57 research outputs found
Series expansion studies of random sequential adsorption with diffusional relaxation
We obtain long series (28 terms or more) for the coverage (occupation
fraction) , in powers of time for two models of random sequential
adsorption with diffusional relaxation using an efficient algorithm developed
by the authors. Three different kinds of analyses of the series are performed
for a wide range of , the rate of diffusion of the adsorbed particles,
to investigate the power law approach of at large times. We find that
the primitive series expansions in time for capture rich short and
intermediate time kinetics of the systems very well. However, we see that the
series are still not long enough to extract the kinetics at large times for
general . We have performed extensive computer simulations employing an
efficient event-driven algorithm to confirm the saturation approach
of at large times for both models, as well as to investigate the short
and intermediate time behaviors of the systems.Comment: 35 pages. revtex. 16 eps figures. Series expansion coefficients are
attached at the end of the soure/LaTex file (named diff.tex). uses fleqn.st
A size-consistent Gr\"uneisen-quasiharmonic approach for lattice thermal conductivity
We propose a size-consistent Gr\"uneisen-quasiharmonic approach (GQA) to
calculate the lattice thermal conductivity where the Gr\"uneisen
parameters that measure the degree of phonon anharmonicity are calculated
directly using first-principles calculations. This is achieved by identifying
and modifying two existing equations related to the Slack formulae for
that suffer from the size-inconsistency problem when dealing with
non-monoatomic primitive cells (where the number of atoms in the primitive cell
is greater than one). In conjunction with other thermal parameters such as
the acoustic Debye temperature that can also be obtained within the
GQA, we predict for a range of materials taken from the diamond,
zincblende, rocksalt, and wurtzite compounds. The results are compared with
that from the experiment and the quasiharmonic Debye model (QDM). We find that
in general the prediction of is rather consistent among the GQA,
experiment, and QDM. However, while the QDM somewhat overestimates the
Gr\"uneisen parameters and hence underestimates for most materials,
the GQA predicts the experimental trends of Gr\"uneisen parameters and
more closely. We expect the GQA with the modified Slack formulae
could be used as an effective and practical predictor for ,
especially for crystals with large .Comment: 8 pages, 6 figure
Linear scaling computation of the Fock matrix. IX. Parallel computation of the Coulomb matrix
We present parallelization of a quantum-chemical tree-code [J. Chem. Phys.
{\bf 106}, 5526 (1997)] for linear scaling computation of the Coulomb matrix.
Equal time partition [J. Chem. Phys. {\bf 118}, 9128 (2003)] is used to load
balance computation of the Coulomb matrix. Equal time partition is a
measurement based algorithm for domain decomposition that exploits small
variation of the density between self-consistent-field cycles to achieve load
balance. Efficiency of the equal time partition is illustrated by several tests
involving both finite and periodic systems. It is found that equal time
partition is able to deliver 91 -- 98 % efficiency with 128 processors in the
most time consuming part of the Coulomb matrix calculation. The current
parallel quantum chemical tree code is able to deliver 63 -- 81% overall
efficiency on 128 processors with fine grained parallelism (less than two heavy
atoms per processor).Comment: 7 pages, 6 figure
In-plane dominant anisotropy stochastic magnetic tunnel junction for probabilistic computing: A Fokker-Planck study
Recently there is considerable interest to realize efficient and low-cost
true random number generators (RNGs) for practical applications. One important
way is through the use of bistable magnetic tunnel junctions (MTJs). Here we
study the magnetization dynamics of an MTJ, with a focus to realize efficient
random bit generation under the assumption that the orientation dependence of
the energy of the nanomagnet is described by two perpendicular in-plane
anisotropies. We find that a high rate of random bit generation is achievable
away from the pure easy-axis situation by tuning a single parameter so
that it is either (a) toward a barrierless-like single easy plane situation
when reduces to zero, or (b) toward a stronger easy plane situation when
becomes increasingly negative where transitions between low energy states
are confined in the stronger easy plane that contains the saddle points. We
find that the MTJs maintain their fast magnetization dynamical characteristics
even in the presence of a magnetic field. Our findings provide a valuable guide
to achieving efficient generation of probabilistic bits for applications in
probabilistic computing.Comment: 10 pages, 5 figure
First-principles study of the lattice dynamics of Sb2S3
We present a lattice dynamics study of orthorhombic antimony sulphide (Sb2S3)
obtained using density-functional calculations in conjunction with the
supercell force-constant method. The effect of Born effective charges is taken
into account using a mixed-space approach, resulting in the splitting of
longitudinal and transverse optical (LO-TO) phonon branches near the zone
center. Zone-center frequencies agree well with Raman scattering experiments.
Due to the slow decay of the interatomic force constants (IFC), a minimal 2x4x2
supercell (Pnma setting) with 320 atoms is crucial for an accurate
determination of the dispersion relations. Smaller supercells result in
artificial acoustic phonon softening and unphysical lifting of degeneracies
along high symmetry directions. We propose a scheme to investigate the
convergence of the IFC with respect to the supercell sizes. The phonon
softening can be attributed to the periodic images that affect the accuracy of
the force constants, and the truncation of long-ranged forces. The
commensuration of the q-vectors with the supercell size is crucial to preserve
degeneracies in Sb2S3 crystals.Comment: 7 pages 4 figures, 3 table
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