13,091 research outputs found
Ab-initio calculation of the effect of stress on the chemical activity of graphene
Graphene layers are stable, hard, and relatively inert. We study how tensile
stress affects and bonds and the resulting change in the
chemical activity. Stress affects more strongly bonds that can become
chemically active and bind to adsorbed species more strongly. Upon stretch,
single C bonds are activated in a geometry mixing and ; an
intermediate state between and bonding. We use ab-initio
density functional theory to study the adsorption of hydrogen on large clusters
and 2D periodic models for graphene. The influence of the exchange-correlation
functional on the adsorption energy is discussed
Role of Rotations on Surface Diffusion of Water Trimers on Pd\{111\}
Diffusion barriers for a cluster of three water molecules on Pd(111) have
been estimated from ab-initio Density Functional Theory. A model for the
diffusion of the trimer based in rotations yields a simple explanation of why
the cluster can diffuse faster than a single water molecule by a factor
. This model is based on the differences between the adsorption
geometry for the three monomers forming the cluster. One member interacts
strongly with the surface and sits closer to the surface (d) while the other
two interact weakly and stay at a larger separation from the surface (u). The
trimer rotates nearly freely around the axis determined by the d monomer.
Translations of the whole trimer imply breaking the strong interaction of the d
monomer with the surface. Alternatively, thermal fluctuations exchange the
actual monomer sitting closer to the surface with a lower energetic cost.
Rotations around different axis introduce a diffusion mechanism where a strong
interaction is kept along the diffusion path between the water molecule
defining the axis of rotation and the Pd underneath.Comment: water ; monomer ; trimer ; water clusters ; diffusion ; rotation
assisted ; Pd\{111\} ; ab-initio ; density functional theor
Mechanical response of a self avoiding membrane: fold collisions and the birth of conical singularities
An elastic membrane that is forced to reside in a container smaller than its
natural size will deform and, upon further volume reduction, eventually
crumple. The crumpled state is characterized by the localization of energy in a
complex network of highly deformed crescent-like regions joined by line ridges.
Previous studies have focused on the onset of the crumpled state by analyzing
the mechanical response and stability of a conical dislocation, while others
have simulated the highly packed regime neglecting the importance of the
connectivity of the membrane. Here we show, through a combination of
experiments, numerical simulations, and analytic approach, that the emergence
of new regions of high stretching is a generic outcome when a self avoiding
membrane is subject to a severe geometrical constraint. We demonstrate that, at
moderate packing fraction, interlayer interactions produce a response
equivalent to the one of a thicker membrane that has the shape of the deformed
one. Evidence is found that friction plays a key role stabilizing the folded
structures.Comment: 10 page
Effects of fluid inertia and turbulence on force coefficients for squeeze film dampers
The effects of fluid inertia and turbulence on the force coefficients of squeeze film dampers are investigated analytically. Both the convective and the temporal terms are included in the analysis of inertia effects. The analysis of turbulence is based on friction coefficients currently found in the literature for Poiseuille flow. The effect of fluid inertia on the magnitude of the radial direct inertia coefficient (i.e., to produce an apparent added mass at small eccentricity ratios, due to the temporal terms) is found to be completely reversed at large eccentricity ratios. The reversal is due entirely to the inclusion of the convective inertia terms in the analysis. Turbulence is found to produce a large effect on the direct damping coefficient at high eccentricity ratios. For the long or sealed squeeze film damper at high eccentricity ratios, the damping prediction with turbulence included is an order of magnitude higher than the laminar solution
Luminous X-ray Flares from Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697
We report results of the first search specifically targeting short-timescale
X-ray flares from low-mass X-ray binaries in an early-type galaxy. A new method
for flare detection is presented. In NGC 4697, the nearest, optically luminous,
X-ray faint elliptical galaxy, 3 out of 157 sources are found to display flares
at >99.95% probability, and all show more than one flare. Two sources are
coincident with globular clusters and show flare durations and luminosities
similar to (but larger than) Type-I X-ray superbursts found in Galactic neutron
star (NS) X-ray binaries (XRBs). The third source shows more extreme flares.
Its flare luminosity (~6E39 erg/s) is very super-Eddington for an NS and is
similar to the peak luminosities of the brightest Galactic black hole (BH)
XRBs. However, the flare duration (~70 s) is much shorter than are typically
seen for outbursts reaching those luminosities in Galactic BH sources.
Alternative models for the flares are considered.Comment: Astrophysical Journal Letters, accepted: 4 page
Crystal structure and electronic states of tripotassium picene
The crystal structure of potassium doped picene with an exact stoichiometry
(K3C22H14, K3picene from here onwards) has been theoretically determined within
Density Functional Theory allowing complete variational freedom of the crystal
structure parameters and the molecular atomic positions. A modified herringbone
lattice is obtained in which potassium atoms are intercalated between two
paired picene molecules displaying the two possible orientations in the
crystal.Along the c-axis, organic molecules alternate with chains formed by
three potassium atoms. The electronic structureof the doped material resembles
pristine picene, except that now the bottom of the conduction band is occupied
by six electrons coming from the ionized K atoms (six per unit cell).
Wavefunctions remain based mainly on picene molecular orbitals getting their
dispersion from intralayer edge to face CH/pi bonding, while eigenenergies have
been modified by the change in the electrostatic potential. The small
dispersion along the c-axis is assigned to small H-H overlap. From the
calculated electronic density of states we expect metallic behavior for
potassium doped picene.Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural
.cif files include
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