Calibration of piezoelectric positioning actuators using a reference voltage-to-displacement transducer based on quartz tuning forks

We use a piezoelectric quartz tuning fork to calibrate the displacement of ceramic piezoelectric scanners which are widely employed in scanning probe microscopy. We measure the static piezoelectric response of a quartz tuning fork and find it to be highly linear, non-hysteretic and with negligible creep. These performance characteristics, close to those of an ideal transducer, make quartz transducers superior to ceramic piezoelectric actuators. Furthermore, quartz actuators in the form of a tuning fork have the advantage of yielding static displacements comparable to those of local probe microscope scanners. We use the static displacement of a quartz tuning fork as a reference to calibrate the three axis displacement of a ceramic piezoelectric scanner. Although this calibration technique is a non-traceable method, it can be more versatile than using calibration grids because it enables to characterize the linear and non-linear response of a piezoelectric scanner in a broad range of displacements, spanning from a fraction of a nanometer to hundreds of nanometers. In addition, the creep and the speed dependent piezoelectric response of ceramic scanners can be studied in detail.Comment: 9 pages, 3 figure

Characterization of single-molecule pentanedithiol junctions by inelastic electron tunneling spectroscopy and first-principles calculations

We study pentanedithiol molecular junctions formed by means of the break-junction technique with a scanning tunneling microscope at low temperatures. Using inelastic electron tunneling spectroscopy and first-principles calculations, the response of the junction to elastic deformation is examined. We show that this procedure makes a detailed characterization of the molecular junction possible. In particular, our results indicate that tunneling takes place through just a single molecule.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B

Origem das variações de cores das escórias do vulcão Sierra Negra das Ilhas Galápagos

Volcanic cinder, also known as scoria, is an extrusive igneous rock that forms when gas-rich magmas of basaltic or andesitic composition cool quickly. It is typically dark in color, ranging from black to red depending on its chemical composition. Sometimes fresh cinder samples show a variety of shiny metallic colors on its surface ranging from blue to gold to silver. The origin of these colors has remained unknown up to now. Cinder samples from an eruptive event occurred in October 2005 have been collected in the surroundings of the Sierra Negra volcano in the Galápagos Islands. The samples’ crystallographic structure, chemical composition, and surface morphology have been analyzed using X-Ray diffractometry (XRD), energy dispersive X-Ray spectroscopy (EDS) and a field gun emission scanning electron microscopy (SEM), respectively. Based on an extensive physical and chemical analysis, we were able to demonstrate that these colors are due to a light interference phenomenon. These results have a great potential to be used for a wide variety of purposes such as determining the temperature and composition of magma and evaluating volcanic samples for planetary studies.Las cenizas volcánicas, también conocidas como escorias, son una roca ígnea extrusiva que se forma cuando los magmas ricos en gas de composición basáltica o andesítica se enfrían rápidamente. Es típicamente de color oscuro, que va del negro al rojo dependiendo de su composición química. A veces las muestras de ceniza fresca muestran una variedad de colores metálicos brillantes en su superficie que van del azul al oro y a la plata, dependiendo de la orientación de la muestra. Hasta ahora, el origen de estos colores ha permanecido desconocido. Muestras de cenizas de un evento eruptivo ocurrido en octubre de 2005, han sido recogidas en los alrededores del volcán Sierra Negra en las Islas Galápagos. La estructura cristalográfica, la composición química y la morfología de la superficie de estas muestras se han analizado utilizando la difracción de rayos X (XRD), la espectroscopia de rayos X de dispersión de energía (EDS) y la microscopia electrónica de barrido con emisión de campo (SEM), respectivamente. Basándonos en un extenso análisis físico y químico, hemos podido demostrar que estos colores se deben a un fenómeno de interferencia de la luz. Estos resultados tienen un gran potencial para ser utilizados para una amplia variedad de propósitos como la determinación de la temperatura y la composición del magma, así como la evaluación de muestras volcánicas para estudios planetarios.As cinzas vulcânicas, também conhecidas como escórias, são uma rocha ígnea extrusiva formada quando os magmas ricos em gás de composição basáltica ou andesítico esfriam rapidamente. É tipicamente de cor escura, que vai do preto ao vermelho, dependendo de sua composição química. Às vezes as amostras de cinza fresca mostram uma variedade de cores metálicas brilhantes em sua superfície que vão do azul ao ouro e à cor prata, dependendo da orientação da amostra. Até o momento, a origem destas cores permanece desconhecida. Amostras de cinzas de um evento eruptivo ocorrido em outubro de 2005, foram recolhidas nos arredores do vulcão Sierra Negra nas Ilhas Galápagos. A estrutura cristalográfica, a composição química e a morfologia da superfície destas amostras foram analisadas por meio da difração de raios X (XRD), da espectroscopia de raios X por energia dispersiva (EDS) e da microscopia eletrônica de varredura com emissão de campo (SEM), respectivamente. Com base em uma extensa análise física e química, pudemos demonstrar que essas cores se devem a um fenômeno de interferência da luz. Esses resultados têm um grande potencial para serem utilizados com uma ampla variedade de propósitos como a determinação da temperatura e a composição do magma, bem como a avaliação de amostras vulcânicas para estudos planetários

On the large N limit, W_\infty Strings, Star products, AdS/CFT Duality, Nonlinear Sigma Models on AdS spaces and Chern-Simons p-branes

