On the correlation between the X-ray absorption chemical shift and the formal valence state in mixed-valence manganites

7 páginas, 7 figuras.Here the correlation between the chemical shift in X-ray absorption spectroscopy, the geometrical structure and the formal valence state of the Mn atom in mixed-valence manganites are discussed. It is shown that this empirical correlation can be reliably used to determine the formal valence of Mn, using either X-ray absorption spectroscopy or resonant X-ray scattering techniques. The difficulties in obtaining a reliable comparison between experimental XANES spectra and theoretical simulations on an absolute energy scale are revealed. It is concluded that the contributions from the electronic occupation and the local structure to the XANES spectra cannot be separated either experimentally or theoretically. In this way the geometrical and electronic structure of the Mn atom in mixed-valence manganites cannot be described as a bimodal distribution of the formal integer Mn3+ and Mn4+ valence states corresponding to the undoped references.This work was supported by the Spanish MICINN FIS2008- 03951 project and Diputación General de Aragón (DGACAMRADS). VC thanks the FPU research grant from MICINN.Peer reviewe

La ruta de señalización WNT en la determinación de progenitores pancreáticos

1 página. IX Jornadas Andaluzas Salud Investiga. Cádiz 20-22 octubre, 2010.Nuestro laboratorio trata de determinar los mecanismos moleculares implicados en la formación de los distintos tipos celulares pancreáticos adultos a partir de células progenitoras. Este conocimiento es fundamental para comprender las bases de enfermedades tan terribles como el cáncer pancreático. Además, es absolutamente necesario para el desarrollo de protocolos de formación in vitro de células beta productoras de insulina, una atractiva estrategia terapéutica para la diabetes. A este respecto, el papel de la ruta de señalización embrionaria Wnt parece ser importante ya que estudios previos han demostrado que la activación de esta ruta es necesaria para la formación de células pancreáticas a partir de células embrionarias. En nuestro proyecto nos hemos planteado determinar el efecto de la ruta Wnt sobre la formación de páncreas sobreactivando dicha ruta en progenitores pancreáticos en el ratón.Peer reviewe

Resonant x-ray scattering in La1-xSr1+xMn0 4 (x 0.5): Incommensurate-lattice modulation vs. Charge-stripe models

Using resonant x-ray scattering at the Mn K-edge, we have investigated the nature of the charge and lattice modulation in the La1-xSr1+xMnO4 (x=0.5 and 0.6) manganites. Resonant reflections (h±e, h±e, 0) and (h±2e, h±2e, 0) of the tetragonal I4/mmm structure with a modulation vector of 2e=l-x were found in the insulating phases of both manganites but the intensity of these reflections is much weaker for La0.4Sr1.6MnO4. Resonant x-ray scattering data for the two samples are well explained by the presence of two types of sinusoidal modulations of the oxygen displacements, transverse and longitudinal to the tetragonal [110] direction. The amplitude of the oxygen displacements for any of the modulations decreases with the hole doping, in agreement with the change from a commensurate (x=0.5) to an incommensurate (x=0.6) ordered phase. The different polarization and azimuthal behaviour of the two sets of resonant reflections rule out any kind of stripe model composed by Mn3+-like and Mn4+-like charge-ordering. The maximum charge disproportionation among the different Mn atoms in the unit cell is about 0.15 e- and 0.04 e- for the x=0.5 and x=0.6 samples, respectively. These results thus confirm the existence of a charge-density-wave ordering in both the commensurate-phase of the half-doped La0.5Sr1.5MnO4 and the incommensurate-phase of the over-doped La0.4Sr1.6MnO4 manganites

Electronic states of R F e2 O4(R=Lu, Yb, Tm, Y) mixed-valence compounds determined by soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism

We here report an investigation of the electronic states in the RFe2O4(R=Lu,Yb,Tm,Y) mixed-valence ferrites by means of soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements together with ab initio theoretical calculations. The presence of Fe+2 and Fe+3 pure ionic species is discarded in the XAS spectra at the OK edge in both experimental data and simulations based on the multiple scattering theory. Similarly, no trace of Fe+2/Fe+3 contributions is detected in the XMCD spectra at the FeK edge. On the other hand, the XAS and XMCD spectra at the FeL2,3 edges can be well described in terms of Fe+2/Fe+3 contributions, and are also supported by multiplet calculations. This finding can be interpreted as the existence of a mixture of 3d5/3d6 configurations at the Fe atoms. Alternative ferrimagnetic spin orderings based on a trimodal Fe valence distribution are also proposed and discussed. Finally, a possible explanation for the strong dependence of the FeL2,3 edges XMCD signal magnitude on both the sample surface preparation and detection method is presented

