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## Anomalous surface lattice dynamics in the low-temperature phase of Ba(Fe1−xCox)2As2

Fonte: National Academy of Sciences
Publicador: National Academy of Sciences

Tipo: Artigo de Revista Científica

Português

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In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe1−xCox)2As2, through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron–phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type...

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## Molecular dynamics calculation of mean square displacement in alkali metals and rare gas solids and comparison with lattice dynamics

Fonte: Brock University
Publicador: Brock University

Tipo: Electronic Thesis or Dissertation

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Molec ul ar dynamics calculations of the mean sq ua re
displacement have been carried out for the alkali metals Na, K
and Cs and for an fcc nearest neighbour Lennard-Jones model
applicable to rare gas solids. The computations for the alkalis
were done for several temperatures for
temperature vol ume a swell as for
the
the
ze r 0 pressure ze ro
zero pressure volume
corresponding to each temperature. In the fcc case, results were
obtained for a wide range of both the temperature and density.
Lattice dynamics calculations of the harmonic and the lowe s t
order anharmonic (cubic and quartic) contributions to the mean
square displacement were performed for the same potential models
as in the molecular dynamics calculations. The Brillouin zone
sums arising in the harmonic and the quartic terms were computed
for very large numbers of points in q-space, and were
extrapolated to obtain results ful converged with respect to
the number of points in the Brillouin zone.An excellent agreement between the
lattice dynamics results was observed
molecular dynamics and
in the case of all the
alkali metals, e~ept for the zero pressure case of CSt where the
difference is about 15 % near the melting temperature. It was
concluded that for the alkalis...

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## Clocking the anisotropic lattice dynamics of multi-walled carbon nanotubes by four-dimensional ultrafast transmission electron microscopy

Fonte: Nature Publishing Group
Publicador: Nature Publishing Group

Tipo: Artigo de Revista Científica

Publicado em 12/02/2015
Português

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Recent advances in the four-dimensional ultrafast transmission electron microscope (4D-UTEM) with combined spatial and temporal resolutions have made it possible to directly visualize structural dynamics of materials at the atomic level. Herein, we report on our development on a 4D-UTEM which can be operated properly on either the photo-emission or the thermionic mode. We demonstrate its ability to obtain sequences of snapshots with high spatial and temporal resolutions in the study of lattice dynamics of the multi-walled carbon nanotubes (MWCNTs). This investigation provides an atomic level description of remarkable anisotropic lattice dynamics at the picosecond timescales. Moreover, our UTEM measurements clearly reveal that distinguishable lattice relaxations appear in intra-tubular sheets on an ultrafast timescale of a few picoseconds and after then an evident lattice expansion along the radial direction. These anisotropic behaviors in the MWCNTs are considered arising from the variety of chemical bonding, i.e. the weak van der Waals bonding between the tubular planes and the strong covalent sp2-hybridized bonds in the tubular sheets.

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## Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Intracellular transport processes driven by molecular motors can be described
by stochastic lattice models of self-driven particles. Here we focus on
bidirectional transport models excluding the exchange of particles on the same
track. We explore the possibility to have efficient transport in these systems.
One possibility would be to have appropriate interactions between the various
motors' species, so as to form lanes. However, we show that the lane formation
mechanism based on modified attachment/detachment rates as it was proposed
previously is not necessarily connected to an efficient transport state and is
suppressed when the diffusivity of unbound particles is finite. We propose
another interaction mechanism based on obstacle avoidance that allows to have
lane formation for limited diffusion. Besides, we had shown in a separate paper
that the dynamics of the lattice itself could be a key ingredient for the
efficiency of bidirectional transport. Here we show that lattice dynamics and
interactions can both contribute in a cooperative way to the efficiency of
transport. In particular, lattice dynamics can decrease the interaction
threshold beyond which lanes form. Lattice dynamics may also enhance the
transport capacity of the system even when lane formation is suppressed.; Comment: 25 pages...

