20,051 research outputs found
Shear-transformation-zone theory of plastic deformation near the glass transition
The shear-transformation-zone (STZ) theory of plastic deformation in
glass-forming materials is reformulated in light of recent progress in
understanding the roles played the effective disorder temperature and entropy
flow in nonequilibrium situations. A distinction between fast and slow internal
state variables reduces the theory to just two coupled equations of motion, one
describing the plastic response to applied stresses, and the other the dynamics
of the effective temperature. The analysis leading to these equations contains,
as a byproduct, a fundamental reinterpretation of the dynamic yield stress in
amorphous materials. In order to put all these concepts together in a realistic
context, the paper concludes with a reexamination of the experimentally
observed rheological behavior of a bulk metallic glass. That reexamination
serves as a test of the STZ dynamics, confirming that system parameters
obtained from steady-state properties such as the viscosity can be used to
predict transient behaviors.Comment: 15 pages, four figure
On the Collapsar Model of Long Gamma-Ray Bursts: Constraints from Cosmic Metallicity Evolution
We explore the consequences of new observational and theoretical evidence
that long gamma-ray bursts prefer low metallicity environments. Using recently
derived mass-metallicity correlations and the mass function from SDSS studies,
and adopting an average cosmic metallicity evolution from \citet{kewley2005}
and \citet{savaglio2005} we derive expressions for the the relative number of
massive stars formed below a given fraction of solar metallicity, ,
as function of redshift. We demonstrate that about 1/10th of all stars form
with . Therefore, a picture where the majority of GRBs form
with is not inconsistent with an empirical global SN/GRB ratio
of 1/1000. It implies that (1) GRB's peak at a significantly higher redshift
than supernovae; (2) massive star evolution at low metallicity may be
qualitatively different and; (3) the larger the low-metallicity bias of GRBs
the less likely binary evolution channels can be significant GRB producers.Comment: 12 pages, 2 figures; accepted as ApJ Lette
Local Geometric Invariants of Integrable Evolution Equations
The integrable hierarchy of commuting vector fields for the localized
induction equation of 3D hydrodynamics, and its associated recursion operator,
are used to generate families of integrable evolution equations which preserve
local geometric invariants of the evolving curve or swept-out surface.Comment: 15 pages, AMSTeX file (to appear in Journal of Mathematical Physics
Excitation Chains at the Glass Transition
The excitation-chain theory of the glass transition, proposed in an earlier
publication, predicts diverging, super-Arrhenius relaxation times and, {\it
via} a similarly diverging length scale, suggests a way of understanding the
relations between dynamic and thermodynamic properties of glass-forming
liquids. I argue here that critically large excitation chains play a role
roughly analogous to that played by critical clusters in the droplet model of
vapor condensation. The chains necessarily induce spatial heterogeneities in
the equilibrium states of glassy systems; and these heterogeneities may be
related to stretched-exponential relaxation. Unlike a first-order condensation
point in a vapor, the glass transition is not a conventional phase
transformation, and may not be a thermodynamic transition at all.Comment: 4 pages, no figure
Representations of molecules and materials for interpolation of quantum-mechanical simulations via machine learning
Computational study of molecules and materials from first principles is a cornerstone of physics, chemistry and materials science, but limited by the cost of accurate and precise simulations. In settings involving many simulations, machine learning can reduce these costs, sometimes by orders of magnitude, by interpolating between reference simulations. This requires representations that describe any molecule or material and support interpolation. We review, discuss and benchmark state-of-the-art representations and relations between them, including smooth overlap of atomic positions, many-body tensor representation, and symmetry functions. For this, we use a unified mathematical framework based on many-body functions, group averaging and tensor products, and compare energy predictions for organic molecules, binary alloys and Al-Ga-In sesquioxides in numerical experiments controlled for data distribution, regression method and hyper-parameter optimization
A microscopic model for solidification
We present a novel picture of a non isothermal solidification process
starting from a molecular level, where the microscopic origin of the basic
mechanisms and of the instabilities characterizing the approach to equilibrium
is rendered more apparent than in existing approaches based on coarse grained
free energy functionals \`a la Landau.
