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## Observation of hexatic liquid vortex matter in YBa2Cu3O7

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/09/2002

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An atomic beam probe is used to study the structure and dynamics of quantized
supercurrent vortex lines in YBa_{2}Cu_{3}O_{7-?} at temperatures up to within
0.7 K below T_{C}. Here we report the direct observation of a vortex
configuration with sample wide bond-orientational order but only short range
translational correlation. The data imply the existence of an intermediate
`hexatic' vortex line liquid phase. We find that the hexatic liquid is in
thermal equilibrium over a narrow temperature range below T_{C} and is quenched
into an immobile hexatic glass at low temperatures.; Comment: Submitted to PRL, 3 figures

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## 2D versus 3D Freezing of a Lennard-Jones Fluid in a Slit Pore: A Molecular Dynamics Study

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 19/02/2009

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We present a computer simulation study of a (6,12)-Lennard-Jones fluid
confined to a slit pore, formed by two uniform planes. These interact via
(3,9)-Lennard-Jones potential with the fluid particles. When the fluid
approaches the liquid-to-solid transition we first observe layering parallel to
the walls. In order to investigate the nature of the freezing transition we
performed a detailed analysis of the bond-orientational order parameter in the
layers. We found no signs of hexatic order which would indicate a melting
scenario of the Kosterlitz-Thouless type. An analysis of the mean-square
displacement shows that the particles can easily move between the layers,
making the crystallization a 3d-like process. This is consistent with the fact
that we observe a considerable hysteresis in the heating-freezing curves,
showing that the crystallization transition proceeds as an activated process.; Comment: submitter to J. Phys.: Condens. Matter

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## Potential-tuning molecular dynamics studies of fusion, and the question of ideal glassformers: (I) The Gay-Berne model

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Chemical Physics

The ability of some liquids to vitrify during supercooling is usually seen as
a consequence of the rates of crystal nucleation (and/or crystal growth)
becoming small- thus a matter of kinetics. However there is evidence, dating
back to the empirics of coal briquetting for maximum trucking efficiency, that
ellipsoids pack efficiently when disordered. Noting that key studies of
non-spherical object packing have never been followed from hard ellipsoids or
spherocylinders (diatomics excepted) into the world of molecules with
attractive forces, we have made a molecular dynamics MD study of crystal
melting and glass formation on the Gay- Berne (G-B) model of ellipsoidal
objects across the aspect ratio range of the hard ellipsoid studies. Here we
report that, in the aspect ratio range of maximum ellipsoid packing efficiency,
various G-B crystalline states, that cannot be obtained directly from the
liquid, disorder spontaneously near 0 K and transform to liquids without any
detectable enthalpy of fusion. Without claiming to have proved the existence of
single component examples, we use the present observations, together with our
knowledge of non-ideal mixing effects, to discuss the probable existence of
"ideal glassformers" - single or multicomponent liquids that vitrify before
ever becoming metastable with respect to crystals. The existence of
crystal-free routes to the glassy state removes any precrystalline fluctuation
perspective from the "glass problem". Unexpectedly we find that liquids with
aspect ratios in the "crystallophobic" range also behave in an unusual
(non-hysteritic) way during temperature cycling through the glass transition.
We link this to the highly volume fraction-sensitive ("fragile") behavior
observed in recent hard dumbbell studies at similar length/diameter ratios.

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## MR Imaging of Reynolds Dilatancy in the Bulk of Smooth Granular Flows

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/04/2007

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Dense granular matter has to expand in order to flow, a phenomenon known as
dilatancy. Here we perform, by means of Magnetic Resonance Imaging, direct
measurements of the evolution of the local packing density of a slow and smooth
granular shear flow generated in a split-bottomed geometry. The dilatancy is
found to be surprisingly strong. The dilated zone follow the region of large
strain rate and slowly spreads as a function of time. This suggests that the
local packing density is governed by the total amount of local strain
experienced since the start of the experiment.; Comment: 4 pages, 3 figs, submitted to PRE

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## Lateral correlation of multivalent counterions is the universal mechanism of charge inversion

