Parametric FEM simulation of composite barrier FTJs under external bias at room temperature

A study on a parametrized model of a composite barrier FTJ (three-interface system, with a non-polar dielectric layer) under an external bias voltage and at room temperature, using FEM-based simulations, was performed. The approach involves the Thomas-Fermi model assuming incomplete screening of polarization charges for building the energy barrier profile, and numerically simulates the electron transport through the barrier by bias-voltage-dependent tunneling, using Tsu-Esaki formulation. That naturally include the temperature dependent contributions to the total current density. The TER coefficient and current densities are computed considering variation of a large set of parameters that describe the composite barrier FTJ system in realistic physical range of values with respect to a reference (prototypical) system. In this study, the parametric simulations were performed starting from selected data reported on the SRO/STO/BTO/SRO heterostructure. The most important results of our work can be stated as follows: i) The FEM simulations prove to be reliable approach when we are interested in the prediction of FTJ characteristics at temperatures close to 300 K, and ii) We show that several configurations with large TER values may be predicted, but at the expense of very low current densities in the ON state. We suggest that the results may be useful for assessing the FTJ performances at ambient temperature, as well as to design preoptimized FTJs by using different combinations of materials to comply with a set of properties of a specific model

Serum concentrations of cytokines and adhesion molecules as markers of endothelial dysfunction

Citokīnu un adhēzijas molekulu koncentrācijas serumā kā endotēlija disfunkcijas pakāpes rādītāji Anotācija Pētījuma mērķis ir novērtēt, kuru ar endotēlija šūnu funkcijām saistīto molekulu koncentrācijas serumā ir pārliecinošākie endotēlija šūnu funkcionālā stāvokļa rādītāji, kā arī noteikt atšķirīgām endoteliālās disfunkcijas attīstības stadijām raksturīgās izmaiņas. Materiāls ietver 234 personas ar plašu endoteliālās funkcijas variabilitāti. Tika noteikta endotēlija vazodilatējošā atbilde uz acetilholīna, siltuma un postokluzīvās cērpjošās asinsplūsmas stimuliem, kā arī ar endotēlija funkcijām saistītu molekulu seruma koncentrācijas. Rezultāti liecina, ka endotēlija šūnu vazodilatējošo aktivitāte pozitīvi korelē ar adiponektīna, IL-10 un IL-4, bet negatīvi – ar sEselektīna, sICAM-1, TNFα un VEGF koncentrācijām, kā arī vairākas būtiskas rādītāju atšķirības četrās atšķirīgas endotēlija disfunkcijas pakāpes pārstāvošajās pētījuma grupās. Atslēgas vārdi: endotēlija disfunkcija, insulīna rezistence, vazodilatācijas testi, adhēzijas molekulas, adiponektīnsSerum concentrations of cytokines and adhesion molecules as markers of endothelial dysfunction Abstract The aim of this study is to select the serum markers that are most convincing for detection of endothelial function as well as to select the markers characteristic for varied degrees of endothelial dysfunction. The study material includes 234 persons with wide variablilty of endothelial dysfunction. The vasodilatory response of endothelium was detected by acetylcholine-, heat- and post-occlusive shear stress-induced vasodilatation tests and serum concentrations of molecular markers associated with endothelial function was detected. The study results show positive correlations of vasodilatory function with adiponectin, IL-10 and IL-4, and negative correlations with sE-selectin, sICAM-1, TNFα and VEGF concentrations as well as several significant differences between the four study groups representing varied degrees of endothelial dysfunction. Key words: endothelial dysfunction, insulin resistance, vasodilatation tests, adhesion molecules, adiponecti

Structural and magnetic investigation of Co-rich Sm–Co unidirectionally solidified alloys

11 figures, 4 tables.Sm(Co, Fe, Cu, Zr)z alloys, with various degrees of crystallographic and magnetic texture, were synthesized by a route based on floating zone melting and unidirectional solidification. The phase composition, the microstructure and magnetisation curves of these materials were investigated by scanning electron microscopy, X-ray diffraction and magnetisation measurements. The trials undertaken by systematically varying the process parameters (heating power, floating zone velocity) allowed us to determine for each particular start composition the solidification regimes which yield materials with 1:5- and/or 2:17-type phases, free of Co metal precipitates, as well as with the crystallographic c-axis, easy magnetisation direction, respectively, oriented parallel or perpendicular to the temperature gradient main direction. The effect of solutionizing and ageing treatments on the microstructure and magnetic properties of the textured alloys was also investigated.The authors are indebted to Professor J. García Ruiz and his group for technical assistance in FZM–DS alloy processing. We acknowledge fruitful discussions with MAVILOR Motors S.A. along this work. The financial support in the frame of the project MAT2005/1272 is also acknowledged.Peer reviewe

