistance at 72 K were observed in the new high T c compound BiCaSrCu x O y . No evidence was found for a sharp resistance drop above 100K as reported by Maeda et al. A maximum in the T c value was measured for a Cu fraction x = 2. Quenching experiments reveal that also in this compound the oxygen content is crucial for a high T c .

A new method of synthesizing high-Tc superconducting materials

Abstract A new sol-gel method for the preparation of the ceramic superconductors has been developed based upon the formation of complexes of the metal cations by Ethylene Diamine Tetraacetic Acid (EDTA). The materials have been characterized by transport, magnetisation, critical field, XRD and SEM measurements. Screen printed thick films made out of the powders have been fabricated and characterized.

Critical fields of the high-Tc BiCaSrCuO compound

Using pulsed magnetic fields up to 27 Tesla, we have measured the temperature dependence of the upper critical field for the Bi-Ca-Sr-Cu-0 compound. From these measurements we infer a superconducting coherence length of the order of 1 nm. Close to T c the appearance of a finite resistance at very low magnetic fields as well as the small critical current density, confirm the importance of Josephson coupling in the Bi-Ca-Sr-Cu-O samples

Anomalous Little-Parks oscillations and field enhanced superconductivity in mesoscopic loops

Quantum interference phenomena have been studied in superconducting Al loops with dimensions smaller than the temperature dependent coherence length ξ(T). Besides the classical Little-Parks (LP) effect, anomalous resistance oscillations have been observed whose amplitude shows an unusual magnetic field and current dependence at low flux quanta. The magnetic field (H) - critical temperature (Tc) phase boundary has been measured for different transport currents jt. A clear field enhancement of Tc is observed at low current densities. The amplitude of the field enhancement of Tc is strongly influenced by the transport current through the loop. Higher jt values induce a transition from the Tc(H) boundary with LP oscillations to a new regime without oscillations. This transition is governed by the interplay between the characteristic superconducting length scale ξ(T), the loop circumference 2πr and the de Broglie wavelength λD ∝ (1/jt), tuned by the transport current.

Properties and microstructure of YBa2Cu3Ox/Ag composites prepared by a sol-gel technique

Abstract YBa 2 Cu 3 O x /Ag composites were prepared by a sol-gel method to assure molecular mixing of components. Up to 0.6 wt% Ag can dissolve in the YBa 2 Cu 3 O x ; this done not affect critical temperature. Superconducting properties could be fully maintained in composites with up to 20 vol% Ag. Critical current density was highest in materials with 15 vol% Ag.

Correlation between critical current and resistivity in YBa2Cu3O7

Abstract Critical current versus temperature measurements were carried out in ceramic YBa 2 Cu 3 O 7 samples. The samples were all prepared using the same sintering procedure, but starting from powders obtained by different wet and dry methods. All samples showed a linear J c vs. T behaviour, typical of Josephson weak-link coupling, with a slope that depends on the preparation method. Using the Ambegaokar-Baratoff relation for J c , the difference in slopes can be attributed to a change in the characteristic value of the junction resistance. This value turns out to be clearly correlated with the resistivity in the normal state.

Anisotropy and magnetic field dependence of critical currents in Pb/Ge multilayers

Abstract We have measured critical currents in Pb/Ge multilayers as .a function of direction and magnitude of a magnetic field perpendicular to the current. The higher J c for a field parallel to the layers indicates flux pinning at the Pb/Ge interfaces. The critical current as a function of a perpendicular field displays a pronounced maximum at low fields. TEM reveals Pb grain sizes of about 1000 A and indicates that the enhanced flux pinning may be explained by grain boundary pinning.

Microstructure and superconducting properties of Bi-Pb-Sr-Ca-Cu-O synthesised by the alkoxy sol-gel method

A sol-gel method for preparing superconducting powders by alkoxy derived materials is described. The Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10 samples were obtained from a solution of nitrates of the cations dissolved in a propylalcohol. A combination of electrical, magnetic, structural and stoichiometric measurements were used to analyse the properties of the samples.