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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We review works on screening of a macroion, such as colloidal particle or
double helix DNA, by multivalent counterions. Multivalent metal ions, charged
micelles, short or long polyelectrolytes can play the role of multivalent
counterions. Due to the strong Coulomb repulsion such multivalent counterions
form a strongly correlated liquid resembling a Wigner crystal at the surface of
the macroion. Even if the macroion is neutralized by this liquid, a newly
arriving counterion creates in the liquid a correlation hole or image which
attracts the ion to the surface. As a result, total charge of adsorbed
counterions can substantially exceed the bare macroion charge, so that the
macroion net charge inverts its sign. We discuss two previously suggested
explanations of charge inversion and show that physics underlying both of them
is intrinsically that of correlations, so that correlation is the universal
force driving charge inversion.; Comment: In Proceedings of Les Houches school on Electrostatic effects in Soft
Matter and Biophysics, October 2000. Many corrections

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## Spontaneous motion in hierarchically assembled active matter

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/01/2013

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Nonlinear Sciences - Chaotic Dynamics#Quantitative Biology - Biomolecules#Quantitative Biology - Cell Behavior

With exquisite precision and reproducibility, cells orchestrate the
cooperative action of thousands of nanometer-sized molecular motors to carry
out mechanical tasks at much larger length scales, such as cell motility,
division and replication. Besides their biological importance, such inherently
non-equilibrium processes are an inspiration for developing biomimetic active
materials from microscopic components that consume energy to generate
continuous motion. Being actively driven, these materials are not constrained
by the laws of equilibrium statistical mechanics and can thus exhibit highly
sought-after properties such as autonomous motility, internally generated flows
and self-organized beating. Starting from extensile microtubule bundles, we
hierarchically assemble active analogs of conventional polymer gels, liquid
crystals and emulsions. At high enough concentration, microtubules form a
percolating active network characterized by internally driven chaotic flows,
hydrodynamic instabilities, enhanced transport and fluid mixing. When confined
to emulsion droplets, 3D networks spontaneously adsorb onto the droplet
surfaces to produce highly active 2D nematic liquid crystals whose streaming
flows are controlled by internally generated fractures and self-healing...

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## Twisting and Bending Stress in DNA Minicircles

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/03/2014

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Quantitative Biology - Biomolecules#Quantitative Biology - Quantitative Methods

The interplay between bending of the molecule axis and appearance of
disruptions in circular DNA molecules, with $\sim 100$ base pairs, is
addressed. Three minicircles with different radii and almost equal content of
AT and GC pairs are investigated. The DNA sequences are modeled by a mesoscopic
Hamiltonian which describes the essential interactions in the helix at the
level of the base pair and incorporates twisting and bending degrees of
freedom. Helix unwinding and bubble formation patterns are consistently
computed by a path integral method that sums over a large number of molecule
configurations compatible with the model potential. The path ensembles are
determined, as a function of temperature, by minimizing the free energy of the
system. Fluctuational openings appear along the helix to release the stress due
to the bending of the molecule backbone. In agreement with the experimental
findings, base pair disruptions are found with larger probability in the
smallest minicircle of \textit{66-bps} whose bending angle is $\sim 6^{o} $.
For this minicircle, a sizeable untwisting is obtained with the helical repeat
showing a step-like increase at $\tau =\,315K$. The method can be generalized
to determine the bubble probability profiles of open ends linear sequences.; Comment: Soft Matter (2014)...

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## Comparison of mean-field theories for vortices in trapped Bose-Einstein condensates

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/08/2001

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We compute structures of vortex configurations in a harmonically trapped
Bose-Einstein condensed atom gas within three different gapless self-consistent
mean-field theories. Outside the vortex core region, the density profiles for
the condensate and the thermal gas are found to differ only by a few percent
between the Hartree-Fock-Bogoliubov-Popov theory and two of its recently
proposed gapless extensions. In the core region, however, the differences in
the density profiles are substantial. The structural differences are reflected
in the energies of the quasiparticle states localized near the vortex core.
Especially, the predictions for the energy of the lowest quasiparticle
excitation differ considerably between the theoretical models investigated.; Comment: 4 pages, 2 figures

