436 research outputs found
Comment on "First order amorphous-amorphous transformation in silica"
In a recent letter (Phys. Rev. Lett. 84, 4629 (2000)), Lacks presents
evidence of a first order amorphous-amorphous transition in silica at T=0. He
calculates the free energy along a path of compression and successive
decompression of a sample of 108 SiO2 units. The free energy of the two
branches cross each other, and this is interpreted as evidence of a first order
transition. We show that this conclusion does not follow from the shown data,
since qualitatively the same phenomenology is obtained in a model where a first
order transition does not exist.Comment: 1 page, 1 figure, to appear in Phys. Rev. Let
Molecular structural order and anomalies in liquid silica
The present investigation examines the relationship between structural order,
diffusivity anomalies, and density anomalies in liquid silica by means of
molecular dynamics simulations. We use previously defined orientational and
translational order parameters to quantify local structural order in atomic
configurations. Extensive simulations are performed at different state points
to measure structural order, diffusivity, and thermodynamic properties. It is
found that silica shares many trends recently reported for water [J. R.
Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate
densities, the distribution of local orientational order is bimodal. At fixed
temperature, order parameter extrema occur upon compression: a maximum in
orientational order followed by a minimum in translational order. Unlike water,
however, silica's translational order parameter minimum is broad, and there is
no range of thermodynamic conditions where both parameters are strictly
coupled. Furthermore, the temperature-density regime where both structural
order parameters decrease upon isothermal compression (the structurally
anomalous regime) does not encompass the region of diffusivity anomalies, as
was the case for water.Comment: 30 pages, 8 figure
Orthorhombic Phase of Crystalline Polyethylene: A Constant Pressure Path Integral Monte Carlo Study
In this paper we present a Path Integral Monte Carlo (PIMC) simulation of the
orthorhombic phase of crystalline polyethylene, using an explicit atom force
field with unconstrained bond lengths and angles. This work represents a
quantum extension of our recent classical simulation (J. Chem. Phys. 106, 8918
(1997)). It is aimed both at exploring the applicability of the PIMC method on
such polymer crystal systems, as well as on a detailed assessment of the
importance of quantum effects on different quantities. We used the
ensemble and simulated the system at zero pressure in the temperature range 25
- 300 K, using Trotter numbers between 12 and 144. In order to investigate
finite-size effects, we used chains of two different lengths, C_12 and C_24,
corresponding to the total number of atoms in the super-cell being 432 and 864,
respectively. We show here the results for structural parameters, like the
orthorhombic lattice constants a,b,c, and also fluctuations of internal
parameters of the chains, such as bond lengths and bond and torsional angles.
We have also determined the internal energy and diagonal elastic constants
c_11, c_22 and c_33. We discuss the temperature dependence of the measured
quantities and compare to that obtained from the classical simulation. For some
quantities, we discuss the way they are related to the torsional angle
fluctuation. In case of the lattice parameters we compare our results to those
obtained from other theoretical approaches as well as to some available
experimental data. In order to study isotope effects, we simulated also a
deuterated polyethylene crystal at a low temperature. We also suggest possible
ways of extending this study and present some general considerations concerning
modeling of polymer crystals.Comment: 18 pages, RevTex, 18 figures, 3 tables, submitted to Phys. Rev.
Locally Preferred Structure and Frustration in Glassforming Liquids: A Clue to Polyamorphism?
We propose that the concept of liquids characterized by a given locally
preferred structure (LPS) could help in understanding the observed phenomenon
of polyamorphism. ``True polyamorphism'' would involve the competition between
two (or more) distinct LPS, one favored at low pressure because of its low
energy and one favored at high pressure because of its small specific volume,
as in tetrahedrally coordinated systems. ``Apparent polyamorphism'' could be
associated with the existence of a poorly crystallized defect-ordered phase
with a large unit cell and small crystallites, which may be illustrated by the
metastable glacial phase of the fragile glassformer triphenylphosphite; the
apparent polyamorphism might result from structural frustration, i. e., a
competition between the tendency to extend the LPS and a global constraint that
prevents tiling of the whole space by the LPS.Comment: 11, 6 figures, Proceedings of the Conference "Horizons in Complex
Systems", Messina; in honor of the 60th birthday of H.E. Stanle
Quasi Harmonic Lattice Dynamics and Molecular Dynamics calculations for the Lennard-Jones solids
We present Molecular Dynamics (MD), Quasi Harmonic Lattice Dynamics (QHLD)
and Energy Minimization (EM) calculations for the crystal structure of Ne, Ar,
Kr and Xe as a function of pressure and temperature. New Lennard-Jones (LJ)
parameters are obtained for Ne, Kr and Xe to reproduce the experimental
pressure dependence of the density. We employ a simple method which combines
results of QHLD and MD calculations to achieve densities in good agreement with
experiment from 0 K to melting. Melting is discussed in connection with
intrinsic instability of the solid as given by the QHLD approximation. (See
http://www.fci.unibo.it/~valle for related papers)Comment: 7 pages, 5 figures, REVte
Electron affinities of the first- and second- row atoms: benchmark ab initio and density functional calculations
A benchmark ab initio and density functional (DFT) study has been carried out
on the electron affinities of the first- and second-row atoms. The ab initio
study involves basis sets of and quality, extrapolations to
the 1-particle basis set limit, and a combination of the CCSD(T), CCSDT, and
full CI electron correlation methods. Scalar relativistic and spin-orbit
coupling effects were taken into account. On average, the best ab initio
results agree to better than 0.001 eV with the most recent experimental
results. Correcting for imperfections in the CCSD(T) method improves the mean
absolute error by an order of magnitude, while for accurate results on the
second-row atoms inclusion of relativistic corrections is essential. The latter