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## Lattice dynamics and a magnetic-structural phase transition in the nickel orthoborate $Ni_{3}(BO_{3})_{2}$

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/09/2015
Português

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Nickel orthoborate $Ni_{3}(BO_{3})_{2}$ having a complex orthorhombic
structure $P_{nnm}$ (#58, Z=2) of the kotoite type is known for quite a long
time as an antiferromagnetic material below $T_{N}$ = 46 K, but up to now its
physical properties including the lattice dynamics have not been explored. Six
magnetic nickel $Ni^{2+}$ ions (S=1) in the unit cell are distributed over the
2a and 4f positions in the centers of distorted $[O_{6}]$ octahedra. The
$[NiO_{6}]$ units are linked via rigid $[BO_{3}]$ groups and these structural
particularities impose restrictions on the lattice dynamics and spin-phonon
interactions. We performed the symmetry analysis of the phonon modes at the
center of the Brillouin zone. The structural parameters and phonon modes were
calculated using Dmol3 program. We report and analyze results of infrared and
Raman studies of phonon spectra measured in all required polarizations. Most of
the even and odd phonons predicted on the basis of the symmetry analysis and
theoretical calculations were reliably identified in the measured spectra.
Absorption measurements in the infrared region showed emergence of several very
narrow and weak phonons at the magnetic ordering temperature $T_{N}$. This
observation proves the existence of a structural phase transition not reported
before which is evidently coupled intrinsically with the magnetic dynamics of
$Ni_{3}(BO_{3})_{2}$. A clear evidence of spin-phonon interaction was observed
for some particular phonons below $T_{N}$.; Comment: 25 pages...

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## Choose interelement coupling to preserve self-adjoint dynamics in multiscale modelling and computation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/11/2008
Português

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Consider the macroscale modelling of microscale spatiotemporal dynamics. Here
we develop a new approach to ensure coarse scale discrete models preserve
important self-adjoint properties of the fine scale dynamics. The first part
explores the discretisation of microscale continuum dynamics. The second
addresses how dynamics on a fine lattice are mapped to lattice a factor of two
coarser (as in multigrids). Such mapping of discrete lattice dynamics may be
iterated to empower us in future research to explore scale dependent emergent
phenomena. The support of dynamical systems, centre manifold, theory ensures
that the coarse scale modelling applies with a finite spectral gap, in a finite
domain, and for all time. The accuracy of the models is limited by the
asymptotic resolution of subgrid coarse scale processes, and is controlled by
the level of truncation. As given examples demonstrate, the novel feature of
the approach developed here is that it ensures the preservation of important
conservation properties of the microscale dynamics.

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## Strong Influence of the diffuse component on the lattice dynamics in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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The temperature and zone dependence of the lattice dynamics in
Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ is characterized using neutron inelastic
scattering. A strong correlation between the diffuse and phonon scattering is
measured. The lattice dynamics in Brillouin zones where the diffuse scattering
is strong is observed to display qualitatively different behavior than those
zones where the diffuse scattering is weak. In the (220) and (200) zones, where
there is a weak diffuse component, the dynamics are well described by coupled
harmonic oscillators. Compared with SrTiO$_{3}$, the coupling is weak and
isotropic, resulting in only a small transfer of spectral weight from one mode
to another. A comparison of the scattering in these zones to the (110) zone,
where a strong diffuse component is present, reveals a strong coupling of the
diffuse (or central) component to the acoustic mode. We speculate that the
coupling to the central peak is the reason for several recent conflicting
interpretations of the lattice dynamics based on data from zones with a strong
diffuse component.; Comment: 7 pages, 7 figures

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## Structure of Defective Crystals at Finite Temperatures: A Quasi-Harmonic Lattice Dynamics Approach

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 26/04/2010
Português

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In this paper we extend the classical method of lattice dynamics to defective
crystals with partial symmetries. We start by a nominal defect configuration
and first relax it statically. Having the static equilibrium configuration, we
use a quasiharmonic lattice dynamics approach to approximate the free energy.
Finally, the defect structure at a finite temperature is obtained by minimizing
the approximate Helmholtz free energy. For higher temperatures we take the
relaxed configuration at a lower temperature as the reference configuration.
This method can be used to semi-analytically study the structure of defects at
low but non-zero temperatures, where molecular dynamics cannot be used. As an
example, we obtain the finite temperature structure of two 180^o domain walls
in a 2-D lattice of interacting dipoles. We dynamically relax both the position
and polarization vectors. In particular, we show that increasing temperature
the domain wall thicknesses increase.

