Polarization-dependence of palladium deposition on ferroelectric lithium niobate (0001) surfaces

We investigate the effect of ferroelectric polarization direction on the geometric properties of Pd deposited on the positive and negative surfaces of LiNbO$_3$ (0001). We predict preferred geometries and diffusion properties of small Pd clusters using density functional theory, and use these calculations as the basis for kinetic Monte Carlo simulations of Pd deposition on a larger scale. Our results show that on the positive surface, Pd atoms favor a clustered configuration, while on the negative surface, Pd atoms are adsorbed in a more dispersed pattern due to suppression of diffusion and agglomeration. This suggests that the effect of LiNbO$_3$ polarization direction on the catalytic activity of Pd [J. Phys. Chem. \textbf{88}, 1148 (1984)] is due, at least in part, to differences in adsorption geometry. Further investigations using these methods can aid the search for catalysts whose activities switch reversibly with the polarization of their ferroelectric substrates

An Examination of the Relationship Between School Climate, Self-Determined Academic Motivation, and Academic Outcomes Among Middle and High School Students

The purpose of this study was to explore connections among student perceptions of specific school climate-factors, self-determined academic motivation, and academic outcomes in a sample of middle and high school students (sixth through eleventh grade). Structural equation modeling techniques were used to identify meaningful grade specific associations within a sample of 2,463 students. The school climate factors of perceived teacher support, peer support, and school bullying emerged as the most salient school climate-based predictors of academic motivation factors. Perceived teacher support positively predicted academic competence and relatedness, whereas school bullying negatively predicted relatedness. Student academic competence, in turn, was found to negatively predict amotivation and to positively predict of GPA, after controlling for previous standardized test scores. Results were found to be consistent across grades. The implications of these findings in regards to school stakeholder practices and future research directions within the school climate and academic motivation literature bases are discussed

Positional Coincidence between the High-latitude Steady Unidentified Gamma-ray Sources and Possibly Merging Clusters of Galaxies

We report an evidence for the first time that merging clusters of galaxies are a promising candidate for the origin of high galactic-latitude, steady unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of eye-selected clusters, we made a matched-filter survey of galaxy clusters over 4\arcdeg \times 4\arcdeg areas around seven steady unidentified EGRET sources at |b|>45\arcdeg together with a 100 \sq \arcdeg area near the South Galactic Pole as a control field. In total, 154 Abell-like cluster candidates and 18 close pairs/groups of these clusters, expected to be possibly merging clusters, were identified within estimated redshift $z_{est}\leq 0.15$. Five among the seven EGRET sources have one or two cluster pairs/groups (CPGs) within 1\arcdeg from them. We assess the statistical significance of this result by several methods, and the confidence level of the real excess is maximally 99.8% and 97.8% in a conservative method. In contrast, we found no significant correlation with single clusters. In addition to the spatial correlation, we also found that the richness of CPGs associated with EGRET sources is considerably larger than those of CPGs in the control field. These results imply that a part of the steady unidentified EGRET sources at high-latitude are physically associated with close CPGs, not with single clusters. We also discuss possible interpretations of these results. We argue that, if these associations are real, they are difficult to explain by hadronic processes, but best explained by the inverse-Compton scattering by high energy electrons accelerated in shocks of cluster formation, as recently proposed.Comment: 9 pages, 2 PostScript figures, uses emulateapj5.sty, added new analysis and discussion, ApJ accepte

Gamma Rays from Clusters and Groups of Galaxies: Cosmic Rays versus Dark Matter

Clusters of galaxies have not yet been detected at gamma-ray frequencies; however, the recently launched Fermi Gamma-ray Space Telescope, formerly known as GLAST, could provide the first detections in the near future. Clusters are expected to emit gamma rays as a result of (1) a population of high-energy primary and re-accelerated secondary cosmic rays (CR) fueled by structure formation and merger shocks, active galactic nuclei and supernovae, and (2) particle dark matter (DM) annihilation. In this paper, we ask the question of whether the Fermi telescope will be able to discriminate between the two emission processes. We present data-driven predictions for a large X-ray flux limited sample of galaxy clusters and groups. We point out that the gamma ray signals from CR and DM can be comparable. In particular, we find that poor clusters and groups are the systems predicted to have the highest DM to CR emission at gamma-ray energies. Based on detailed Fermi simulations, we study observational handles that might enable us to distinguish the two emission mechanisms, including the gamma-ray spectra, the spatial distribution of the signal and the associated multi-wavelength emissions. We also propose optimal hardness ratios, which will help to understand the nature of the gamma-ray emission. Our study indicates that gamma rays from DM annihilation with a high particle mass can be distinguished from a CR spectrum even for fairly faint sources. Discriminating a CR spectrum from a light DM particle will be instead much more difficult, and will require long observations and/or a bright source. While the gamma-ray emission from our simulated clusters is extended, determining the spatial distribution with Fermi will be a challenging task requiring an optimal control of the backgrounds.Comment: revised to match resubmitted version, 35 pages, 16 figures: results unchanged, some discussion added and unnecessary text and figures remove

Observation-based assessment of stratospheric fractional release, lifetimes, and ozone depletion potentials of ten important source gases

Estimates of the recovery time of stratospheric ozone heavily rely on the exact knowledge of the processes that lead to the decomposition of the relevant halogenated source gases. Crucial parameters in this context are fractional release factors (FRFs) as well as stratospheric lifetimes and ozone depletion potentials (ODPs). We here present data from the analysis of air samples collected between 2009 and 2011 on board research aircraft flying in the mid- and high-latitude stratosphere and infer the above-mentioned parameters for ten major source gases: CFCl3 (CFC-11), CF2Cl2 (CFC-12), CF2ClCFCl2 (CFC-113), CCl4 (carbon tetrachloride), CH3CCl3 (methyl chloroform), CHF2Cl (HCFC-22), CH3CFCl2 (HCFC-141b), CH3CF2Cl (HCFC-142b), CF2ClBr (H-1211), and CF3Br (H-1301). The inferred correlations of their FRFs with mean ages of air reveal less decomposition as compared to previous studies for most compounds. When using the calculated set of FRFs to infer equivalent stratospheric chlorine, we find a reduction of more than 20% as compared to the values inferred in the most recent Scientific Assessment of Ozone Depletion by the World Meteorological Organisation (WMO, 2011). We also note that FRFs and their correlations with mean age are not generally time-independent as often assumed. The stratospheric lifetimes were calculated relative to that of CFC-11. Within our uncertainties the ratios between stratospheric lifetimes inferred here agree with the values in recent WMO reports except for CFC-11, CFC-12 and CH3CCl3. Finally, we calculate lower ODPs than recommended by WMO for six out of ten compounds, with changes most pronounced for the three HCFCs. Collectively these newly calculated values may have important implications for the severity and recovery time of stratospheric ozone loss