Electrical resistivity of RFe11.5Ta0.5 and RFe11.3W0.7 alloys

We have measured the electrical resistivity of RFe11.5Ta0.5 and RFe11.3W0.7 (R = Tb, Dy, Ho, Er, and Lu) polycrystalline systems over a temperature range of 4–700 K. The overall behaviour of the resistivity is determined mainly by the transition-metal ions. However, some interesting features are related to rare-earth atoms. At low temperatures, the resistivity, ρ, increases with temperature as AT2, where the coefficient A varies systematically with the total spin of rare-earth ions. We also find that ρ depends on the M element in our RFe12−xMx iron-rich compounds. Close to the Curie temperature, the resistivities of the alloys studied clearly show a change in the slope of ρ versus T. Saturation values of the high temperature magnetic resistivity follow the prediction by de Gennes and Friedel for spin disorder scattering.We acknowledge support from Ministerio de Ciencia y Tecnologia of Spain through grant MAT02/166.Peer reviewe

Powder neutron diffraction of Nd3Co13 xNixB2 compounds

We report results of a powder neutron diffraction study on the crystal structure and local magnetic moments in Nd3Co13 xNixB2 compounds with Nd3Ni13B2 type structure. The largest 3d magnetic moment resides at the 4 h sites. Nd magnetic moment is reduced with respect to the free ion value. Preferential Ni substitution for Co is also found at the 4h site. The refined magnetization direction is consistent with the T x phase diagram derived from magnetization dat

Mössbauer spectral study of RFe11.3W0.7 compounds (R=Dy, Ho, Er, and Lu)

12 pages, 7 figures, 4 tables.-- PACS: 75.50.Bb; 76.80.+yThe series of compounds RFe11.3W0.7, where R=Dy, Ho, Er, and Lu, has been measured by Mössbauer spectroscopy between 15 and 295 K. A model which takes into account the Wigner–Seitz cell local environment of each iron site, the distribution of tungsten atoms on the 8i site, and the temperature-dependent magnetic structures of the compounds has been used to analyse the spectra. The assignment and the temperature dependencies of the hyperfine fields and the isomer shifts are in complete agreement with the Wigner–Seitz cell analysis of the three iron sites in RFe11.3W0.7 compounds. The plausible orientations of the iron magnetic moments in the canted magnetic phase of ErFe11.3W0.7 compound have been established by a thorough Mössbauer spectral analysis. The Mössbauer spectra of DyFe11.3W0.7 clearly show the influence of the two spin re-orientations occurring in this compound and indicate the directions of the iron magnetic moments in both canted and planar magnetic arrangements.The full financial support through the CERES program of the Romanian Ministry of Education and Science, under Contract no. 30/2001, and the partial one via the Spanish Ministry of Science and Technology, projects MAT02/166 and MAT00/0107-P4-02, are deeply acknowledged.Peer reviewe

Structure and electrical behavior of silicon nanowires prepared by MACE process

We report on the structure and electrical characteristics of silicon nanowire arrays prepared by metal assisted chemical etching (MACE) method, investigated by cross-sectional scanning electron microscopy (SEM) and high resolution X-ray diffraction (HR-XRD) methods. SEM micrographs show arrays of merged parallel nanowires, with lengths of 700 nm and 1000 nm, resulted after 1.5 min and 5 min etching time, respectively. X-ray reciprocal space maps (RSMs) around Si (004) reciprocal lattice point indicate the presence of 0D structural defects rather than of extended defects. The photoluminescence spectra exhibit emission bands at 1.70 eV and 1.61 eV, with intensity significantly higher in the case of longer wires and associated with the more defected surface. The transient photoluminescence spectroscopy reveals average lifetime of 60 $\mu$s and 111 $\mu$s for the two SiNW arrays, which correlate with a larger density of defects states in the latest case. The I-V characteristics of the nanowires, show a memristive behavior with the applied voltage sweep rate in the range 5V/s - 0.32V/s. We attribute this behavior to trap states which control the carrier concentration, and model this effect using an equivalent circuit. Photogeneration processes under excitation wavelengths in visible domain, 405 nm - 650 nm, and under light intensity in the range 20 - 100 mW/cm$^2$ provided a further insight into the trap states.Comment: 23 pages 18 figure

Heat capacity measurements of itinerant electron magnetism in Y3Ni13−xCOxB2 system

4 pages, 4 figures, 1 table.-- PACS classification codes: 75.20.En; 75.40.CxThe heat capacity of the Y3Ni13−xCoxB2 series has been measured from 300 mK to RT. The magnetic ordering phase transitions have been characterized as second-order type and the Tc's determined. The electronic contribution to the low-temperature heat capacity for x=0 yields an electronic constant γ=54 mJ mol K2, which is higher than those of YNi5 and YNi4B, proving experimentally that its density of states at the Fermi surface is larger than in those other compounds. The substitution of Ni by Co increases γ linearly. Electronic band calculations could explain these features.Financial support from Spanish MEC (projects MAT05- 01272) is acknowledged. The DGA is acknowledged for the IMANA project E34.Peer reviewe