Descripción: Resistive random access memory based on the resistive switching phenomenon is emerging as a strong candidate for next generation non-volatile memory. So far, the resistive switching effect has been observed in many transition metal oxides, including strongly correlated ones, such as, cuprate superconductors, colossal magnetoresistant manganites and Mott insulators. However, up to now, no clear evidence of the possible relevance of strong correlation effects in the mechanism of resistive switching has been reported. Here, we study Pr 0.7 Ca0.3 MnO3, which shows bipolar resistive switching. Performing micro-spectroscopic studies on its bare surface we are able to track the systematic electronic structure changes in both, the low and high resistance state. We find that a large change in the electronic conductance is due to field-induced oxygen vacancies, which drives a Mott metal-insulator transition at the surface. Our study demonstrates that strong correlation effects may be incorporated to the realm of the emerging oxide electronics.

Descripción: Multilevel resistance states in silver-manganite interfaces are studied both experimentally and through a realistic model that includes as a main ingredient the oxygen vacancies diffusion under applied electric fields. The switching threshold and amplitude studied through hysteresis switching loops are found to depend critically on the initial state. The associated vacancy profiles further unveil the prominent role of the effective electric field acting at the interfaces. While experimental results validate main assumptions of the model, the simulations allow to disentangle the microscopic mechanisms behind the resistive switching in metal-transition metal oxide interfaces. © 2010 American Institute of Physics.

Descripción: The study of mineral assemblages and metamorphic facies in rocks exposed on the eastern margin of Brazo Huemul, Nahuel Huapi Lake, allowed the reconsideration of the temporal succession of units whose outcrops are isolated and difficult to locate due to the exuberant forest that covers the southestern part of Neuquén province. The rocks exposed in the study area are mainly of tonalitic composition and have been correlated with the Subcordilleran Patagonian Batholith because they are crosscut by two different types of dikes. One group of dikes shows an albite -chlorite - epidote - cummingtonite/grunerite -hornblende mineral assemblage as a consequence of a thermal metamorphic event transitional between albite - epidote hornfels facies and hornblende hornfels conditions. This process has been attributed to the emplacement of the Cordilleran Patagonian Batholith of Cretaceous age. Because of this, the hornfels dikes were correlated with Montes de Oca Formation. The other group of dikes shows a chlorite + pumpellyite + kumontite + chlorite/esmectite + iron oxides assemblage indicating metamorphism in zeolite facies of high temperature or, sometimes shows propilitic alteration and these dikes have been correlated with the Ventana Formation. The differences between these two types of dikes allow the discussion of the relative age of the tonalitic rocks that host them. In this paper it is proposed that this tonalitic body is of Low to Middle Jurassic age. © 2006 Asociación Geológica Argentina.

Descripción: In this paper the mineralogy and fluid inclusion of the upper sector of an epithermal manganese deposit are described. The study area comprises the central sector of a bigger mineralised zone where non-ore mineral are the most common ones and are composed of barite, calcite and opal with fewer amounts of chalcedony, quartz and manganese oxides. Their textures and their assemblages allow us to determine several events of precipitation. The open spaces between barite crystals were filled by calcite and opal which are commoner species toward the end of the mineralization process. Chalcedony and quartz only occur inside a few small cavities. Fluid inclusions were studied in barite, calcite and quartz samples. They are composed of H 2O, NaCl and fewer amounts of CaCl 2 and MgCl 2. Calcite and barite samples have an average salinity of about 4.18 %wt NaCleq., meanwhile quartz evidences salinities around 5.26%wt NaCleq. Homogenization temperatures are also different for each species: quartz shows an average temperature of about 148°C, meanwhile barite and calcite seem to be a little higher (184° and 171°C respectively). All the data compiled in this paper suggest that this district represents one of the uppermost sectors of the ore deposit. The high amount of non-ore minerals evidences that this area, in contrast with the others ones, was especially active during the end of the mineralization process.

Descripción: A comprehensive study of nonstoichiometry titanium oxide thin films (TiO x , 0.3≤x≤2) prepared by ion beam deposition technique is reported. The physical properties of the material are studied by ultraviolet and x-ray photoelectron, Raman, and Fourier transform infrared spectroscopies, and atomic force microscopy. An abrupt transition from metallic characteristics to a wide gap semiconductor is observed in a very narrow range of oxygen variation. Concomitantly with this change the crystal structure and morphology suffer remarkable physical properties modifications. This transformation is ascribed to surface-volume energy minimization due to the influence of oxygen determining the size of the TiO 2 particles during coalescence. © 2010 American Institute of Physics.

