8,564 research outputs found
Bi-simulation Between P Colonies and P Systems with Multi-stable Catalysts
International audienc
Clathrate type 2 hydrate formation in vacuo under astrophysical conditions
The properties of clathrate hydrates were used to explain the complex and poorly understood physical processes taking place within cometary nuclei and other icy solar system bodies. Most of all the experiments previously conducted used starting compositions which would yield clathrate types I hydrates. The main criterion for type I vs. type II clathrate hydrate formation is the size of the guest molecule. The stoichiometry of the two structure types is also quite different. In addition, the larger molecules which would form type II clathrate hydrates typically have lower vapor pressures. The result of these considerations is that at temperatures where we identified clathrate formation (120-130 K), it is more likely that type II clathrate hydrates will form. We also formed clathrate II hydrates of methanol by direct vapor deposition in the temperature range 125-135 K
Low-pressure clathrate-hydrate formation in amorphous astrophysical ice analogs
In modeling cometary ice, the properties of clathrate hydrates were used to explain anomalous gas release at large radial distances from the Sun, and the retention of particular gas inventories at elevated temperatures. Clathrates may also have been important early in solar system history. However, there has never been a reasonable mechanism proposed for clathrate formation under the low pressures typical of these environments. For the first time, it was shown that clathrate hydrates can be formed by warming and annealing amorphous mixed molecular ices at low pressures. The complex microstructures which occur as a result of clathrate formation from the solid state may provide an explanation for a variety of unexplained phenomena. The vacuum and imaging systems of an Hitachi H-500H Analytical Electron Microscope was modified to study mixed molecular ices at temperatures between 12 and 373 K. The resulting ices are characterized by low-electron dose Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED). The implications of these results for the mechanical and gas release properties of comets are discussed. Laboratory IR data from similar ices are presented which suggest the possibility of remotely observing and identifying clathrates in astrophysical objects
Earth Matter Effects in Detection of Supernova Neutrinos
We calculated the matter effect, including both the Earth and supernova, on
the detection of neutrinos from type II supernovae at the proposed Daya Bay
reactor neutrino experiment. It is found that apart from the dependence on the
flip probability P_H inside the supernova and the mass hierarchy of neutrinos,
the amount of the Earth matter effect depends on the direction of the incoming
supernova neutrinos, and reaches the biggest value when the incident angle of
neutrinos is around 93^\circ. In the reaction channel \bar{\nu}_e + p --> e^+ +
n the Earth matter effect can be as big as about 12%. For other detection
processes the amount of the Earth matter effect is a few per cent.Comment: 13 pages, 5 figure
A next-to-leading order analysis of deeply virtual Compton scattering
We present a complete, next-to-leading-order (NLO), leading-twist QCD
analysis of deeply virtual Compton scattering (DVCS) observables, in the scheme, and in the kinematic ranges of the H1, ZEUS and HERMES
experiments. We use a modified form of Radyushkin's ansatz for the input model
for the generalized parton distributions. We present results for leading order
(LO) and NLO for representative observables and find that they compare
favourably to the available data.Comment: 5 pages, 2 figures, revtex, published version, we modify Radyushkin's
ansatz for the GPDs to correct for finite hadronic mass effects, and, using
the latest MRST PDFs, now agree with the H1 data (modified figs). Typo in
Eq.(3) correcte
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
Ensemble learning of linear perceptron; Online learning theory
Within the framework of on-line learning, we study the generalization error
of an ensemble learning machine learning from a linear teacher perceptron. The
generalization error achieved by an ensemble of linear perceptrons having
homogeneous or inhomogeneous initial weight vectors is precisely calculated at
the thermodynamic limit of a large number of input elements and shows rich
behavior. Our main findings are as follows. For learning with homogeneous
initial weight vectors, the generalization error using an infinite number of
linear student perceptrons is equal to only half that of a single linear
perceptron, and converges with that of the infinite case with O(1/K) for a
finite number of K linear perceptrons. For learning with inhomogeneous initial
weight vectors, it is advantageous to use an approach of weighted averaging
over the output of the linear perceptrons, and we show the conditions under
which the optimal weights are constant during the learning process. The optimal
weights depend on only correlation of the initial weight vectors.Comment: 14 pages, 3 figures, submitted to Physical Review
From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization
A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images
Planar model system of the Phillips (Cr/SiO<sub>2</sub>) catalyst based on a well-defined thin silicate film
The Phillips catalyst (Cr/SiO2) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years. However, the atomic structure of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ~1007 cm-1 typical for Cr=O vibrations. In addition, CO titration experiments suggested also the presence of "naked" Cr, which transforms into mono- and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions
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