Well-ordered [(YBa2Cu3O7)nY/(PrBa2Cu3O7)nPr)]m superlattices with high superconducting critical temperatures were prepared by sequential reactive dc sputtering. The superconducting critical current density at 4.5 K reaches values as high as 1.1×107 A/cm2 for a sample with nY = 2 and nPr = 1 and 3.8×106 A/cm2 for nY = 1 and nPr = 3, indicating that the coupling of the CuO2 layers is not necessary for high-jc values. The angular dependence jc(φ) is determined by the component of the magnetic field perpendicular to the a-b planes and shows that these superlattices are two-dimensional systems.
We investigated the transport properties of high-quality YB+ Cu3 0, /PrB+ Cu3 O7 superlattices. The exceptional structural order of the superlattices resulted in satellite peaks up to the ninth order in X-ray diffraction diagrams and high T, values. We find high superconducting critical transport current densities j , even for ultrafine modulated superlattices which proves the existence of nearly continuous YB+Cu307 layers. The activation energy U is found to be constant or to have a linear temperature dependence over a wide temperature range.All Cu-based high-T, superconductors have a layered structure containing superconducting Cu02-planes. The resulting anisotropy of these materials depends on the coupling of the superconducting Cu02-planes. The preparation of YBa, Cu3 O7 /PrBa, Cu3 O7 superlattices (YBCO/PrBCO) allows us to vary the coupling between neighbouring superconducting Cu02-planes in a single system from the rather strongly coupled bulk-YBCO all the way to completely decoupled YBCO-layers of unit cell thickness. Therefore this system offers the possibility to study the transition from an anisotropic 3-dimensional to a 2-dimensional superconductor in a nearly ideal way. Many groups prepared and measured superlattices of this kind [1-4], but though the general results concerning the T, depression with decoupling and the anisotropy of the transport properties are in good agreement, the exact values still show a strong scattering due to nonideal sample preparation. Therefore all values published have to be regarded as lower bounds for an ideal superlattice and improvement of superlattice preparation remains a challenging task for the future.This letter presents new results concerning transport properties of YBCO/PrBCO superlattices with very thin YBCO-sheets. We prepared our samples by high-pressure d.c.-sputtering in pure oxygen atmosphere from stoichiometric YBCO and PrBCO targets onto heated SrTiOs substrates. Details of the sample preparation are described elsewhere [4]. The onset temperatures T,,hd of the inductive superconducting transitions as determined by ax.-susceptibility measurements are shown in fig. 1 for (ma, Cu3 07)% /(PrBa, Cu3 07)%Jnz superlattices. They coincide with the zero resistance temperatures Tc,o of the films. In the notation above ny and npr are the numbers of unit cells of the individual layers and m is the number of supercells in these superlattice films. For
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