International School for Advanced Studies

About: International School for Advanced Studies is a education organization based out in Trieste, Friuli-Venezia Giulia, Italy. It is known for research contribution in the topics: Galaxy & Dark matter. The organization has 3751 authors who have published 13433 publications receiving 588454 citations. The organization is also known as: SISSA & Scuola Internazionale Superiore di Studi Avanzati.

Creating single-atom Pt-ceria catalysts by surface step decoration

Abstract: Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces--monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt(2+) ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt(2+) and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

Numerical study of halo concentrations in dark-energy cosmologies

Abstract: We study the concentration parameters, their mass dependence and redshift evolution, of dark-matter halos in different dark-energy cosmologies with constant and time-variable equation of state, and compare them with standard ACDM and OCDM models. We find that previously proposed algorithms for predicting halo concentrations can be well adapted to dark-energy models. When centred on the analytically expected values, halo concentrations show a log-normal distribution with a uniform standard deviation of ∼0.2. The dependence of averaged halo concentrations on mass and redshift permits a simple fit of the form (1 + z) c = c 0 (M/M 0 ) α , with α -0.1 throughout. We find that the cluster concentration depends on the dark energy equation of state at the cluster formation redshift z coll through the linear growth factor D + (z coll ). As a simple correction accounting for dark-energy cosmologies, we propose scaling c 0 from ACDM with the ratio of linear growth factors, c 0 → c 0 D + (Z coll )/D +.ACDM (z coll ).

Planck intermediate results: V. Pressure profiles of galaxy clusters from the Sunyaev-Zeldovich effect

Abstract: Taking advantage of the all-sky coverage and broad frequency range of the Planck satellite, we study the Sunyaev-Zeldovich (SZ) and pressure profiles of 62 nearby massive clusters detected at high significance in the 14-month nominal survey. Careful reconstruction of the SZ signal indicates that most clusters are individually detected at least out to R500. By stacking the radial profiles, we have statistically detected the radial SZ signal out to 3R500, i.e., at a density contrast of about 50-100, though the dispersion about the mean profile dominates the statistical errors across the whole radial range. Our measurement is fully consistent with previous Planck results on integrated SZ fluxes, further strengthening the agreement between SZ and X-ray measurements inside R500. Correcting for the effects of the Planck beam, we have calculated the corresponding pressure profiles. This new constraint from SZ measurements is consistent with the X-ray constraints from xmm in the region in which the profiles overlap (i.e., [0.1-1] R500), and is in fairly good agreement with theoretical predictions within the expected dispersion. At larger radii the average pressure profile is shallower than the predictions. Combining the SZ and X-ray observed profiles into a joint fit to a generalised pressure profile gives best-fit parameters [P0, c500, gamma, alpha, beta] = [6.41, 1.81, 0.31, 1.33, 4.13]. Using a reasonable hypothesis for the gas temperature in the cluster outskirts we reconstruct from our stacked pressure profile the gas mass fraction profile out to 3R500. Within the temperature driven uncertainties, our Planck constraints are compatible with the cosmic baryon fraction and expected gas fraction in halos.

Perturbations of spacetime : gauge transformations and gauge invariance at second order and beyond

Abstract: We consider in detail the problem of gauge dependence that exists in relativistic perturbation theory, going beyond the linear approximation and treating second- and higher-order perturbations. We first derive some mathematical results concerning the Taylor expansion of tensor fields under the action of one-parameter families (not necessarily groups) of diffeomorphisms. Secondly, we define gauge invariance to an arbitrary order n. Finally, we give a generating formula for the gauge transformation to an arbitrary order and explicit rules to second and third order. This formalism can be used in any field of applied general relativity, such as cosmological and black hole perturbations, as well as in other spacetime theories. As a specific example, we consider here second-order perturbations in cosmology, assuming a flat Robertson - Walker background, giving explicit second-order transformations between the synchronous and the Poisson (generalized longitudinal) gauges.