Critical current densities ${(j}_c)$ and pinning forces ${(f}_p)$ in superconducting Pb/Ge multilayers and single WGe films are strongly enhanced by introducing regular arrays of submicron holes (“antidot lattices”) acting as artificial pinning centers. Comparative measurements of $j_c$ and $f_p$ for several well-defined antidot diameters $D$ have shown that pinning centers with a size considerably larger than the temperature-dependent coherence length $\xi \mathrm{}\left(T\right)\mathrm{}$ are much more efficient than those with a size close to $\xi \mathrm{}\left(T\right).$ Moreover, the antidot size realizing the optimum pinning is field-dependent: we need smaller antidots to optimize pinning in lower fields and larger antidots for optimum pinning in higher fields. Crossover between different pinning regimes is controlled by the saturation number $n_s$ that defines the largest possible number of flux lines trapped by an antidot. In dependence upon the $n_s$ value, we have observed various composite flux lattices with vortices at antidots and interstices ${(n}_s\approx 1),$ multiquanta vortex lattices ${(n}_s>1),$ and finally we have reached the limit of superconducting networks at $n_s\gg 1.$

We have studied sharp cusplike magnetization (M) anomalies, appearing at matching fields ${\mathit{H}}_{\mathit{m}}$=m${\mathrm{\phi }}_0$/S in superconducting films with sufficiently large antidots, forming a regular lattice with a unit-cell area S. Exactly at H=${\mathit{H}}_{\mathit{m}}$ each antidot pins the same quantized flux m${\mathrm{\phi }}_0$. This m${\mathrm{\phi }}_0$-flux-line lattice has magnetization M(${\mathit{H}}_{\mathit{m}}$) ∝-m${\mathrm{\phi }}_0$/${\mathrm{\Lambda }}^2$, where Λ is the penetration depth in the film. Between the matching fields ${\mathit{H}}_{\mathit{m}}$<H<${\mathit{H}}_{\mathit{m}+1\mathrm{}}$ the M(H) curve follows a simple M∞-ln(H-${\mathit{H}}_{\mathit{m}}$) dependence. As a result, the whole magnetization curve M(H) can now be successfully described by the simple expression, derived for interacting multiquanta vortices in the London limit. In higher fields H≳${\mathit{H}}_{{\mathit{n}}_{\mathit{s}}}$, when the occupancy of the antidots reaches the saturation number ${\mathit{n}}_{\mathit{s}}$=r/2ξ(T), determined by the antidot radius r and temperature-dependent coherence length ξ(T), the ${\mathrm{\phi }}_0$ vortices begin to fill interstices, thus forming composite flux-line lattices with m${\mathrm{\phi }}_0$ vortices strongly pinned by antidots and ${\mathrm{\phi }}_0$ vortices weakly pinned by interstices. © 1996 The American Physical Society.

Our understanding of the gas exchange mechanisms in plant organs critically depends on insights in the three-dimensional (3-D) structural arrangement of cells and voids. Using synchrotron radiation x-ray tomography, we obtained for the first time high-contrast 3-D absorption images of in vivo fruit tissues of high moisture content at 1.4-μm resolution and 3-D phase contrast images of cell assemblies at a resolution as low as 0.7 μm, enabling visualization of individual cell morphology, cell walls, and entire void networks that were previously unknown. Intercellular spaces were always clear of water. The apple (Malus domestica) cortex contains considerably larger parenchyma cells and voids than pear (Pyrus communis) parenchyma. Voids in apple often are larger than the surrounding cells and some cells are not connected to void spaces. The main voids in apple stretch hundreds of micrometers but are disconnected. Voids in pear cortex tissue are always smaller than parenchyma cells, but each cell is surrounded by a tight and continuous network of voids, except near brachyssclereid groups. Vascular and dermal tissues were also measured. The visualized network architecture was consistent over different picking dates and shelf life. The differences in void fraction (5.1% for pear cortex and 23.0% for apple cortex) and in gas network architecture helps explain the ability of tissues to facilitate or impede gas exchange. Structural changes and anisotropy of tissues may eventually lead to physiological disorders. A combined tomography and internal gas analysis during growth are needed to make progress on the understanding of void formation in fruit.

We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a $2\times 2$-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.