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## Shear shocks in fragile networks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/07/2013

Relevância na Pesquisa

55.53%

A minimal model for studying the mechanical properties of amorphous solids is
a disordered network of point masses connected by unbreakable springs. At a
critical value of its mean connectivity, such a network becomes fragile: it
undergoes a rigidity transition signaled by a vanishing shear modulus and
transverse sound speed. We investigate analytically and numerically the linear
and non-linear visco-elastic response of these fragile solids by probing how
shear fronts propagate through them. Our approach, that we tentatively label
shear front rheology, provides an alternative route to standard oscillatory
rheology. In the linear regime, we observe at late times a diffusive broadening
of the fronts controlled by an effective shear viscosity that diverges at the
critical point. No matter how small the microscopic coefficient of dissipation,
strongly disordered networks behave as if they were over-damped because energy
is irreversibly leaked into diverging non-affine fluctuations. Close to the
transition, the regime of linear response becomes vanishingly small: the
tiniest shear strains generate strongly non-linear shear shock waves
qualitatively different from their compressional counterparts in granular
media. The inherent non-linearities trigger an energy cascade from low to high
frequency components that keep the network away from attaining the quasi-static
limit. This mechanism...

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## The Jamming point street-lamp in the world of granular media

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/05/2013

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The Jamming of soft spheres at zero temperature, the J-point, has been
extensively studied both numerically and theoretically and can now be
considered as a safe location in the space of models, where a street lamp has
been lit up. However, a recent work by Ikeda et al, 2013 reveals that, in the
Temperature/Packing fraction parameter space, experiments on colloids are
actually rather far away from the scaling regime illuminated by this lamp. Is
it that the J-point has little to say about real system? What about granular
media? Such a-thermal, frictional, systems are a-priori even further away from
the idealized case of thermal soft spheres. In the past ten years, we have
systematically investigated horizontally shaken grains in the vicinity of the
Jamming transition. We discuss the above issue in the light of very recent
experimental results. First, we demonstrate that the contact network exhibits a
remarkable dynamics, with strong heterogeneities, which are maximum at a
packing fraction phi star, distinct and smaller than the packing fraction phi
dagger, where the average number of contact per particle starts to increase.
The two cross-overs converge at point J in the zero mechanical excitation
limit. Second, a careful analysis of the dynamics on time scales ranging from a
minute fraction of the vibration cycle to several thousands of cycles allows us
to map the behaviors of this shaken granular system onto those observed for
thermal soft spheres and demonstrate that some light of the J-point street-lamp
indeed reaches the granular universe.; Comment: 20 pages...

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## Monte Carlo and Molecular Dynamics Simulation of the Glass Transition of Polymers

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/09/1998

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Two coarse-grained models for polymer chains in dense glass-forming polymer
melts are studied by computer simulation: the bond-fluctuation model on a
simple cubic lattice, where a bond-length potential favors long bonds, is
treated by dynamic Monte Carlo methods, and a bead-spring model in the
continuum with a Lennard-Jones potential between the beads is treated by
Molecular Dynamics. While the dynamics of both models differ for short length
scales and associated time scales, on mesoscopic spatial and temporal scales
both models behave similarly. In particular, the mode coupling theory of the
glass transition can be used to interpret the slowing down of the undercooled
polymer melt. For the off-lattice model, the approach to the critical point of
mode coupling is both studied for constant pressure and for constant volume.
The lattice model allows a test of the Gibbs-Di Marzio entropy theory of the
glass transition, and our finding is that although the entropy does decrease
significantly, there is no ``entropy catastrophe'', where the fluid entropy
would turn negative. Finally, an outlook on confinement effects on the glass
transition in thin film geometry is given.; Comment: 11 pages of REVTeX, 6 PostScript figures, submitted to J. Phys.:
Condens. Matter

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## Depinning and dynamics of vortices confined in mesoscopic flow channels