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## Lattice Fluid Dynamics from Perfect Discretizations of Continuum Flows

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We use renormalization group methods to derive equations of motion for large
scale variables in fluid dynamics. The large scale variables are averages of
the underlying continuum variables over cubic volumes, and naturally live on a
lattice. The resulting lattice dynamics represents a perfect discretization of
continuum physics, i.e. grid artifacts are completely eliminated. Perfect
equations of motion are derived for static, slow flows of incompressible,
viscous fluids. For Hagen-Poiseuille flow in a channel with square cross
section the equations reduce to a perfect discretization of the Poisson
equation for the velocity field with Dirichlet boundary conditions. The perfect
large scale Poisson equation is used in a numerical simulation, and is shown to
represent the continuum flow exactly. For non-square cross sections we use a
numerical iterative procedure to derive flow equations that are approximately
perfect.; Comment: 25 pages, tex., using epsfig, minor changes, refernces added

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## Symmetry-mode-based classical and quantum mechanical formalism of lattice dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We present classical and quantum mechanical descriptions of lattice dynamics,
from the atomic to the continuum scale, using atomic scale symmetry modes and
their constraint equations. This approach is demonstrated for a one-dimensional
chain and a two-dimensional square lattice with a monatomic basis. For the
classical description, we find that rigid modes, in addition to the
distortional modes found before, are necessary to describe the kinetic energy.
The long wavelength limit of the kinetic energy terms expressed in terms of
atomic scale modes is shown to be consistent with the continuum theory, and the
leading order corrections are obtained. For the quantum mechanical description,
we find conjugate momenta for the atomic scale symmetry modes. In direct space,
graphical rules for their commutation relations are obtained. Commutation
relations in the reciprocal space are also calculated. As an example, phonon
modes are analyzed in terms of symmetry modes. We emphasize that the approach
based on atomic scale symmetry modes could be useful, for example, for the
description of multiscale lattice dynamics and the dynamics near structural
phase transition.

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## Direct characterization of photo-induced lattice dynamics in BaFe2As2

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/12/2014
Português

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Ultrafast light pulses can modify the electronic properties of quantum
materials by perturbing the underlying, intertwined degrees of freedom. In
particular, iron-based superconductors exhibit a strong coupling among
electronic nematic fluctuations, spins, and the lattice, serving as a
playground for ultrafast manipulation. Here we use time-resolved x-ray
scattering to measure the lattice dynamics of photo-excited BaFe2As2. Upon
optical excitation, no signature of an ultrafast change of the crystal symmetry
is observed, but the lattice oscillates rapidly in time due to the coherent
excitation of an A1g mode that modulates the Fe-As-Fe bond angle. We directly
quantify the coherent lattice dynamics and show that even a small photo-induced
lattice distortion can induce notable changes in the electronic and magnetic
properties. Our analysis implies that transient structural modification can
generally be an effective tool for manipulating the electronic properties of
multi-orbital systems, where electronic instabilities are sensitive to the
orbital character of bands near the Fermi level.

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## Caricature of Hydrodynamics for Lattice Dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/10/2011
Português

