Jacobo Santamaria

“Charge Leakage” at LaMnO3/SrTiO3 Interfaces

We report on the charge transfer at the interface between a band (SrTiO3) and a Mott insulator (LaMnO3) in epitaxial superlattices. We have used combined atomic resolution electron microscopy and spectroscopy, synchrotron X ray reciprocal space maps and magneto transport measurements, to characterize the interface properties. The LaMnO3 layers are always started and terminated in (LaO) planes, giving an overall electron doping to the system. However, the direction of charge leakage is determined by the manganite to titanate thickness ratio in a way controlled by the different epitaxial strain patterns. This result may provide a clue to optimize oxide devices such as magnetic tunnel junctions and field effect transistors whose operation is determined by the interface properties.

Nanoscale analysis of YBa2Cu3O7−x/La0.67Ca0.33MnO3 interfaces

The structure of interfaces in superconducting/ferromagnetic YBa2Cu3O7−x/La0.67Ca0.33MnO3 superlattices has been analyzed by scanning transmission electron microscopy and high spatial resolution electron energy loss spectroscopy. Individual layers are flat over long lateral distances. The interfaces are coherent, free of defects, exhibiting no roughness, and are located at the BaO plane of the superconductor. Concerning chemical disorder, EELS measurements show the absence of measurable chemical interdiffusion within experimental error bars.

Tailoring Disorder and Dimensionality: Strategies for Improved Solid Oxide Fuel Cell Electrolytes

Reducing the operation temperature of solid oxide fuel cells is a major challenge towards their widespread use for power generation. This has triggered an intense materials research effort involving the search for novel electrolytes with higher ionic conductivity near room temperature. Two main directions are being currently followed: the use of doping strategies for the synthesis of new bulk materials and the implementation of nanotechnology routes for the fabrication of artificial nanostructures with improved properties. In this paper, we review our recent work on solid oxide fuel cell electrolyte materials in these two directions, with special emphasis on the importance of disorder and reduced dimensionality in determining ion conductivity. Substitution of Ti for Zr in the A(2)Zr(2-) (y)Ti(y)O(7) (A = Y, Dy, and Gd) series, directly related to yttria stabilized zirconia (a common fuel cell electrolyte), allows controlling ion mobility over wide ranges. In the second scenario we describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating strontium titanate and yttria stabilized zirconia ultrathin films. We conclude that cooperative effects in oxygen dynamics play a primary role in determining ion mobility of bulk and artificially nanolayered materials and should be considered in the design of new electrolytes with enhanced conductivity.

Optical analysis of absorbing thin films: application to ternary chalcopyrite semiconductors

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.

METAL-INSULATOR TRANSITION IN SRRUO3 INDUCED BY ION IRRADIATION

We have studied the effect of He+ irradiation on the electrical resistivity and Curie temperature of ferromagnetic SrRuO3 thin films. An evolution from metallic to insulating behavior is observed when He+ 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 (similar to 160 K) 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.

A simple closed-form expression for the X-ray reflectivity from multilayers with cumulative roughness

Abstract We present a simple closed-form expression for the reflectivity from a multilayer which includes the effects of absorption, refraction, surface and substrate reflections but neglects dynamical effects. This expression reproduces the exact dynamical calculation except for the regions near the critical angle and for intense Bragg reflections. The expression is generalized to include cumulative interface roughness which follows a t 1 2 power-law growth.

Non-Debye conductivity relaxation in the non-Arrhenius Li0.5La0.5TiO3 fast ionic conductor. A nuclear magnetic resonance and complex impedance study

Abstract An analysis of electrical conductivity relaxation and nuclear magnetic resonance spin-lattice relaxation has been conducted to study the dynamics of the ionic diffusion process in the crystalline ionic conductor Li 0.5 La 0.5 TiO 3 . Dc conductivity shows a non-Arrhenius temperature dependence, similar to the one recently reported for some ionic conducting glasses. Taking into account this non-Arrhenius dependence for the relaxation times, spin-lattice and conductivity relaxations are compared in the same frequency and temperature ranges. A single “stretched exponential” relaxation function of the form f ( t )=exp(−( t / τ ) β ), with β =0.4, can be used to describe both relaxation processes in which correlation times differ in the preexponential factor.

Lithium intercalation in FeOCl revisited

The reaction of iron oxychloride with n-butyl-lithium has been re-examined, important changes being found in the structure and properties of the lithiated compounds as compared to the pristine material. Lithium intercalation by these means increases several orders of magnitude the electrical conductivity of FeOCl although the intercalate remains semiconductor. From the structural and microstructural point of view the lithiation results in complex changes, including inter alia staging, superstructures, etc., and it seems to be quite an heterogeneous process.

USE OF KRAMERS-KRONIG TRANSFORMS FOR THE TREATMENT OF ADMITTANCE SPECTROSCOPY DATA OF P-N JUNCTIONS CONTAINING TRAPS

The use of Kramers–Kronig transforms is proposed for the treatment of admittance spectroscopy data of junctions when significant shunt conductance or series resistance is present. An algorithm has been implemented to calculate the transformations numerically and the validity of the method developed has been tested using simulated data. Two experimental systems, p‐n junctions into InP made by ion implantation, and atomic‐layer‐epitaxy‐grown CdS/CdTe heterojunctions, have been characterized using this procedure.

Analytical distributions of relaxation times for the description of electrical conductivity relaxation in ionic conductors

Abstract Electrical conductivity relaxation was measured in three different ionic conductors (yttria-stabilized zirconia, Li0.5La0.5TiO3 and Li0.5.5Na0.5La(Cr04)2), using the admittance spectroscopy technique. A study of the frequency dependence of the dielectric modulus is presented, which shows that the Havriliak-Negami dielectric functions describe the relaxation process accurately. This allows the use of an analytical distribution of relaxation times to determine the time decay function from admittance data in the frequency domain. Electrical conductivity relaxation of the measured ionic conductors has been analysed using this method showing strong non-Debye frequency behaviour and ‘stretched’-exponential Kohlrausch-Williams-Watts time decay functions. The results obtained are interpreted in terms of cooperative effects or correlation among ions. Activation energies for long- and short-range ion motion are deduced.