130 research outputs found
Continuum field description of crack propagation
We develop continuum field model for crack propagation in brittle amorphous
solids. The model is represented by equations for elastic displacements
combined with the order parameter equation which accounts for the dynamics of
defects. This model captures all important phenomenology of crack propagation:
crack initiation, propagation, dynamic fracture instability, sound emission,
crack branching and fragmentation.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett. Additional
information can be obtained from http://gershwin.msd.anl.gov/theor
Atomic Scale Modelling of Two-Dimensional Molecular Self-Assembly on a Passivated Si Surface
International audienceThe self-assembly of two-dimensional (2D) molecular structures on a solid surface relies on the subtle balance between non covalent intermolecular and molecule-surface forces. The energetics of 2D molecular lattices forming different patterns on a passivated semiconductor surface are here investigated by a combination of atomistic simulation methods. Density-functional theory provides structure and charges of the molecules, while metadynamics with empirical forces provides a best guess for the lowest-energy adsorption sites of single molecules and dimers. Subsequently, molecular dynamics simulations of extended molecular assemblies with empirical forces yield the most favorable lattice structures at finite temperature and pressure.The theoretical results are in good agreement with scanning tunneling microscopy observations of self-assembled molecular monolayers on a B-doped Si(111) surface, thus allowing to rationalize the competition of long-range dispersion forces between the molecules and the surface. Such a result demonstrates the interest of this predictive approach for further progress in supramolecular chemistry on semiconductor surface
The Influence of Molecular Adsorption on Elongating Gold Nanowires
Using molecular dynamics simulations, we study the impact of physisorbing
adsorbates on the structural and mechanical evolution of gold nanowires (AuNWs)
undergoing elongation. We used various adsorbate models in our simulations,
with each model giving rise to a different surface coverage and mobility of the
adsorbed phase. We find that the local structure and mobility of the adsorbed
phase remains relatively uniform across all segments of an elongating AuNW,
except for the thinning region of the wire where the high mobility of Au atoms
disrupts the monolayer structure, giving rise to higher solvent mobility. We
analyzed the AuNW trajectories by measuring the ductile elongation of the wires
and detecting the presence of characteristic structural motifs that appeared
during elongation. Our findings indicate that adsorbates facilitate the
formation of high-energy structural motifs and lead to significantly higher
ductile elongations. In particular, our simulations result in a large number of
monatomic chains and helical structures possessing mechanical stability in
excess of what we observe in vacuum. Conversely, we find that a molecular
species that interacts weakly (i.e., does not adsorb) with AuNWs worsens the
mechanical stability of monatomic chains.Comment: To appear in Journal of Physical Chemistry
Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions
Using an updated simulation tool, we examine molecular junctions comprised of
benzene-1,4-dithiolate bonded between gold nanotips, focusing on the importance
of environmental factors and inter-electrode distance on the formation and
structure of bridged molecules. We investigate the complex relationship between
monolayer density and tip separation, finding that the formation of
multi-molecule junctions is favored at low monolayer density, while
single-molecule junctions are favored at high density. We demonstrate that tip
geometry and monolayer interactions, two factors that are often neglected in
simulation, affect the bonding geometry and tilt angle of bridged molecules. We
further show that the structures of bridged molecules at 298 and 77 K are
similar.Comment: To appear in ACS Nano, 30 pages, 5 figure
New insight on the nature of cosmic reionizers from the CEERS survey
The Epoch of Reionization (EoR) began when galaxies grew in abundance and
luminosity, so their escaping Lyman continuum (LyC) radiation started ionizing
the surrounding neutral intergalactic medium (IGM). Despite significant recent
progress, the nature and role of cosmic reionizers are still unclear: in order
to define them, it would be necessary to directly measure their LyC escape
fraction (). However, this is impossible during the EoR due to the
opacity of the IGM. Consequently, many efforts at low and intermediate redshift
have been made to determine measurable indirect indicators in high-redshift
galaxies so that their can be predicted. This work presents the
analysis of the indirect indicators of 62 spectroscopically confirmed
star-forming galaxies at from the Cosmic Evolution Early
Release Science (CEERS) survey, combined with 12 sources with public data from
other JWST-ERS campaigns. From the NIRCam and NIRSpec observations, we measured
their physical and spectroscopic properties. We discovered that on average
star-forming galaxies are compact in the rest-frame UV ( 0.4
kpc), are blue sources (UV- slope -2.17), and have a predicted
of about 0.13.