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The lattice dynamics in $\mathbb{Z}^d$, $d\ge1$, is considered. The initial
data are supposed to be random function. We introduce the family of initial
measures $\{\mu_0^\epsilon,\epsilon>0\}$ depending on a small scaling parameter
$\epsilon$. We assume that the measures $\mu_0^\epsilon$ are locally
homogeneous for space translations of order much less than $\epsilon^{-1}$ and
nonhomogeneous for translations of order $\epsilon^{-1}$. Moreover, the
covariance of $\mu_0^\epsilon$ decreases with distance uniformly in $\epsilon$.
Given $\tau\in\mathbb{R}\setminus 0$, $r\in\mathbb{R}^d$, and $\kappa>0$, we
consider the distributions of random solution in the time moments
$t=\tau/\epsilon^\kappa$ and at lattice points close to
$[r/\epsilon]\in\mathbb{Z}^d$. The main goil is to study the asymptotics of
these distributions as $\epsilon\to0$ and derive the limit hydrodynamic
equations of the Euler or Navier-Stokes type. The similar results are obtained
for lattice dynamics in the half-space $\mathbb{Z}^d_+$.; Comment: 43 pages

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## Nonlinear Fractional Dynamics on a Lattice with Long Range Interactions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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A unified approach has been developed to study nonlinear dynamics of a 1D
lattice of particles with long-range power-law interaction. A classical case is
treated in the framework of the generalization of the well-known
Frenkel-Kontorova chain model for the non-nearest interactions. Quantum
dynamics is considered following Davydov's approach for molecular excitons. In
the continuum limit the problem is reduced to dynamical equations with
fractional derivatives resulting from the fractional power of the long-range
interaction. Fractional generalizations of the sine-Gordon, nonlinear
Schrodinger, and Hilbert-Schrodinger equations have been found. There exists a
critical value of the power s of the long-range potential. Below the critical
value (s<3) we obtain equations with fractional derivatives while for s>3 we
have the well-known nonlinear dynamical equations with space derivatives of
integer order. Long-range interaction impact on the quantum lattice propagator
has been studied. We have shown that the quantum exciton propagator exhibits
transition from the well-known Gaussian-like behavior to a power-law decay due
to the long-range interaction. A link between 1D quantum lattice dynamics in
the imaginary time domain and a random walk model has been discussed.; Comment: 28 pages...

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## The ideal energy of classical lattice dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Nonlinear Sciences - Cellular Automata and Lattice Gases#Computer Science - Information Theory#General Relativity and Quantum Cosmology#Quantum Physics

We define, as local quantities, the least energy and momentum allowed by
quantum mechanics and special relativity for physical realizations of some
classical lattice dynamics. These definitions depend on local rates of
finite-state change. In two example dynamics, we see that these rates evolve
like classical mechanical energy and momentum.; Comment: 12 pages, 4 figures, includes revised portion of arXiv:0805.3357

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## Clocking the anisotropic lattice dynamics of multi-walled carbon nanotubes by four-dimensional ultrafast transmission electron microscopy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/10/2014
Português

Relevância na Pesquisa

47.144004%

Recent advances in the four-dimensional ultrafast transmission electron
microscope (4D-UTEM) with combined spatial and temporal resolutions have made
it possible to directly visualize structural dynamics of materials at the
atomic level. Herein, we report on our development on a 4D-UTEM which can be
operated properly on either the photo-emission or the thermionic mode. We
demonstrate its ability to obtain sequences of snapshots with high spatial and
temporal resolutions in the study of lattice dynamics of the multi-walled
carbon nanotubes (MWCNTs). This investigation provides an atomic level
description of remarkable anisotropic lattice dynamics at the picosecond
timescales. Moreover, our UTEM measurements clearly reveal that distinguishable
lattice relaxations appear in intra-tubular sheets on an ultrafast timescale of
a few picoseconds and after then an evident lattice expansion along the radical
direction. These anisotropic behaviors in the MWCNTs are considered arising
from the variety of chemical bonding, i.e. the weak van der Waals bonding
between the tubular planes and the strong covalent sp2-hybridized bonds in the
tubular sheets.; Comment: 23pages,5figures,includes 4pages supplementary information

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## Lattice dynamics of coesite

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

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The lattice dynamics of coesite has been studied by a combination of diffuse
x-ray scattering, inelastic x-ray scattering and an ab initio lattice dynamics
calculation. The combined technique gives access to the full lattice dynamics
in harmonic description and thus eventually provides detailed information on
the elastic properties, the stability and metastability of crystalline systems.
The experimentally validated calculation was used for the investigation of
eigenvectors, mode character and their influence on the density of vibrational
states. High symmetry sections of the reciprocal space distribution of diffuse
scattering and inelastic x-ray scattering spectra as well as the density of
vibrational states and the dispersion relation are reported and compared to the
calculation. A critical point at the zone boundary is found to contribute
strongly to the main peak of the low energy part in the density of vibrational
states. Comparison with the most abundant SiO2 polymorph - alpha-quartz -
reveals similarities and distinct differences in the low-energy vibrational
properties.