It is shown that the large $N$ limit of SU(N) YM in $curved$ $m$-dim backgrounds can be subsumed by a higher $m+n$ dimensional gravitational theory which can be identified to an $m$-dim generally invariant gauge theory of diffs $N$, where $N$ is an $n$-dim internal space (Cho, Sho, Park, Yoon). Based on these findings, a very plausible geometrical interpretation of the $AdS/CFT$ correspondence could be given. Conformally invariant sigma models in $D=2n$ dimensions with target non-compact SO(2n,1) groups are reviewed. Despite the non-compact nature of the SO(2n,1), the classical action and Hamiltonian are positive definite. Instanton field configurations are found to correspond geometrically to conformal ``stereographic'' mappings of $R^{2n}$ into the Euclidean signature $AdS_{2n}$ spaces. The relation between Self Dual branes and Chern-Simons branes, High Dimensional Knots, follows. A detailed discussion on $W_\infty$ symmetry is given and we outline the Vasiliev procedure to construct an action involving higher spin massless fields in $AdS_4$. This $AdS_4$ spacetime higher spin theory should have a one-to-one correspondence to noncritical $W_\infty$ strings propagating on $AdS_4 \times S^7$.Comment: 43 pages, Tex fil

Evaluación de la harina de pejibaye (Bactris gasipaes) en dietas para gansos durante el período de inicio

Se evaluó la sustitución de maíz por harina de pejibaye en dietas para gansos en el período de inicio (0-4 semanas). En el experimento se evaluó consumo de alimento,  ganancia de peso, conversión alimenticia, conversión calórica y porcentaje de mortalidad de 224 gansos de 1 día de edad. Las aves sin sexar eran de las razas Tolouse, Embden, China y Africana , los cuales se distribuyeron en cuatro tratamientos, los cuales fueron la sustitución del maíz en la dieta control por harina de pejibaye en  niveles de 0, 25, 50 y 75%, en raciones isoproteicas pero no isocalóricas, durante cuatro semanas que duró la prueba. En el ensayo se determinó un consumo menor de alimento y una  ganancia  de peso menor conforme aumentó el nivel de sustitución de maíz por harina de pejibaye.  Sin embargo, en las variables, conversión alimenticia y costos de alimentación/kg de carne de ganso en pie,  los cuales son  parámetros muy importantes para determinar la eficiencia de las dietas, se encontraron diferencias estadísticamente significativas en los tratamientos cuando el nivel de sustitución de maíz por harina de pejibaye fue inferior o igual al 50 % en las dietas de los gansos

Carbon tips as electrodes for single-molecule junctions

The following article appeared in Applied Physics Letters vol 99,12 (2011): 123105 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/12/10.1063/1.3643031?showFTTab=true&containerItemId=content/aip/journal/aplWe study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a goldelectrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.Acknowledges fellowship support from the Comunidad de Madrid (Spain). This work was supported by MICINN (Spain) through the programs MAT2008-01735 and CONSOLIDER-INGENIO-2010 CSD-2007-00010, by Comunidad de Madrid through the program NANOBIOMAGNET S2009/MAT1726, by the EU through the networks BIMORE (MRTN-CT-2006-035859),and by the EC through the network FP7 ITN “FUNMOLS”Project Number 212942

Tailoring Organic-Organic Poly(vinylpyrrolidone) Microparticles and Fibers with Multiwalled Carbon Nanotubes for Reinforced Composites

Polymeric-based microparticles and fibers are tailorable for a wide range of common industrial and biomedical applications, while multiwalled carbon nanotubes (MWCNTs) are among the most useful macromolecules based on their outstanding electronic, mechanical, and optical properties at the nanoscale. If one combines these nanostructures with various polymeric precursors, their range of potential applications becomes even greater. One of the simplest and most affordable methods for fabricating micro- and nanostructures is electrospinning. Herein we demonstrate how MWCNTs may be used to produce tailor-made organic-organic poly(vinylpyrrolidone) (PVP) microparticles and fibers via electrospinning by studying their structural, vibrational, rheological, and mechanical properties' dependence on their solvent (ethanol (EtOH) or dimethylformamide (DMF)) and resulting morphology. Specifically, we find clear differences in morphologies from perfectly spherical and isolated microparticles to fibers mats, or a combination of fibers with entangled beads, with solvent type and concentration. On the basis of our findings, we propose that the mechanism governing the shape and size of the particles is a competition between the solvent's surface tension, dielectric constant, and viscoelastic properties. We show, based on both our experimental results and density functional theory (DFT) calculations, that OH functionalization of the MWCNTs is essential for achieving high PVP coverages and promoting the stability of the resulting PVP/MWCNT nanocomposite. Finally, by fabricating PVP/MWCNT fiber mats, we demonstrate that low concentrations (0.01-0.1 wt %) of MWCNTs led to a qualitative improvement (â250%) in the resulting mechanical properties, i.e., a reinforced composite. These results show how by controlling the solvent's dielectric constant, surface tension, and polymer concentration, one may produce tailor-made polymeric nanomaterials in combination with other organic/inorganic nanoparticles, i.e., silver, gold, or carbon allotropes, for next-generation applications

Design of a Trans-Horizon radio link for ultra high and super high frequencies

Cost considerations and availability of frequencies condition the feasibility of links at distances in which there is no line of sight between antennas. These trans-horizon links require precise design procedures to determine whether the intensity of the signal and its noise ratio allow reception with reasonable quality. The present work describes a procedure for radio electric link calculation to transmit voice and data signals through bandwidth at ultra high and super high frequencies.Presentado en el V Workshop Arquitectura, Redes y Sistemas Operativos (WARSO)Red de Universidades con Carreras en Informática (RedUNCI