730 research outputs found
Platonic crystal with low-frequency locally resonant snail structures. Wave trapping, transmission amplification and shielding
We propose a new type of platonic crystal. The proposed microstructured plate
includes snail resonators with low-frequency resonant vibrations. The
particular dynamic effect of the resonators are highlighted by a comparative
analysis of dispersion properties of homo- geneous and perforated plates.
Analytical and numerical estimates of classes of standing waves are given and
the analysis on a macrocell shows the possibility to obtain localization, wave
trapping and edge waves. Applications include transmission amplification within
two plates separated by a small ligament. Finally we proposed a design
procedure to suppress low frequency flexural vibration in an elongated plate
implementing a by-pass system re- routing waves within the mechanical system.Comment: 11 figures (20 files
Conduction in jammed systems of tetrahedra
Control of transport processes in composite microstructures is critical to
the development of high performance functional materials for a variety of
energy storage applications. The fundamental process of conduction and its
control through the manipulation of granular composite attributes (e.g., grain
shape) are the subject of this work. We show that athermally jammed packings of
tetrahedra with ultra-short range order exhibit fundamentally different
pathways for conduction than those in dense sphere packings. Highly resistive
granular constrictions and few face-face contacts between grains result in
short-range distortions from the mean temperature field. As a consequence,
'granular' or differential effective medium theory predicts the conductivity of
this media within 10% at the jamming point; in contrast, strong enhancement of
transport near interparticle contacts in packed-sphere composites results in
conductivity divergence at the jamming onset. The results are expected to be
particularly relevant to the development of nanomaterials, where nanoparticle
building blocks can exhibit a variety of faceted shapes.Comment: 9 pages, 10 figure
Trapped Modes and Steered Dirac Cones in Platonic Crystals
This paper discusses the properties of flexural waves obeying the biharmonic
equation, propagating in a thin plate pinned at doubly-periodic sets of points.
The emphases are on the properties of dispersion surfaces having the Dirac cone
topology, and on the related topic of trapped modes in plates with a finite set
(cluster) of pinned points. The Dirac cone topologies we exhibit have at least
two cones touching at a point in the reciprocal lattice, augmented by another
band passing through the point. We show that the Dirac cones can be steered
along symmetry lines in the Brillouin zone by varying the aspect ratio of
rectangular lattices of pins, and that, as the cones are moved, the involved
band surfaces tilt. We link Dirac points with a parabolic profile in their
neighbourhood, and the characteristic of this parabolic profile decides the
direction of propagation of the trapped mode in finite clusters.Comment: 21 pages, 12 figure
Berezinskii-Kosterlitz-Thouless Type Scenario in Molecular Spin Liquid CrO
The spin relaxation in chromium spinel oxides CrO ( Mg,
Zn, Cd) is investigated in the paramagnetic regime by electron spin resonance
(ESR). The temperature dependence of the ESR linewidth indicates an
unconventional spin-relaxation behavior, similar to spin-spin relaxation in the
two-dimensional (2D) chromium-oxide triangular lattice antiferromagnets. The
data can be described in terms of a generalized Berezinskii-Kosterlitz-Thouless
(BKT) type scenario for 2D systems with additional internal symmetries. Based
on the characteristic exponents obtained from the evaluation of the ESR
linewidth, short-range order with a hidden internal symmetry is suggested.Comment: 7 pages, 4 figure
Basic Understanding of Condensed Phases of Matter via Packing Models
Packing problems have been a source of fascination for millenia and their
study has produced a rich literature that spans numerous disciplines.
Investigations of hard-particle packing models have provided basic insights
into the structure and bulk properties of condensed phases of matter, including
low-temperature states (e.g., molecular and colloidal liquids, crystals and
glasses), multiphase heterogeneous media, granular media, and biological
systems. The densest packings are of great interest in pure mathematics,
including discrete geometry and number theory. This perspective reviews
pertinent theoretical and computational literature concerning the equilibrium,
metastable and nonequilibrium packings of hard-particle packings in various
Euclidean space dimensions. In the case of jammed packings, emphasis will be
placed on the "geometric-structure" approach, which provides a powerful and
unified means to quantitatively characterize individual packings via jamming
categories and "order" maps. It incorporates extremal jammed states, including
the densest packings, maximally random jammed states, and lowest-density jammed
structures. Packings of identical spheres, spheres with a size distribution,
and nonspherical particles are also surveyed. We close this review by
identifying challenges and open questions for future research.Comment: 33 pages, 20 figures, Invited "Perspective" submitted to the Journal
of Chemical Physics. arXiv admin note: text overlap with arXiv:1008.298
Exact Constructions of a Family of Dense Periodic Packings of Tetrahedra
The determination of the densest packings of regular tetrahedra (one of the
five Platonic solids) is attracting great attention as evidenced by the rapid
pace at which packing records are being broken and the fascinating packing
structures that have emerged. Here we provide the most general analytical
formulation to date to construct dense periodic packings of tetrahedra with
four particles per fundamental cell. This analysis results in six-parameter
family of dense tetrahedron packings that includes as special cases recently
discovered "dimer" packings of tetrahedra, including the densest known packings
with density . This study strongly suggests that
the latter set of packings are the densest among all packings with a
four-particle basis. Whether they are the densest packings of tetrahedra among
all packings is an open question, but we offer remarks about this issue.
Moreover, we describe a procedure that provides estimates of upper bounds on
the maximal density of tetrahedron packings, which could aid in assessing the
packing efficiency of candidate dense packings.Comment: It contains 25 pages, 5 figures
The Geometry of Slow Structural Fluctuations in a Supercooled Binary Alloy
The liquid structure of a glass-forming binary alloy is studied using
molecular dynamics simulations. The analysis combines common neighbour analysis
with the geometrical approach of Frank and Kasper to establish that the
supercooled liquid contains extended clusters characterised by the same short
range order as the crystal. Fluctuations in these clusters exhibit strong
correlations with fluctuations in the inherent structure energy. The steep
increase in the heat capacity on cooling is, thus, directly coupled to the
growing fluctuations of the Frank-Kasper clusters. The relaxation of particles
in the clusters dominates the slow tail of the self-intermediate scattering
function
A Collection of Challenging Optimization Problems in Science, Engineering and Economics
Function optimization and finding simultaneous solutions of a system of
nonlinear equations (SNE) are two closely related and important optimization
problems. However, unlike in the case of function optimization in which one is
required to find the global minimum and sometimes local minima, a database of
challenging SNEs where one is required to find stationary points (extrama and
saddle points) is not readily available. In this article, we initiate building
such a database of important SNE (which also includes related function
optimization problems), arising from Science, Engineering and Economics. After
providing a short review of the most commonly used mathematical and
computational approaches to find solutions of such systems, we provide a
preliminary list of challenging problems by writing the Mathematical
formulation down, briefly explaning the origin and importance of the problem
and giving a short account on the currently known results, for each of the
problems. We anticipate that this database will not only help benchmarking
novel numerical methods for solving SNEs and function optimization problems but
also will help advancing the corresponding research areas.Comment: Accepted as an invited contribution to the special session on
Evolutionary Computation for Nonlinear Equation Systems at the 2015 IEEE
Congress on Evolutionary Computation (at Sendai International Center, Sendai,
Japan, from 25th to 28th May, 2015.
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