Spin-polarized transport and Andreev reflection in semiconductor/superconductor hybrid structures

We show that spin-polarized electron transmission across semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on the degree of spin polarization as well as the strengths of potential and spin-flip scattering at the interface. We demonstrate that increasing the Fermi velocity mismatch in the Sm and S regions can lead to enhanced junction transparency in the presence of spin polarization. We find that the Andreev reflection amplitude at the superconducting gap energy is a robust measure of the spin polarization magnitude, being independent of the strengths of potential and spin-flip scattering and the Fermi velocity of the superconductor.Comment: 4 pages, 2 figure

UV+IR Star Formation Rates: Hickson Compact Groups with Swift and Spitzer

We present Swift UVOT (1600-3000A) 3-band photometry for 41 galaxies in 11 nearby (<4500km/s) Hickson Compact Groups (HCGs) of galaxies. We use the uvw2-band (2000A) to estimate the dust-unobscured component, SFR_UV, of the total star-formation rate, SFR_T. We use Spitzer MIPS 24-micron photometry to estimate SFR_IR, the dust-obscured component of SFR_T. We obtain SFR_T=SFR_UV+SFR_IR. Using 2MASS K_s band based stellar mass, M*, estimates, we calculate specific SFRs, SSFR=SFR_T/M*. SSFR values show a clear and significant bimodality, with a gap between low (<~3.2x10^-11 / yr) and high SSFR (>~1.2x10^-10 / yr) systems. All galaxies with MIR activity index a_IRAC 0) are in the high- (low-) SSFR locus, as expected if high levels of star-formation power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. All elliptical/S0 galaxies are in the low-SSFR locus, while 22 out of 24 spirals/irregulars are in the high-SSFR locus, with two borderline cases. We divide our sample into three subsamples (I, II and III) according to decreasing HI-richness of the parent galaxy group to which a galaxy belongs. Consistent with the SSFR and a_IRAC bimodality, 12 out of 15 type-I (11 out of 12 type-III) galaxies are in the high- (low-) SSFR locus, while type II galaxies span almost the full range of SSFR values. Unlike HCG galaxies, galaxies in a comparison quiescent SINGS sub-sample are continuously distributed both in SSFR and a_IRAC. Any uncertainties can only further enhance the SSFR bimodality. These results suggest that an environment characterized by high galaxy number-densities and low galaxy velocity-dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star-formation processes in galaxies and favoring a fast transition to quiescence.(abridged)Comment: Accepted by ApJ. [8 Tables, 16 Figures. Color figures have reduced size for ArXiv - emulateapj v. 2/16/10

An Improved Description of the Dielectric Breakdown in Oxides Based on a Generalized Weibull distribution

In this work, we address modal parameter fluctuations in statistical distributions describing charge-to-breakdown $(Q_{BD})$ and/or time-to-breakdown $(t_{BD})$ during the dielectric breakdown regime of ultra-thin oxides, which are of high interest for the advancement of electronic technology. We reobtain a generalized Weibull distribution ($q$-Weibull), which properly describes $(t_{BD})$ data when oxide thickness fluctuations are present, in order to improve reliability assessment of ultra-thin oxides by time-to-breakdown $(t_{BD})$ extrapolation and area scaling. The incorporation of fluctuations allows a physical interpretation of the $q$-Weibull distribution in connection with the Tsallis statistics. In support to our results, we analyze $t_{BD}$ data of SiO$_2$-based MOS devices obtained experimentally and theoretically through a percolation model, demonstrating an advantageous description of the dielectric breakdown by the $q$-Weibull distribution.Comment: 5 pages, 3 figure

Precise radial velocities of giant stars. III. Spectroscopic stellar parameters

Context: A radial velocity survey of about 380 G and K giant stars is ongoing at Lick observatory. For each star we have a high signal to noise ratio template spectrum, which we use to determine spectroscopic stellar parameters. Aim: The aim of this paper is to present spectroscopic stellar parameters, i.e. effective temperature, surface gravity, metallicity and rotational velocity for our sample of G and K giant stars. Methods: Effective temperatures, surface gravities and metallicities are determined from the equivalent width of iron lines. Rotational velocities are determined from the full width at half maximum (FWHM) of moderate spectral lines. A calibration between the FWHM and total broadening (rotational velocity and macro turbulence) is obtained from stars in common between our sample and the sample from Gray (1989). Results: The metallicity we derive is essentially equal to the literature values, while the effective temperature and surface gravity are slightly higher by 56 K and 0.15 dex, respectively. Our rotational velocities are comparable with the ones obtained by Gray (1989), but somewhat higher than the ones obtained by de Medeiros & Mayor (1999), consistent with the different diagnostics used. Conclusions: We are able to determine spectroscopic stellar parameters for about 380 G and K giant stars in a uniform way (112 stars are being analysed spectroscopically for the first time). For stars available in the literature, we find reasonable agreement between literature values and values determined in the present work. In addition, we show that the metallicity enhancement of companion hosting stars might also be valid for giant stars, with the planet-hosting giants being 0.13 +/- 0.03 dex (i.e. 35 +/- 10%) more metal-rich than our total sample of stars.Comment: 16 pages, 8 figures, 4 tables, accepted for publication by Astronomy and Astrophysic

Green's function probe of a static granular piling

We present an experiment which aim is to investigate the mechanical properties of a static granular assembly. The piling is an horizontal 3D granular layer confined in a box, we apply a localized extra force at the surface and the spatial distribution of stresses at the bottom is obtained (the mechanical Green's function). For different types of granular media, we observe a linear pressure response which profile shows one peak centered at the vertical of the point of application. The peak's width increases linearly when increasing the depth. This green function seems to be in -at least- qualitative agreement with predictions of elastic theory.Comment: 9 pages, 3 .eps figures, submitted to PR