Double stripe ordering in Nd0.5Sr0.5MnO3 determined by resonant soft x-ray scattering

7 pages, 8 figures.-- PACS number(s): 75.47.Lx, 78.70.Ck, 71.30.+hWe have studied the low-temperature ordered phase of Nd0.5Sr0.5MnO3 by resonant soft x-ray scattering at the Mn L2,3 edges. Strong resonances were observed at (0 0) and ( 0 0) reflections, which are resolved in momentum space. Azimuthal scans reveal a periodicity for both resonances, the minimum intensity being almost zero. The two reflections show a very different energy dependence which extends in the whole energy region of the Mn L-edge. The (0 0) reflection originates from the anisotropy induced on the d-symmetry projected density of states owing to the local tetragonal distortion at one of the two crystallographic inequivalent Mn sites (the so-called Mn3+ ions in the conventional ionic description). This is similar to the resonance observed at the Mn K edge. We propose that the ( 0 0) reflection, which has not its counterpart either on the Mn K-edge resonant scattering or on the magnetic scattering, is originated by anisotropy of the d-symmetry projected density of the so-called Mn4+ sublattice. In this way, the latter reflection could denote a pure orbital ordering in this sublattice.We are thankful for the financial support from the Spanish CICyT Project No. MAT2005-04562 and from D.G.A. We also acknowledge ESRF and ID08 beamline for granting beam time.Peer reviewe

Valence change of praseodymium in Pr0.5Ca0.5CoO3 investigated by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(+5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomic-like 4f1 states (Pr4+) and f- symmetry states on the O 2p valence band (Pr3+L states) indicative of a strong 4f- 2p interaction.Comment: 19 pages (.doc), 4 figures, Phys. Rev. B, in pres

UV photoprocessing of CO2 ice: a complete quantification of photochemistry and photon-induced desorption processes

Ice mantles that formed on top of dust grains are photoprocessed by the secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons induce photochemistry and desorption of ice molecules. Experimental simulations dedicated to ice analogs under astrophysically relevant conditions are needed to understand these processes. We present UV-irradiation experiments of a pure CO2 ice analog. Calibration of the QMS allowed us to quantify the photodesorption of molecules to the gas phase. This information was added to the data provided by the FTIR on the solid phase to obtain a complete quantitative study of the UV photoprocessing of an ice analog. Experimental simulations were performed in an ultra-high vacuum chamber. Ice samples were deposited onto an infrared transparent window at 8K and were subsequently irradiated with a microwave-discharged hydrogen flow lamp. After irradiation, ice samples were warmed up until complete sublimation was attained. Photolysis of CO2 molecules initiates a network of photon-induced chemical reactions leading to the formation of CO, CO3 ,O2 , and O3 . During irradiation, photon-induced desorption of CO and, to a lesser extent, O2 and CO2 took place through a process called indirect desorption induced by electronic transitions (DIET), with maximum photodesorption yields (Ypd) of 1.2 x 10-2 molecules/incident photon , 9.3 x 10-4 molecules/incident photon , and 1.1 x 10-4 molecules/incident photon , respectively. Calibration of mass spectrometers allows a direct quantification of photodesorption yields instead of the indirect values that were obtained from infrared spectra in most previous works. Supplementary information provided by infrared spectroscopy leads to a complete quantification, and therefore a better understanding, of the processes taking place in UV-irradiated ice mantles

Pretensado y comportamiento mecánico del pericardio de ternera utilizado en la construcción de bioprótesis cardíacas