A comparison of our results to models and predictions as well as an
estimation of the ionizing budget suggests that low-mass galaxies with UV
magnitudes fainter than that we currently do not characterize
with JWST observations probably played a key role in the process of
reionization.Comment: 14 pages, 11 figures, submitted to A&
Elasticity, Stability and Ideal Strength of -SiC in plane-wave-based ab initio calculations
On the basis of the pseudopotential plane-wave(PP-PW) method and the
local-density-functional theory(LDFT), this paper studies energetics,
stress-strain relation, stability and ideal strength of -SiC under
various loading modes, where uniform uniaxial extension and tension, biaxial
proportional extension are considered along directions [001] and [111]. The
lattice constant, elastic constants and moduli of equilibrium state are
calculated, and the results agree well with the experimental data. As the four
Si-C bonds along directions [111], [11], [11] and
[11] are not the same under the loading along [111], internal
relaxation and the corresponding internal displacements must be considered. We
find that, at the beginning of loading, the effect of internal displacement
through shuffle and glide plane diminishes the difference among the four Si-C
bonds length, but will increase the difference at the subsequent loading, which
will result in a crack nucleated on \{111\} shuffle plane and a subsequently
cleavage fracture. Thus the corresponding theoretical strength is 50.8 GPa,
which agrees well with the recent experiment value, 53.4 GPa. However, with the
loading along [001], internal relaxation is not important for tetragonal
symmetry. Elastic constants during the uniaxial tension along [001] are
calculated. Based on the stability analysis with stiffness coefficients, we
find that the spinodal and Born instabilities are triggered almost at the same
strain, which agrees with the previous molecular dynamics simulation. During
biaxial proportional extension, stress and strength vary proportionally with
the biaxial loading ratio at the same longitudinal strain.Comment: 9 pages, 10 figure
Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at to from CEERS
We analyze the evolution of massive (log [] )
galaxies at 4--8 selected from the JWST Cosmic Evolution Early Release
Science (CEERS) survey. We infer the physical properties of all galaxies in the
CEERS NIRCam imaging through spectral energy distribution (SED) fitting with
dense basis to select a sample of high redshift massive galaxies. Where
available we include constraints from additional CEERS observing modes,
including 18 sources with MIRI photometric coverage, and 28 sources with
spectroscopic confirmations from NIRSpec or NIRCam wide-field slitless
spectroscopy. We sample the recovered posteriors in stellar mass from SED
fitting to infer the volume densities of massive galaxies across cosmic time,
taking into consideration the potential for sample contamination by active
galactic nuclei (AGN). We find that the evolving abundance of massive galaxies
tracks expectations based on a constant baryon conversion efficiency in dark
matter halos for 1--4. At higher redshifts, we observe an excess
abundance of massive galaxies relative to this simple model. These higher
abundances can be explained by modest changes to star formation physics and/or
the efficiencies with which star formation occurs in massive dark matter halos,
and are not in tension with modern cosmology.Comment: 20 pages, 10 figure
CEERS: Diversity of Lyman-Alpha Emitters during the Epoch of Reionization
We analyze rest-frame ultraviolet to optical spectra of three -
galaxies whose Ly-emission lines were previously detected with
Keck/MOSFIRE observations, using the JWST/NIRSpec observations from the Cosmic
Evolution Early Release Science (CEERS) survey. From NIRSpec data, we confirm
the systemic redshifts of these Ly emitters, and emission-line ratio
diagnostics indicate these galaxies were highly ionized and metal poor. We
investigate Ly line properties, including the line flux, velocity
offset, and spatial extension. For the one galaxy where we have both NIRSpec
and MOSFIRE measurements, we find a significant offset in their flux
measurements ( greater in MOSFIRE) and a marginal difference in
the velocity shifts. The simplest interpretation is that the Ly
emission is extended and not entirely encompassed by the NIRSpec slit. The
cross-dispersion profiles in NIRSpec reveal that Ly in one galaxy is
significantly more extended than the non-resonant emission lines. We also
compute the expected sizes of ionized bubbles that can be generated by the
Ly sources, discussing viable scenarios for the creation of sizable
ionized bubbles (1 physical Mpc). The source with the highest-ionization
condition is possibly capable of ionizing its own bubble, while the other two
do not appear to be capable of ionizing such a large region, requiring
additional sources of ionizing photons. Therefore, the fact that we detect
Ly from these galaxies suggests diverse scenarios on escape of
Ly during the epoch of reionization. High spectral resolution spectra
with JWST/NIRSpec will be extremely useful for constraining the physics of
patchy reionization.Comment: Submitted to ApJ (18 pages, 7 figures, 2 tables
The Physical Conditions of Emission-Line Galaxies at Cosmic Dawn from JWST/NIRSpec Spectroscopy in the SMACS 0723 Early Release Observations
We present rest-frame optical emission-line flux ratio measurements for five
galaxies observed by the JWST Near-Infared Spectrograph (NIRSpec) in the
SMACS 0723 Early Release Observations. We add several quality-control and
post-processing steps to the NIRSpec pipeline reduction products in order to
ensure reliable relative flux calibration of emission lines that are closely
separated in wavelength, despite the uncertain \textit{absolute}
spectrophotometry of the current version of the reductions. Compared to
galaxies in the literature, the galaxies have similar
[OIII]5008/H ratios, similar [OIII]4364/H
ratios, and higher (0.5 dex) [NeIII]3870/[OII]3728
ratios. We compare the observations to MAPPINGS V photoionization models and
find that the measured [NeIII]3870/[OII]3728,
[OIII]4364/H, and [OIII]5008/H emission-line
ratios are consistent with an interstellar medium that has very high ionization
(, units of cm~s), low metallicity (), and very high pressure (, units of
cm). The combination of [OIII]4364/H and
[OIII](4960+5008)/H line ratios indicate very high electron
temperatures of , further implying metallicities of
with the application of low-redshift calibrations for
``-based'' metallicities. These observations represent a tantalizing new
view of the physical conditions of the interstellar medium in galaxies at
cosmic dawn.Comment: Accepted for publication in AAS Journals. 14 pages, 6 figures, 3
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