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/11/2004

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We study the behavior of vortex matter in artificial flow channels confined
by pinned vortices in the channel edges (CE's). The critical current $J_s$ is
governed by the interaction with static vortices in the CE's. We study
structural changes associated with (in)commensurability between the channel
width $w$ and the natural row spacing $b_0$, and their effect on $J_s$. The
behavior depends crucially on the presence of disorder in the CE arrays. For
ordered CE's, maxima in $J_s$ occur at matching $w=nb_0$ ($n$ integer), while
for $w\neq nb_0$ defects along the CE's cause a vanishing $J_s$. For weak CE
disorder, the sharp peaks in $J_s$ at $w=nb_0$ become smeared via nucleation
and pinning of defects. The corresponding quasi-1D $n$ row configurations can
be described by a (disordered)sine-Gordon model. For larger disorder and
$w\simeq nb_0$, $J_s$ levels at $\sim 30 %$ of the ideal lattice strength
$J_s^0$. Around 'half filling' ($w/b_0 \simeq n\pm 1/2$), disorder causes new
features, namely {\it misaligned} defects and coexistence of $n$ and $n \pm 1$
rows in the channel. This causes a {\it maximum} in $J_s$ around mismatch,
while $J_s$ smoothly decreases towards matching due to annealing of the
misaligned regions. We study the evolution of static and dynamic structures on
changing $w/b_0$...

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## From Vulcanization to Isotropic and Nematic Rubber Elasticity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/11/2004

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A Landau theory is constructed for the vulcanization transition in
cross-linked polymer systems with spontaneous nematic ordering. The
neo-classical theory of the elasticity of nematic elastomers is derived via the
minimization of this Landau free energy; this neo-classical theory contains the
classical theory of rubber elasticity as its isotropic limit. Our work not only
reveals the statistical-mechanical roots of these elasticity theories, but also
demonstrates that they are applicable to a wide class of random solids. It also
constitutes a starting-point for the investigation of sample-to-sample
fluctuations in various forms of vulcanized matter.; Comment: 7 pages, 1 eps figure, submited to Europhys. Lett

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## Integration through transients for Brownian particles under steady shear

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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Starting from the microscopic Smoluchowski equation for interacting Brownian
particles under stationary shearing, exact expressions for shear-dependent
steady-state averages, correlation and structure functions, and
susceptibilities are obtained, which take the form of generalized Green-Kubo
relations. They require integration of transient dynamics. Equations of motion
with memory effects for transient density fluctuation functions are derived
from the same microscopic starting point. We argue that the derived formal
expressions provide useful starting points for approximations in order to
describe the stationary non-equilibrium state of steadily sheared dense
colloidal dispersions.; Comment: 17 pages, Submitted to J. Phys.: Condens. Matter; revised version
with minor corrections

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## Dynamics of Self-Threading Ring Polymers in a Gel

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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We study of the dynamics of ring polymers confined to diffuse in a background
gel at low concentrations. We do this in order to probe the inter-play between
topology and dynamics in ring polymers. We develop an algorithm that takes into
account the possibility that the rings hinder their own motion by passing
through themselves, i.e. "self-threading". Our results suggest that the number
of self-threadings scales extensively with the length of the rings and that
this is substantially independent of the details of the model. The slowing down
of the rings' dynamics is found to be related to the fraction of segments that
can contribute to the motion. Our results give a novel perspective on the
motion of ring polymers in gel, for which a complete theory is still lacking,
and may help us to understand the irreversible trapping of ring polymers in gel
electrophoresis experiments.; Comment: 11 pages,14 figures, Soft Matter 2014

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## Gel-forming patchy colloids and network glass formers: Thermodynamic and Dynamic analogies

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/11/2007

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55.53%

This article discusses recent attempts to provide a deeper understanding of
the thermoreversible "gel" state of colloidal matter and to unravel the
analogies between gels at the colloidal level and gels at the molecular level,
commonly known as network-forming strong liquids. The connection between
gel-forming patchy colloids and strong liquids is provided by the limited
valence of the inter-particle interactions, i.e. by the presence of a limit in
the number of bonded nearest neighbors.; Comment: Proceeding of Stat-Phys 2007. EPJB in press