Este trabajo analiza las propiedades mecánicas de un tejido biológico, pericardio de bovino empleado como biomaterial en la fabricación de bioprótesis cardíacas. Para ello se sometió pericardio de ternera a un estrés de tracción, uniaxial, en escalones de carga, con retirada de dicha carga tras cada escalón. Los escalones de carga fueron de aproximadamente 2,3 y 8 MPa sucesivamente, continuándose finalmente el ensayo hasta la rotura. Se ensayaron 32 muestras, 16 tratadas con glutaraldehído como grupo de control y 16 tratadas con cloroformo/metanol. La resistencia media a rotura estuvo entre 19.14 MPa y 32.60 MPa sin encontrar diferencias estadísticamente significativas en la comparación de las series ensayadas. Se estudio también el estrés remanente tras cada escalón de carga, siendo este constante en su valor para cada ensayo. Para un valor medio de estrés inicial de 2.14 MPa, el estrés remanente medio fue de 0.36 MPa (16.82 % del estrés inicial) muy similar al estrés máximo de trabajo descrito para un velo valvular. La aplicación de un estrés progresivo produce una disminución de la deformación con pérdida de la elasticidad ,pero un comportamiento mas homogéneo del pericardio. Este hallazgo no se afectó, por la región de pericardio de donde se obtuvo la muestra o por tratamiento químico recibido. Estos ensayos permiten, mediante una recta de regresión Xi=f(Ymax.),conocer la deformación (Xi) que puede sufrir el pericardio de ternera, tras un estrés de pretensado previo (Ymax.), cuando posteriormente se le somete a un estrés.Peer Reviewe

Shards: An optical spectro-photometric survey of distant galaxies

Pérez-González et al.We present the Survey for High-z Absorption Red and Dead Sources (SHARDS), an ESO/GTC Large Program carried out using the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias (GTC). SHARDS is an ultra-deep optical spectro-photometric survey of the GOODS-N field covering 130 arcmin2 at wavelengths between 500 and 950 nm with 24 contiguous medium-band filters (providing a spectral resolution R ∿ 50). The data reach an AB magnitude of 26.5 (at least at a 3σ level) with sub-arcsec seeing in all bands. SHARDS' main goal is to obtain accurate physical properties of intermediate- and high-z galaxies using well-sampled optical spectral energy distributions (SEDs) with sufficient spectral resolution to measure absorption and emission features, whose analysis will provide reliable stellar population and active galactic nucleus (AGN) parameters. Among the different populations of high-z galaxies, SHARDS' principal targets are massive quiescent galaxies at z > 1, whose existence is one of the major challenges facing current hierarchical models of galaxy formation. In this paper, we outline the observational strategy and include a detailed discussion of the special reduction and calibration procedures which should be applied to the GTC/OSIRIS data. An assessment of the SHARDS data quality is also performed. We present science demonstration results on the detection and study of emission-line galaxies (star-forming objects and AGNs) at z = 0-5. We also analyze the SEDs for a sample of 27 quiescent massive galaxies with spectroscopic redshifts in the range 1.0 < z ≲ 1.4. We discuss the improvements introduced by the SHARDS data set in the analysis of their star formation history and stellar properties. We discuss the systematics arising from the use of different stellar population libraries, typical in this kind of study. Averaging the results from the different libraries, we find that the UV-to-MIR SEDs of the massive quiescent galaxies at z = 1.0-1.4 are well described by an exponentially decaying star formation history with scale Ï„ = 100-200 Myr, age around 1.5-2.0 Gyr, solar or slightly sub-solar metallicity, and moderate extinction, A(V) ∿ 0.5 mag. We also find that galaxies with masses above M* are typically older than lighter galaxies, as expected in a downsizing scenario of galaxy formation. This trend is, however, model dependent, i.e., it is significantly more evident in the results obtained with some stellar population synthesis libraries, and almost absent in others. © 2013. The American Astronomical Society. All rights reserved.We acknowledge support from the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA2009-07723-E and AYA2009-10368. SHARDS has been funded by the Spanish MICINN/MINECO under the Consolider-Ingenio 2010 Program grant CSD2006-00070: First Science with the GTC. O.G.-M., C.M.-T., J.M.R.-E., and J.R.-Z. wish to acknowledge support from grant AYA2010-21887-C04-04. A.A.-H. and A.H.-C. acknowledge financial support from the Universidad de Cantabria through the Augusto G. Linares Program.Peer Reviewe