John (Jack) W. Ekin

Bio: John (Jack) W. Ekin is an academic researcher. The author has contributed to research in topics: Josephson effect & Diffraction. The author has an hindex of 2, co-authored 3 publications receiving 213 citations.

Josephson Junction Model of Critical Current in Granular Y1Ba2Cu3O7-δ

Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

Critical-current diffraction patterns of grain-boundary Josephson weak links.

Abstract: We discuss the diffraction patterns and other characteristics of the critical current as a function of magnetic field in grain-boundary Josephson barriers. Diffraction patterns occur not just for {ital SIS} junctions but for all types of Josephson links, including {ital SNS} junctions, which may be present at grain boundaries in high-{Tc} superconductors. We discuss the generality of the Airy diffraction pattern, which is expected to characterize grain-boundary barriers in bulk material more accurately than the Fraunhofer pattern. The transport critical-current density in many bulk, granular high-{ital T}{sub {ital c}} superconductors has a power-law dependence on very low magnetic fields, characteristic of averaged diffraction patterns, and cannot be fitted by an exponential magnetic-field dependence, which may result from the material properties of the barriers.

Critical-Current Diffraction Patterns of Grain-Boundary Josephson Weak-Links

Abstract: We discuss the diffraction patterns and other characteristics of the critical current as a function of magnetic field in grain-boundary Josephson barriers. Diffraction patterns occur not just for {ital SIS} junctions but for all types of Josephson links, including {ital SNS} junctions, which may be present at grain boundaries in high-{Tc} superconductors. We discuss the generality of the Airy diffraction pattern, which is expected to characterize grain-boundary barriers in bulk material more accurately than the Fraunhofer pattern. The transport critical-current density in many bulk, granular high-{ital T}{sub {ital c}} superconductors has a power-law dependence on very low magnetic fields, characteristic of averaged diffraction patterns, and cannot be fitted by an exponential magnetic-field dependence, which may result from the material properties of the barriers.

The Flux-Line Lattice in Superconductors

Abstract: Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

Magneto-optical studies of current distributions in high-Tc superconductors

Abstract: In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

AC susceptibility of high temperature superconductors in a critical state model

Abstract: A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

Airy Pattern, Weak-Link Modeling of Critical Currents in High-Tc Superconductors

Abstract: We have measured the transport critical current density at very low magnetic fields in samples of superconducting bulk sintered Y 1 Ba 2 Cu 3 O x , Y 1 Ba 2 Cu 4 O x obtained from several sources. The results are analyzed at low fields (≤10 mT) with a statistical model which assumes that the current is limited by Josephson weak links (SNS or SIS Josephson junctions or microbridges) whose locations are to be determined. Each weak link is assumed to be described by an Airy current-field pattern rather than a Fraunhofer pattern. The former has a better theoretical foundation and is in better agreement with the data, varying as H -3 2 upon averaging. The fitting procedure yields the average cross sectional area of the weak links. By assuming the link thickness to be twice the London penetration depth at 77 K, we find that the average linear dimensions of the links are in all cases comparable to the grain dimensions. The quantitative analysis also confirms the percolation concept, in which a subset of weakest links controls the transport current.