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...

We present complex admittance measurements on single-crystal yttria-stabilized zirconia and polycrystalline Li 0.5 La 0.5 TiO 3 over the frequency range 5 Hz to 30 MHz and at temperatures ranging between 150 and 650 K. Electric-field relaxation in both fast ionic conductors can be described using Kohlrausch-Williams-Watts decay functions, but departures are observed at high frequencies and low temperatures. Electric modulus data obey the Dixon-Nagel scaling that has been proposed to be universal in describing the relaxation processes in supercooled liquids. Our data provide broader universality to the Dixon-Nagel scaling, and are interpreted in terms of the influence of mobile ions positional disorder on the relaxation dynamics. ͓S0163-1829͑98͒06701-0͔Much interest has been paid in recent years to the study of the effect of ion-ion or ion-lattice correlations on the dynamic response of ionic conductors. In this field, electrical conductivity relaxation from complex admittance measurements is one of the most frequently used tools to characterize the effect of cooperativity on ion motion. It turns out that many-body interactions give rise to non-Debye response functions, 1-3 and these functions are usually well reproduced using stretched exponential relaxation functions of the Kohlrausch-Williams-Watts ͑KWW͒ ͑Ref. 4͒ kind, i.e., (t)ϭe Ϫ(t/ ) ␤ , with the exponent ␤ comprised between 0 and 1.KWW functions have proven to describe the relaxation processes in very different classes of disordered systems ranging from electric-field relaxation in glasses, 5 or stress relaxation in glasses, 6 to dielectric relaxation in glassforming polymers 7 and supercooled liquids, 8 etc., at least over a few orders of magnitude of frequency around the relaxation peak. However, a description of the relaxation process over wide frequency and temperature ranges is often impossible using a KWW function: significant departures from this behavior have been frequently observed at short times compared with the relaxation time . 9 In this context an universal scaling of the ␣ relaxation in supercooled liquids has been proposed, 8 showing that data of different materials over wide frequency and temperature ranges fall on a single scaling curve. The scaling curve clearly shows the departure from the KWW behavior at high frequencies. This scaling has also shown to be valid in other systems like orientationally disordered crystals, 10 and spin glasses. 11 In this paper we present data of conductivity relaxation in two crystalline ionic conductors: single-crystal yttriastabilized zirconia and polycrystalline Li 0.5 La 0.5 TiO 3 . We show that electric modulus plots may also be normalized using the Dixon-Nagel scaling procedure. 8 It points to a broader degree of universality of this scaling plot and, besides, may shed some light on the nature of the ionic conduction process in crystalline solids and the role played by the positional disorder of mobile ions in fast ionic conductors. 12 We have measured complex admittance over the frequency range 5 ...

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 ...

The refractive index n and the absorption coefficient a of radio frequency sputtered CuGaSe(2) and CuInSe(2) thin films were obtained by means of transmissivity (T) and reflectivity (R) measurements at normal incidence. The optical properties were determined from the rigorous expressions for the transmission and the reflection in an air/film/(glass)substrate/air multilayer system. The solutions to this system of equations are not unique, and the physically meaningful solution is identified by trying different thicknesses in the numerical approach. Usually, nonacceptable n dispersion curves are found for all thicknesses. To be able to obtain a good n dispersion curve and, therefore, a correct absorption coefficient, we propose a simple modification of the equations for R and T through a factor called the coherence factor (CF). Because of the surface roughness and the nonuniformity of n and alpha, the light rays that reflect internally in the interface between the substrate and the film have a random difference in opt cal path. The CF accounts for this effect. This modification leads to an unambiguous and accurate determination of the optical properties and thickness of thin films for all wavelengths where transmission is not negligible. The CF is shown to be greatly dependent on the thickness of the film. This method can be used even when the R and T spectra do not have interference fringes. This method is applied successfully to the optical analyses, in the 0.4-2.5-mum wavelength range, of CuInSe(2) and CuGaSe(2) ternary chalcopyrite thin films deposited onto glass substrates by radio-frequency sputtering.

We have studied the effect of He+ irradiation with doses in the range 1012–2×1015 cm−2 on two high-temperature superconducting structures: YBa2Cu3O7−x (YBCO) films and YBa2Cu3O7−x/PrBa2Cu3O7 (YBCO/PBCO) superlattices. In particular we have focused on superlattices [YBCON/PBCOM]1000 Å with N=1, 8 unit cells of YBCO, and M=5 unit cells of PBCO, with a total thickness of 1000 Å. The analysis is presented in terms of depression of the critical temperature (Tc) and modification of the crystalline structure using X ray refinement technique. Single films show a systematic increase in the c-lattice parameter upon irradiation, which is not observed in thin one unit cell YBCO layers in superlattice structures. However, Tc depression resulting from irradiation is deeper in superlattices. These results are explained considering the strained nature of the as-grown [YBCON/PBCO5]1000 Å superlattices with low values of N. Both structures show persistent photoconductivity, indicating that defects are related to oxygen displacements in the Cu–O chains.