Element abundances in the metal rich open cluster NGC6253

We have carried out a big FLAMES survey of 10 Galactic open clusters aiming at different goals. One of them is the determination of chemical abundances, in order to put constraints on the radial metallicity gradient in the disk and its evolution. One of the sample clusters is the very metal rich NGC 6253. We have obtained UVES high resolution spectra of seven candidate cluster members (from the turn off up to the red clump) with the goal of determining the chemical composition of NGC 6253 and to investigate its origin and role in the interpretation of the radial metallicity gradient in the disk. Equivalent width analysis and spectral synthesis were performed using MOOG and Kurucz model atmospheres. We derived abundances of Fe, alpha- and Fe-peak elements, the light element Na and the s-process element Ba. Excluding two likely non-members and the clump giant, whose metallicity from equivalent widths is overestimated, we find an average [Fe/H]=+0.36+/-0.07 (rms) for the cluster. For most of the other elements we derive solar abundance ratios.Comment: accepted by A&A (02/01/2007), 21 pages, 11 ps figure

Theory of electrical spin injection: Tunnel contacts as a solution of the conductivity mismatch problem

Theory of electrical spin injection from a ferromagnetic (FM) metal into a normal (N) conductor is presented. We show that tunnel contacts (T) can dramatically increase spin injection and solve the problem of the mismatch in the conductivities of a FM metal and a semiconductor microstructure. We also present explicit expressions for the spin-valve resistance of FM-T-N- and FM-T-N-T-FM-junctions with tunnel contacts at the interfaces and show that the resistance includes both positive and negative contributions (Kapitza resistance and injection conductivity, respectively).Comment: 4 pages, to appear in Phys. Rev. B (rapid communications

A new interpretation of total column BrO during Arctic spring

Emission of bromine from sea-salt aerosol, frost flowers, ice leads, and snow results in the nearly complete removal of surface ozone during Arctic spring. Regions of enhanced total column BrO observed by satellites have traditionally been associated with these emissions. However, airborne measurements of BrO and O3 within the convective boundary layer (CBL) during the ARCTAS and ARCPAC field campaigns at times bear little relation to enhanced column BrO. We show that the locations of numerous satellite BrO "hotspots" during Arctic spring are consistent with observations of total column ozone and tropopause height, suggesting a stratospheric origin to these regions of elevated BrO. Tropospheric enhancements of BrO large enough to affect the column abundance are also observed, with important contributions originating from above the CBL. Closure of the budget for total column BrO, albeit with significant uncertainty, is achieved by summing observed tropospheric partial columns with calculated stratospheric partial columns provided that natural, short-lived biogenic bromocarbons supply between 5 and 10 ppt of bromine to the Arctic lowermost stratosphere. Proper understanding of bromine and its effects on atmospheric composition requires accurate treatment of geographic variations in column BrO originating from both the stratosphere and troposphere. Copyright 2010 by the American Geophysical Union

Adverse childhood experiences and suicide attempts in morbidly obese adults

Introdução: As tentativas de suicídio surgem frequentemente associadas a problemas alimentares, tanto anorexia quanto bulimia. Do mesmo modo, tem-se verifi cado uma elevada ocorrência de suicídio entre obesos. Investigações têm mostrado que a adversidade na infância pode ser um fator de risco para as tentativas de suicídio. Objetivos: Caracterizar e compreender a relação entre experiências de adversidade na infância e tentativas de suicídio em 100 obesos mórbidos candidatos a cirurgia bariátrica. Métodos: Um total de 100 pacientes foram selecionados de setembro de 2007 a outubro de 2007 e de janeiro de 2008 a janeiro de 2009, sendo que 20 pacientes eram do sexo feminino. A média de idade era de 38,89±9,87 anos, e a média do peso máximo era de 136,43±14 kg. O Questionário da História de Adversidade na Infância foi utilizado para avaliar experiências adversas. Resultados: 88% dos pacientes relataram a existência de pelo menos uma experiência de adversidade na infância, e 25% relataram já ter realizado pelo menos uma tentativa de suicídio. A adversidade na infância esteve associada a um risco aumentado para realizar tentativas de suicídio (odds ratio = 2,026). Conclusão: Esses dados devem ser levados em consideração na avaliação e no acompanhamento desses pacientes.Introduction: Suicide attempts are often associated with eating disorders, both anorexia and bulimia. Likewise, a high incidence of suicide has been observed among obese patients. Previous studies have shown that adverse experiences in childhood may be a risk factor for suicide attempts. Objectives: To characterize and to understand the relationship between adverse experiences and suicide attempts in 100 morbidly obese patients referred for bariatric surgery. Methods: A total of 100 patients were selected from September 2007 to October 2007 and from January 2008 to January 2009. Of these, 20 patients were females. Mean age was 38.89±9.87 years, and mean maximum weight was 136.43±14 kg. The Portuguese version of the Family ACE (Adverse Childhood Experiences) Questionnaire was used to assess the occurrence of adverse events. Results: 88% of the patients reported the existence of at least one adverse experience in childhood, and 25% reported at least one previous suicide attempt. Adversity in childhood was associated with an increased risk for suicide attempts (odds ratio = 2.026). Conclusion: These data should be taken into account in the assessment and monitoring of these patients.Fundação para a Ciência e a Tecnologia (FCT); (SFRH/BD/37069/2007)