are significantly overestimated at the SCF level; for accurate spin-orbit
splitting constants of second-row atoms inclusion of (2s,2p) correlation is
essential. In the DFT calculations it is found that results for the 1st-row
atoms are very sensitive to the exchange functional, while those for second-row
atoms are rather more sensitive to the correlation functional. While the LYP
correlation functional works best for first-row atoms, its PW91 counterpart
appears to be preferable for second-row atoms. Among ``pure DFT'' (nonhybrid)
functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991
correlation) puts in the best overall performance. The best results overall are
obtained with the 1-parameter hybrid modified Perdew-Wang (mPW1) exchange
functionals of Adamo and Barone [J. Chem. Phys. {\bf 108}, 664 (1998)], with
mPW1LYP yielding the best results for first-row, and mPW1PW91 for second-row
atoms. Indications exist that a hybrid of the type mPW1LYP +
mPW1PW91 yields better results than either of the constituent functionals.Comment: Phys. Rev. A, in press (revised version, review of issues concerning
DFT and electron affinities added
Sheared Solid Materials
We present a time-dependent Ginzburg-Landau model of nonlinear elasticity in
solid materials. We assume that the elastic energy density is a periodic
function of the shear and tetragonal strains owing to the underlying lattice
structure. With this new ingredient, solving the equations yields formation of
dislocation dipoles or slips. In plastic flow high-density dislocations emerge
at large strains to accumulate and grow into shear bands where the strains are
localized. In addition to the elastic displacement, we also introduce the local
free volume {\it m}. For very small the defect structures are metastable
and long-lived where the dislocations are pinned by the Peierls potential
barrier. However, if the shear modulus decreases with increasing {\it m},
accumulation of {\it m} around dislocation cores eventually breaks the Peierls
potential leading to slow relaxations in the stress and the free energy
(aging). As another application of our scheme, we also study dislocation
formation in two-phase alloys (coherency loss) under shear strains, where
dislocations glide preferentially in the softer regions and are trapped at the
interfaces.Comment: 16pages, 11figure
Diffusion and viscosity in a supercooled polydisperse system
We have carried out extensive molecular dynamics simulations of a supercooled
polydisperse Lennard-Jones liquid with large variations in temperature at a
fixed pressure. The particles in the system are considered to be polydisperse
both in size and mass. The temperature dependence of the dynamical properties
such as the viscosity () and the self-diffusion coefficients () of
different size particles is studied. Both viscosity and diffusion coefficients
show super-Arrhenius temperature dependence and fit well to the well-known
Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range
investigated, the value of the Angell's fragility parameter (D )
classifies the present system into a strongly fragile liquid. The critical
temperature for diffusion () increases with the size of the
particles. The critical temperature for viscosity () is larger than
that for the diffusion and a sizeable deviations appear for the smaller size
particles implying a decoupling of translational diffusion from viscosity in
deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian
behavior at low temperatures where a highly nonlinear dependence on size is
observed. An inspection of the trajectories of the particles shows that at low
temperatures the motions of both the smallest and largest size particles are
discontinuous (jump-type). However, the crossover from continuous Brownian to
large length hopping motion takes place at shorter time scales for the smaller
size particles.Comment: Revtex4, 7 pages, 8 figure
Plastic Flow in Two-Dimensional Solids
A time-dependent Ginzburg-Landau model of plastic deformation in
two-dimensional solids is presented. The fundamental dynamic variables are the
displacement field \bi u and the lattice velocity {\bi v}=\p {\bi u}/\p t.
Damping is assumed to arise from the shear viscosity in the momentum equation.
The elastic energy density is a periodic function of the shear and tetragonal
strains, which enables formation of slips at large strains. In this work we
neglect defects such as vacancies, interstitials, or grain boundaries. The
simplest slip consists of two edge dislocations with opposite Burgers vectors.
The formation energy of a slip is minimized if its orientation is parallel or
perpendicular to the flow in simple shear deformation and if it makes angles of
with respect to the stretched direction in uniaxial stretching.
High-density dislocations produced in plastic flow do not disappear even if
the flow is stopped. Thus large applied strains give rise to metastable,
structurally disordered states. We divide the elastic energy into an elastic
part due to affine deformation and a defect part. The latter represents degree
of disorder and is nearly constant in plastic flow under cyclic straining.Comment: 16pages, Figures can be obtained at
http://stat.scphys.kyoto-u.ac.jp/index-e.htm
Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly
We investigate the phase behavior of a single-component system in 3
dimensions with spherically-symmetric, pairwise-additive, soft-core
interactions with an attractive well at a long distance, a repulsive soft-core
shoulder at an intermediate distance, and a hard-core repulsion at a short
distance, similar to potentials used to describe liquid systems such as
colloids, protein solutions, or liquid metals. We showed [Nature {\bf 409}, 692
(2001)] that, even with no evidences of the density anomaly, the phase diagram
has two first-order fluid-fluid phase transitions, one ending in a
gas--low-density liquid (LDL) critical point, and the other in a
gas--high-density liquid (HDL) critical point, with a LDL-HDL phase transition
at low temperatures. Here we use integral equation calculations to explore the
3-parameter space of the soft-core potential and we perform molecular dynamics
simulations in the interesting region of parameters. For the equilibrium phase
diagram we analyze the structure of the crystal phase and find that, within the
considered range of densities, the structure is independent of the density.
Then, we analyze in detail the fluid metastable phases and, by explicit
thermodynamic calculation in the supercooled phase, we show the absence of the
density anomaly. We suggest that this absence is related to the presence of
only one stable crystal structure.Comment: 15 pages, 21 figure
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