9,622 research outputs found
First principles study of adsorbed Cu_n (n=1-4) microclusters on MgO(100): structural and electronic properties
We present a density functional study of the structural and electronic
properties of small Cu_n (n=1,4) aggregates on defect-free MgO(100). The
calculations employ a slab geometry with periodic boundary conditions,
supercells with up to 76 atoms, and include full relaxation of the surface
layer and of all adsorbed atoms. The preferred adsorption site for a single Cu
adatom is on top of an oxygen atom. The adsorption energy and Cu-O distance are
E_S-A = 0.99 eV and d_S-A = 2.04 Angstroems using the Perdew-Wang gradient
corrected exchange correlation functional. The saddle point for surface
diffusion is at the "hollow" site, with a diffusion barrier of around 0.45 eV.
For the adsorbed copper dimer, two geometries, one parallel and one
perpendicular to the surface, are very close in energy. For the adsorbed Cu_3,
a linear configuration is preferred to the triangular geometry. As for the
tetramer, the most stable adsorbed geometry for Cu_4 is a rhombus. The
adsorption energy per Cu atom decreases with increasing the size of the
cluster, while the Cu-Cu cohesive energy increases, rapidly becoming more
important than the adsorption energy.Comment: Major revision, Latex(2e) document, 23 pages, 11 figures, accepted
for publication in J. of Chem. Phys., paper available at
http://irrmawww.epfl.ch/vm/vm_wor
The old anticentre open cluster Berkeley 32: membership and fundamental parameters
We have obtained medium-low resolution spectroscopy and BVI CCD imaging of
Berkeley 32, an old open cluster which lies in the anticentre direction. From
the radial velocities of 48 stars in the cluster direction we found that 31 of
them, in crucial evolutionary phases, are probable cluster members, with an
average radial velocity of +106.7 (sigma = 8.5) km/s. From isochrone fitting to
the colour magnitude diagrams of Berkeley 32 we have obtained an age of 6.3
Gyr, (m-M)0 = 12.48 and E(B-V) = 0.10. The best fit is obtained with Z=0.008. A
consistent distance, (m-M)0 ~= 12.6 +/- 0.1, has been derived from the mean
magnitude of red clump stars with confirmed membership; we may assume (m-M)0 ~=
12.55 +/- 0.1. The colour magnitude diagram of the nearby field observed to
check for field stars contamination looks intriguingly similar to that of the
Canis Major overdensity.Comment: MNRAS, in press. Degraded resolution for Fig.
Radial velocities and membership of stars in the old, distant open cluster Berkeley 29
Multi slit spectroscopy at the Telescopio Nazionale Galileo was employed to
measure radial velocities for 20 stars in the direction of the old open cluster
Berkeley 29, the farthest known in our Galaxy. Membership information was
derived for stars along all the red giant branch, in particular near its tip,
and on the red clump. The sample of bona-fide cluster members was used to
revise the cluster distance to about 15 kpc, on the basis of an empirical
comparison with the red clump in open clusters with known distances. A
metallicity [Fe/H] = -0.74 +/- 0.18 was also estimated using the colours of
spectroscopically confirmed red giant stars.Comment: 7 pages, 5 figures (fig. 1 at low-res, but JPEG version included
too), accepted for publication in A&
Photometric and spectroscopic study of the intermediate-age open cluster NGC 2355
In this paper we analyse the evolutionary status and properties of the old
open cluster NGC 2355, located in the Galactic anticentre direction, as a part
of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the
Bessel , , and filters. The cluster parameters have been obtained
using the synthetic colour-magnitude diagram (CMD) method, as done in other
papers of this series. Additional spectroscopic observations with FIES@NOT of
three giant stars were used to determine the chemical properties of the
cluster. Our analysis shows that NGC 2355 has metallicity slightly less than
solar, with [Fe/H] dex, age between 0.8 and 1 Gyr, reddening
in the range 0.14 and 0.19 mag, and distance modulus of about 11 mag.
We also investigated the abundances of O, Na, Al, , iron-peak, and
neutron capture elements, showing that NGC 2355 falls within the abundance
distribution of similar clusters (same age and metallicity). The Galactocentric
distance of NGC~2355 places it at the border between two regimes of metallicity
distribution; this makes it an important cluster for the study of the chemical
properties and evolution of the disc.Comment: 20 pages, 11 figures, Accepted on MNRA
Functional compartmentalization of Rad9 and Hus1 reveals diverse assembly of the 9-1-1 complex components during the DNA damage response in Leishmania
The Rad9-Rad1-Hus1 (9-1-1) complex is a key component in the coordination of DNA damage sensing, cell cycle progression and DNA repair pathways in eukaryotic cells. This PCNA-related trimer is loaded onto RPA-coated single stranded DNA and interacts with ATR kinase to mediate effective checkpoint signaling to halt the cell cycle and to promote DNA repair. Beyond these core activities, mounting evidence suggests that a broader range of functions can be provided by 9-1-1 structural diversification. The protozoan parasite Leishmania is an early-branching eukaryote with a remarkably plastic genome, which hints at peculiar genome maintenance mechanisms. Here, we investigated the existence of homologs of the 9-1-1 complex subunits in L. major and found that LmRad9 and LmRad1 associate with chromatin in response to replication stress and form a complex in vivo with LmHus1. Similar to LmHus1, LmRad9 participates in telomere homeostasis and in the response to both replication stress and double strand breaks. However, LmRad9 and LmHus1-deficient cells present markedly opposite phenotypes, which suggest their functional compartmentalization. We show that some of the cellular pool of LmRad9 forms an alternative complex and that some of LmHus1 exists as a monomer. We propose that the diverse assembly of the Leishmania 9-1-1 subunits mediates functional compartmentalization, which has a direct impact on the response to genotoxic stress
Stacked analysis of earthquake sequences: statistical space-time definition of clustering and Omori law behavior
The definition of the aftershocks sequence is still a debated topic. We here propose a study of the
spatial and temporal variation of the earthquakes clustering and rate decay. We used five different
seismic catalogues, characterized by specific spatial and magnitude ranges. They are respectively:
world one, for a global analysis, Greek, Japanese, Californian and Italian regional catalogues in
order to investigate different seismo-tectonic settings.