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## Local stationarity for lattice dynamics in the harmonic approximation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/05/2005
Português

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We consider the lattice dynamics in the harmonic approximation for We
consider the lattice dynamics in the harmonic approximation for a simple
hypercubic lattice with arbitrary unit cell. The initial data are random
according to a probability measure which enforces slow spatial variation on the
linear scale $\epsilon^{-1}$. We establish two time regimes. For times of order
$\epsilon^{-\gamma}$, $0<\gamma<1$, locally the measure converges to a Gaussian
measure which is space-time stationary with a covariance inherited from the
initial (in general, non-Gaussian) measure. For times of order $\epsilon^{-1}$
this local space covariance changes in time and is governed by a semiclassical
transport equation.

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## Atomistic Coupling between Magnetization and Lattice Dynamics from First Principles

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We formulate a new scheme to study the combined magnetization and lattice
dynamics in magnets, so-called magnetoelastics. The coupling between
magnetization and lattice are considered through an expansion of
electron-phonon coupling, while the magnetization is coupled to electrons
through sd-like interaction. We show the that the time-scale of the
magnetization dynamics due to coupling between magnetic degrees of freedom and
electronic degrees of freedom can be transferred to lattice degrees of freedom
and therefore can give rise to lattice dynamics on the same time-scale. This
opens a new route to understand and treat ultrafast lattice dynamics induced by
magnetization dynamics. We also show that all the parameters necessary to
simulate this coupled lattice and magnetization dynamics can be obtained from
first-principles.; Comment: The paper's been withdrawn due inconsistencies in the model
definition. This is has been corrected in a different submission

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## Data for "Lattice dynamics and electron-phonon coupling calculations using non-diagonal supercells"

Fonte: Universidade de Cambridge
Publicador: Universidade de Cambridge

Tipo: Relatório
Formato: CASTEP input files are compatible with CASTEP version 8.0 and later. For information on how to obtain a copy of CASTEP visit: http://www.castep.org -- The LTE research code was originally written by Neil Drummond and has been modified by Jonathan Lloyd-Wi

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All relevant data present in "Lattice dynamics and electron-phonon coupling calculations using non-diagonal supercells"; This record will be updated with publication details.; This work was supported by the EPSRC [grant numbers EP/J017639/1 and EP/K013564/1].

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## Numerical schemes for coarse-graining of stochastic lattice dynamics

Fonte: University of Delaware
Publicador: University of Delaware

Tipo: Tese de Doutorado

Português

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Plechac, Petr; This dissertation is focused on numerical schemes of coarse-graining (CG) for stochastic many-body microscopic models with short- and long-range interactions on 1-dimensional lattice systems. In this thesis, we focus on the numerical schemes of the coarse-graining (CG) Monte Carlo (CGMC) algorithms for 1) equilibrium states of 1-dimensional Ising-type models and 2) evolution of the dynamics on the path-wise level. Microscopic computational models for stochastic many particle systems such as Monte Carlo (MC) algorithms are typically formulated in terms of simple rules describing interactions between individual particles or spin variables. Due to the large size of particles and interactions between them, it represents a costly computational task for the direct numerical simulations. In contrast, the CGMC algorithms decrease the CPU times and substantial accelerate the resolution of the dynamics. In this thesis, I investigate the role of multi-body interactions in the construction of CG dynamics and demonstrate the efficiency and reliability of the two-body and three-body coarse-grained schemes. Furthermore, I explore the micro-macro parareal algorithm to study the evolution of dynamics of the stochastic lattice models described by continuous time Markov chains. Finally...

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