The structure of high quality ͓YBCO N ͞PBCO M ͔ 1000 ± A superlattices, with N ranging between 1 and 12 unit cells and M 5 unit cells, grown by high oxygen pressure sputtering, is analyzed. Intracell atomic structure of the layers along the c axis and disorder at interfaces is investigated using an x-ray refinement technique. Negligible roughness, step disorder, and interdiffusion are found at the interfaces. Epitaxial mismatch strain results in a surprising reorganization of interatomic distances for the thinnest YBCO layers, which seems to correlated with the decrease in the critical temperature. Intracell structure is invoked as an additional source of T c changes in very thin YBCO layers. PACS numbers: 74.76.Bz, 61.10.Nz, 68.65. + g Since the discovery of the high T c superconductivity, structure has been recognized to play a crucial role towards the understanding of its nature and mechanisms. It has been known for years that distortions arising from cation substitution can produce significant changes in T c [1], and recent experiments on doped La 2 CuO 4 superconductors at constant carrier concentration show a clear dependence of T c on lattice strains [2]. A great effort has been put in structure determination under hydrostatic pressure [3]. Epitaxial stress in thin films offers a simple way to arrive at a strain pattern not attainable under hydrostatic pressure [4]: According to the Poisson effect, film growth on a substrate with slightly smaller (larger) in-plane lattice parameters may lead to a compression (expansion) in the ab plane that can result in an expansion (contraction) in the out-ofplane direction. Uniaxial epitaxial strain, together with Poisson's ratios, has been addressed before [5]. However, the general applicability of the Poisson effect to thin films is still doubtful [6], especially in these highly anisotropic materials. Anyway, Locquet et al. [7] have been able to double the critical temperature in the La 1.9 Sr 0.1 CuO 4 high T c superconductor using mismatch strain. They show that compressive epitaxial strain in-plane can generate much larger increases in T c than those obtained by comparable hydrostatic pressures, and their claim is that the distance relevant to the mechanism of the superconductivity being modified is the separation between consecutive CuO 2 planes. Mismatch strain constitutes an alternative way to change the intracell distances which may be "relevant" to the mechanism of superconductivity, but a quantitative structure analysis of strained films is necessary. X-ray diffraction is a widely used technique to analyze structure, which supplies structural information averaged over a length scale (structural coherence length) which may be around a hundred angstroms. The extraction of quantitative information requires the fit of the diffraction pattern to a structure model containing a large number of parameters in these complex materials, and, therefore, results may not be very reliable for single epitaxial films, which usually show a reduced number of diffraction peak...

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The properties of symmetric 24 • and asymmetric 45 • [100] tilt YBa 2 Cu 3 O 7−x bicrystal grain boundary junctions (GBJs) have been extensively investigated. A large number of junctions, with dimensions ranging from 2 to 20 µm, have been fabricated and characterized at 4.2 K. Experimental data have been compared with [001] tilt bicrystal and [100] tilt biepitaxial YBa 2 Cu 3 O 7−x junctions. [100] GBJs show high I c R n products in the range 5-10 mV at T = 4.2 K, these being slightly sensitive to the critical current density values. A direct tunnelling mechanism for both Cooper pair and quasiparticles has been evidenced. The reduced I c R n values of asymmetric 45 • with respect to symmetric 24 • [100] tilt GBJs have been accounted for by considering a d-wave symmetry of the order parameter using the Sigrist-Rice equation.

We have studied the effect of He ϩ irradiation on the electrical resistivity and Curie temperature of ferromagnetic SrRuO 3 thin films. An evolution from metallic to insulating behavior is observed when He ϩ ion fluence is increased, suggesting a metal-insulator transition. Damage by ion irradiation produces a strong decrease of the Curie temperature. On the other hand, no significant change in T c ͑ϳ160 K͒ takes place in fresh samples grown at different substrate temperatures. We discuss the possible correlation between structural changes induced by irradiation, which reflect in an increase of the pseudocubic lattice parameter, and the observed depression of T c . © 1998 American Institute of Physics. ͓S0003-6951͑98͒03349-X͔ SrRuO 3 is an orthorhombically distorted perovskite ͑space group Pbnm͒, and the only example of ferromagnetic ordering ͑with a Curie temperature T c , of 165 K͒ in conducting 4d transition-metal oxides. 1,2 This compound has a 4d 4 low spin configuration (Sϭ1) and is believed to have a narrow * band resulting from Ru t 2g and O 2 p orbitals, which governs magnetic ordering. 3,4 The actual nature of magnetism in this material, usually considered as an example of pure itinerant magnetism, 5 is not fully understood yet. This system can be included in the group of the so-called ''bad metals,'' 5,6 strongly correlated electron systems showing remarkable electrical and magnetic properties. A common feature of these materials is the high value of their roomtemperature electrical resistivities, close to the theoretical limit for the metallic state ͑Ioffe-Regel limit͒. From the point of view of possible technological applications, both the crystallographic structure ͑pseudocubic perovskite͒ and lattice parameters of this material, close to those of ͑YBCO͒, make it a promising candidate for the fabrication of ͑SNS͒ Josephson junctions of high-T c superconducting oxides. 7 In addition, its conducting character makes it a suitable material for electrodes in other types of devices based on perovskite oxides, as epitaxial conducting oxide-ferroelectric or superconduction-ferroelectric heterostructures. 8 Structural distortions are known to play a central role in the magnetic properties of SrRuO 3 . The absence of ferromagnetic ordering in the isostructural CaRuO 3 has been explained in terms of a stronger orthorhombical distortion in this compound which changes the sign of the magnetic interaction. 9 Both hydrostatic and chemical pressure resulting from the partial substitution of Sr 2ϩ ͑0.62 A͒ by the bigger Ca 2ϩ ͑1.06 A͒ give rise to a decrease of the Curie temperature, but the system remains metallic. This has been explained in terms of the magnetism of this compound being very sensitive to the Ru-Ru distance. 10 On the other hand, substitution of Ru by Ti up to 20% rapidly reduces the critical temperature and causes an increase of the resistivity which undergoes a crossover from metallic to semiconducting behavior at low temperatures. Once again, this has been proposed to occur as a result of local ...

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