A study of the effect of correlated ion motion on the electrical conductivity relaxation in single-crystalline yttria-stabilized zirconia is presented. Complex admittance in the radio frequency range show power-law dependencies in the real part of the conductivity at high frequencies of the form n and asymmetric electric modulus plots as a result of correlations. An analysis of the frequency dependence of the electric modulus is conducted to obtain time decay functions of the form exp͓Ϫ(t/ ) ␤ ͔ from an analytical distribution of relaxation times. Correlation times, and parameters n and ␤ characterizing the relaxation in time and frequency domains are compared to show the equivalence of time and frequency representations. The common origin of ac and dc processes is discussed in view of the frequency dependence of the complex conductivity. From a macroscopic activation energy for ion motion Eϭ1.16 eV and a ␤ value of 0.43, a single-ion microscopic activation energy E a ϭ0.5 eV is obtained as ␤E according to Ngai's coupling model. The microscopic activation energy is related to the association energy of oxygen vacancies.

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...

Lithium ionic conductivity of Li 0.5 La 0.5 TiO 3 has been studied using nuclear magnetic resonance ͑NMR͒ and admittance spectroscopy ͑AS͒ techniques. Spin-lattice relaxation and electrical conductivity relaxation are well described in terms of stretched-exponential correlation functions in the time domain of the form (t)ϭexp"Ϫ(t/) ␤ …, but showing different relaxation times scales ( 0 ϭ1.4ϫ10 Ϫ11 s from NMR and 0 ϭ10 Ϫ14 s from AS͒, and activation energies ͑0.15 and 0.4 eV, respectively͒. Different ␤ exponents, 1 from spin lattice relaxation and 0.4 from electric-field relaxation have been also deduced. A microscopic activation energy for lithium motion of 0.15 eV is deduced from both techniques. Discrepancies between both techniques are analyzed and discussed in terms of frequency-dependent correlation effects. ͓S0163-1829͑96͒00225-1͔

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 ...

A set of capacitance measurements is proposed to identify the different contributions to the junction capacitance (diffusion capacitance and depletion layer capacitance) of p-n Si diodes. By measuring the C-f and C-V characteristics of Si commercial diodes, we fully characterize the AC behaviour of such devices. At reverse bias voltages, only the depletion layer capacitance is present and it is frequency independent in our range of measurement (up to 13 MHz). From C-V characteristics, we deduce a linearly graded junction nature and a built-in value of 0.59 ± 0.02 V. At forward bias voltages and frequencies lower than 100 kHz, both the diffusion and the depletion layer capacitances contribute to the junction capacitance. A simple calculation allows us to obtain a value for the average minority carrier lifetime of tau = (4 ± 2) × 10-6 s. Finally, a voltage dependence of exp(qV/2kT) is deduced for the diffusion capacitance.