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## Momentum-Transfer to and Elementary-Excitations of a Bose-Einstein Condensate by a Time-Dependent Optical Potential

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/07/2002

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We present results of calculations on Bose-Einstein condensed $^{87}$Rb atoms
subjected to a moving standing-wave light-potential of the form $V_L(z,t) =
V_0(t) \cos(q z-\omega t)$. We calculate the mean-field dynamics (the order
paramter) of the condensate and determine the resulting condensate momentum in
the $z$ direction, $P_z(q,\omega,V_0,t_p)$, where $V_0$ is the peak optical
potential strength and $t_p$ is the pulse duration. Although the local density
approximation for the Bogoliubov excitation spectral distribution is a good
approximation for very low optical intensities, long pulse duration and
sufficiently large values of the wavevector $q$ of the light-potential, for
small $q$, short duration pulses, or for not-so-low intensities, the local
density perturbative description of the excitation spectrum breaks down badly,
as shown by our results.; Comment: 8 pages, 7 figures

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## Molecular Structure and Electrophysical Properties of Pentacene Thioderivatives

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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Possible conformations of the thioderivatives of pentacene (Pn) have been
considered. The absorption spectra of polythiopentacene (PTPn) solutions and
films have been studied. PTPn is revealed to be a mixture of Pn thioderivatives
with different numbers of S atoms. After this mixture having been condensed in
vacuum onto quartz substrates, its main components are tetrathiopentacene
(TTPn) and hexathiopentacene (HTPn). The position of the maximum in the
long-wave absorption bands of Pn thioderivatives is a linear function of the
number of valence electrons in S atoms, which take part in the conjugation with
the {\pi}-system of the pentacene frame of PTPn molecules. The analysis of the
photocurrent and capacitor photovoltage (CPV) spectra in the range of the first
electron transitions in PTPn has shown that the photoconductivity is of the
hole type and is caused by the dissociation of excitons at the electron capture
centers. The frontal CPV is caused by the Dember photovoltage ({\phi}D), and
the back one by the surface-barrier photovoltage ({\phi}b).

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## Theory of ice premelting in porous media

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

Premelting describes the confluence of phenomena that are responsible for the
stable existence of the liquid phase of matter in the solid region of its bulk
phase diagram. Here we develop a theoretical description of the premelting of
water ice contained in a porous matrix, made of a material with a melting
temperature substantially larger than ice itself, to predict the amount of
liquid water in the matrix at temperatures below its bulk freezing point. Our
theory combines the interfacial premelting of ice in contact with the matrix,
grain boundary melting in the ice, and impurity and curvature induced
premelting, the latter occurring in regions which force the ice-liquid
interface into a high curvature configuration. These regions are typically
found at points where the matrix surface is concave, along contact lines of a
grain boundary with the matrix, and in liquid veins. Both interfacial
premelting and curvature induced premelting depend on the concentration of
impurities in the liquid, which, due to the small segregation coefficient of
impurities in ice are treated as homogeneously distributed in the premelted
liquid. Our principal result is an equation for the fraction of liquid in the
porous medium as a function of the undercooling...

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## Structural diversity and the role of particle shape and dense fluid behavior in assemblies of hard polyhedra

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/02/2012

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55.53%

A fundamental characteristic of matter is its ability to form ordered
structures under the right thermodynamic conditions. Predicting these
structures - and their properties - from the attributes of a material's
building blocks is the holy grail of materials science. Here, we investigate
the self-assembly of 145 hard convex polyhedra whose thermodynamic behavior
arises solely from their anisotropic shape. Our results extend previous works
on entropy-driven crystallization by demonstrating a remarkably high propensity
for self-assembly and an unprecedented structural diversity, including some of
the most complex crystalline phases yet observed in a non-atomic system. In
addition to 22 Bravais and non-Bravais crystals, we report 66 plastic crystals
(both Bravais and topologically close-packed), 21 liquid crystals (nematic,
smectic, and columnar), and 44 glasses. We show that from simple measures of
particle shape and local order in the disordered fluid, the class of ordered
structure can be predicted.; Comment: 21 pages, 4 figures

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