A stacking procedure has been applied to characterize a typical sequence behavior and allowing the
evaluation of changes over time intervals
Earthquake sound perception
Sound is an effect produced by almost all earthquakes. Using a web-based questionnaire on earthquake effects that included questions relating to seismic sound, we collected 77,000 responses for recent shallow Italian earthquakes. An analysis of audibility attenuation indicated that the decrease of the percentage of respondents hearing the sound was proportional to the logarithm of the epicentral distance and linearly dependent on earthquake magnitude, in accordance with the behavior of ground displacement. Even if this result was based on Italian data, qualitative agreement with the results of theoretical displacement, and of a similar study based on French seismicity suggests wider validity. We also found that, given earthquake magnitude, audibility increased together with the observed macroseismic intensity, leading to the possibility of accounting for sound audibility in intensity assessment. Magnitude influenced this behavior, making small events easier to recognize, as suggested by their frequency content
How Observer Conditions Impact Earthquake Perception
Intensity scales define the criteria used to determine different levels of shaking in relation to environmental effects. Objective evaluations of low intensity degrees based on transient effects may be difficult. In particular, estimations for the number of people feeling an earthquake are critical, and are qualitatively described by words such as “few”, “many”, and “most” for determining various intensity levels. In general, such qualitative amounts are converted into specific percentages for each macroseismic scale. Additionally, estimations of macroseismic intensity are influenced by variables that are mentioned in macroseismic scale degree descriptions. For example, the Mercalli-Cancani-Sieberg (MCS; Sieberg, 1930) and the Modified Mercalli Intensity (MMI) scales (Wood and Neumann, 1931) describe the intensity II as “Felt only by a few people, extremely susceptible, in perfectly quiet situations, almost always on the upper floors of buildings”. Another example is the European Macroseismic Scale (EMS) (Grunthal, 1998) that describes the intensity V as “felt indoors by most, outdoors by few. Many sleeping people awake”.
In this work, we focus on two variables referred to as people’s physical “situation” (what were you doing?), here categorized as “sleeping”, “at rest”, or “in motion”; and the observer’s “location”, here categorized as “higher floors”, “lower floors”, and “outdoors”. Both variables have a partial influence on intensity assessments because they condition vibration perception. However, it is important to study, using an experimental method, the weights of these variables in the quantification of felt effects. Musson (2005a) also recognized the influence of such conditions on the number of people feeling an earthquake, stating that the proportion of people in different conditions “are generally difficult to quantify in any case”. Today, we have a large amount of data available through the macroseismic web site “haisentitoilterremoto” associated with specific observer conditions. Using this data, a study of these effects is possible. For this analysis, we placed attention on transitory effects that, in the past, could not be easily studied due to the intrinsic difficulty in collecting this type of data. The aim of this work was to specifically analyze and quantify how the observer’s “situation” and “location” influence earthquake perception suggesting a new scale description that can be easily used for low intensity estimation
Luther-Emery Phase and Atomic-Density Waves in a Trapped Fermion Gas
The Luther-Emery liquid is a state of matter that is predicted to occur in
one-dimensional systems of interacting fermions and is characterized by a
gapless charge spectrum and a gapped spin spectrum. In this Letter we discuss a
realization of the Luther-Emery phase in a trapped cold-atom gas. We study by
means of the density-matrix renormalization-group technique a two-component
atomic Fermi gas with attractive interactions subject to parabolic trapping
inside an optical lattice. We demonstrate how this system exhibits compound
phases characterized by the coexistence of spin pairing and atomic-density
waves. A smooth crossover occurs with increasing magnitude of the atom-atom
attraction to a state in which tightly bound spin-singlet dimers occupy the
center of the trap. The existence of atomic-density waves could be detected in
the elastic contribution to the light-scattering diffraction pattern.Comment: 10 pages, 3 figures, 1 Table, submitted to Phys. Rev. on July 25th
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Double layer in ionic liquids: Overscreening vs. crowding
We develop a simple Landau-Ginzburg-type continuum theory of solvent-free
ionic liquids and use it to predict the structure of the electrical double
layer. The model captures overscreening from short-range correlations, dominant
at small voltages, and steric constraints of finite ion sizes, which prevail at
large voltages. Increasing the voltage gradually suppresses overscreening in
favor of the crowding of counterions in a condensed inner layer near the
electrode. The predicted ion profiles and capacitance-voltage relations are
consistent with recent computer simulations and experiments on room-temperature
ionic liquids, using a correlation length of order the ion size.Comment: 4 pages + supplementary informatio
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