Descripción: Continuum solvent models have become a standard technique in the context of electronic structure calculations, yet no implementations have been reported capable to perform molecular dynamics at solid-liquid interfaces. We propose here such a continuum approach in a density functional theory framework using plane-wave basis sets and periodic boundary conditions. Our work stems from a recent model designed for Car-Parrinello simulations of quantum solutes in a dielectric medium [D. A. Scherlis, J. Chem. Phys. 124, 074103 (2006)], for which the permittivity of the solvent is defined as a function of the electronic density of the solute. This strategy turns out to be inadequate for systems extended in two dimensions: the dependence of the dielectric function on the electronic density introduces a new term in the Kohn-Sham potential, which becomes unphysically large at the interfacial region, seriously affecting the convergence of the self-consistent calculations. If the dielectric medium is properly redefined as a function of the atomic coordinates, a good convergence is obtained and the constant of motion is conserved during the molecular dynamics simulations. The Poisson problem is solved using a multigrid method, and in this way Car-Parrinello molecular dynamics simulations of solid-liquid interfaces can be performed at a very moderate computational cost. This scheme is employed to investigate the acid-base equilibrium at the TiO2 -water interface. The aqueous behavior of titania surfaces has stimulated a large amount of experimental research, but many open questions remain concerning the molecular mechanisms determining the chemistry of the interface. Here we make an attempt to answer some of them, putting to the test our continuum model. © 2009 American Institute of Physics.

Descripción: Magnetic elastic structured composites were prepared by using CoFe 2O4 ferromagnetic and superparamagnetic nanoparticles as fillers in polydimethylsiloxane (PDMS) matrixes, which were cured in the presence of a uniform magnetic field. Cobalt-iron oxide nanoparticles of three different average sizes (between 2 and 12 nm) were synthesized and characterized. The smallest nanoparticles presented superparamagnetic behavior, with a blocking temperature of approximately 75 K, while larger particles are already blocked at room temperature. Macroscopically structured-anisotropic PDMS-CoFe2O4 composites were obtained when curing the dispersion of the nanoparticles in the presence of a uniform magnetic field (0.3 T). The formation of the particle's chains (needles) orientated in the direction of the magnetic field was observed only when loading with the larger magnetically blocked nanoparticles. The SEM images show that the needles are formed by groups of nanoparticles which retain their original average size. The Young's moduli of the structured composites are four times larger when measured along the oriented needles than in the perpendicular direction. Magnetization (VSM) and ferromagnetic resonance curves of the structured composites were determined as a function of the relative orientation between the needles and the probe field. The remanence magnetization was 30 higher when measured parallel to the needles, while the coercive field remains isotropic. These observations are discussed in terms of the individual nanoparticle's properties and its aggregation in the composites. © 2011 American Institute of Physics.

Descripción: Although transcriptional effects of thyroid hormones have substantial influence on oxidative metabolism, how thyroid sets basal metabolic rate remains obscure. Compartmental localization of nitric-oxide synthases is important for nitric oxide signaling. We therefore examined liver neuronal nitric-oxide synthase-α (nNOS) subcellular distribution as a putative mechanism for thyroid effects on rat metabolic rate. At low 3,3′,5-triiodo-L-thyronine levels, nNOS mRNA increased by 3-fold, protein expression by one-fold, and nNOS was selectively translocated to mitochondria without changes in other isoforms. In contrast, under thyroid hormone administration, mRNA level did not change and nNOS remained predominantly localized in cytosol. In hypothyroidism, nNOS translocation resulted in enhanced mitochondrial nitric-oxide synthase activity with low O2 uptake. In this context, NO utilization increased active O2 species and peroxynitrite yields and tyrosine nitration of complex I proteins that reduced complex activity. Hypothyroidism was also associated to high phospho-p38 mitogen-activated protein kinase and decreased phospho-extracellular signal-regulated kinase 1/2 and cyclin D1 levels. Similarly to thyroid hormones, but without changing thyroid status, nitric-oxide synthase inhibitor Nω-nitro-L-arginine methyl ester increased basal metabolic rate, prevented mitochondrial nitration and complex I derangement, and turned mitogen-activated protein kinase signaling and cyclin D1 expression back to control pattern. We surmise that nNOS spatial confinement in mitochondria is a significant downstream effector of thyroid hormone and hypothyroid phenotype. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.