The system is composed by a lattice of Potts spins, which change their state
according to the stochastic dynamics proposed some time ago by Creutz. Such a
method is extended to include the presence of latent heat and thermal
conduction.
Not only the model agrees with previous continuum treatments, but it allows
to introduce in a consistent fashion the microscopic stochastic fluctuations.
These play an important role in nucleating the growing solid phase in the melt.
The approach is also very satisfactory from the quantitative point of view
since the relevant growth regimes are fully characterized in terms of scaling
exponents.Comment: 7 pages Latex +3 figures.p
Dynamics of Shear-Transformation Zones in Amorphous Plasticity: Formulation in Terms of an Effective Disorder Temperature
This investigation extends earlier studies of a shear-transformation-zone
(STZ) theory of plastic deformation in amorphous solids. My main purpose here
is to explore the possibility that the configurational degrees of freedom of
such systems fall out of thermodynamic equilibrium with the heat bath during
persistent mechanical deformation, and that the resulting state of
configurational disorder may be characterized by an effective temperature. The
further assumption that the population of STZ's equilibrates with the effective
temperature allows the theory to be compared directly with experimentally
measured properties of metallic glasses, including their calorimetric behavior.
The coupling between the effective temperature and mechanical deformation
suggests an explanation of shear-banding instabilities.Comment: 29 pages, 11 figure
Evolution, Explosion and Nucleosynthesis of Core Collapse Supernovae
We present a new set of presupernova evolutions and explosive yields of
massive stars of initial solar composition (Y=0.285, Z=0.02) in the mass range
13-35 Msun. All the models have been computed with the latest version (4.97) of
the FRANEC code that now includes a nuclear network extending from neutrons to
Mo98. The explosive nucleosynthesis has been computed twice: a first one with
an hydro code and a second one following the simpler radiation dominated shock
approximation (RDA).Comment: 20 pages, 10 figures, 12 tables. Accepted for publication on Ap
The Supernova Channel of Super-AGB Stars
We study the late evolution of solar metallicity stars in the transition
region between white dwarf formation and core collapse. This includes the
super-asymptotic giant branch (super-AGB, SAGB) stars, which have massive
enough cores to ignite carbon burning and form an oxygen-neon (ONe) core. The
most massive SAGB stars have cores that may grow to the Chandrasekhar mass
because of continued shell-burning. Their cores collapse, triggering a so
called electron capture supernovae (ECSN). From stellar evolution models we
find that the initial mass range for SAGB evolution is 7.5 ... 9.25\msun. We
perform calculations with three different stellar evolution codes to
investigate the sensitivity of this mass range to some of the uncertainties in
current stellar models. The mass range significantly depends on the treatment
of semiconvective mixing and convective overshooting. To consider the effect of
a large number of thermal pulses, as expected in SAGB stars, we construct
synthetic SAGB models that include a semi-analytical treatment of dredge-up,
hot-bottom burning, and thermal pulse properties. This synthetic model enables
us to compute the evolution of the main properties of SAGB stars from the onset
of thermal pulses until the core reaches the Chandrasekhar mass or is uncovered
by the stellar wind. Thereby, we determine the stellar initial mass ranges that
produce ONe-white dwarfs and electron-capture supernovae. The latter is found
to be 9.0 ... 9.25\msun for our fiducial model, implying that electron-capture
supernovae would constitute about 4% of all supernovae in the local universe.
Our synthetic approach allows us to explore the uncertainty of this number
imposed by uncertainties in the third dredge-up efficiency and ABG mass loss
rate. We find for ECSNe a upper limit of ~20% of all supernovae (abridged).Comment: 13 pages, 16 figures, submitted to ApJ, uses emulateap
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