369 research outputs found
Novel Features Arising in the Maximally Random Jammed Packings of Superballs
Dense random packings of hard particles are useful models of granular media
and are closely related to the structure of nonequilibrium low-temperature
amorphous phases of matter. Most work has been done for random jammed packings
of spheres, and it is only recently that corresponding packings of nonspherical
particles (e.g., ellipsoids) have received attention. Here we report a study of
the maximally random jammed (MRJ) packings of binary superdisks and
monodispersed superballs whose shapes are defined by |x_1|^2p+...+|x_2|^2p<=1
with d = 2 and 3, respectively, where p is the deformation parameter with
values in the interval (0, infinity). We find that the MRJ densities of such
packings increase dramatically and nonanalytically as one moves away from the
circular-disk and sphere point. Moreover, the disordered packings are
hypostatic and the local arrangements of particles are necessarily nontrivially
correlated to achieve jamming. We term such correlated structures "nongeneric".
The degree of "nongenericity" of the packings is quantitatively characterized
by determining the fraction of local coordination structures in which the
central particles have fewer contacting neighbors than average. We also show
that such seemingly special packing configurations are counterintuitively not
rare. As the anisotropy of the particles increases, the fraction of rattlers
decreases while the minimal orientational order increases. These novel
characteristics result from the unique rotational symmetry breaking manner of
the particles.Comment: 20 pages, 8 figure
Solid molecular hydrogen: The Broken Symmetry Phase
By performing constant-pressure variable-cell ab initio molecular dynamics
simulations we find a quadrupolar orthorhombic structure, of symmetry,
for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150
GPa. We present results for the equation of state, lattice parameters and
vibronic frequencies, in very good agreement with experimental observations.
Anharmonic quantum corrections to the vibrational frequencies are estimated
using available data on H2 and D2. We assign the observed modes to specific
symmetry representations.Comment: 5 pages (twocolumn), 4 Postscript figures. To appear in Phys. Rev.
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Crystalline inclusion of wheel-and-axle diol hosts featuring benzo[b]thiophene units as a lateral construction element
By applying the âwheel-and-axleâ host concept and incorporating a previously developed heteroaromatic substitution strategy, a new type of diol host featuring two di(benzo[b]thien-2-yl)hydroxymethyl units attached to both ends of a central ethynylene (3) and 1,4-phenylene (4) moiety is reported. The syntheses of the host compounds are described, and solvent inclusion formation via crystallization has extensively been studied showing a remarkable inclusion capability of the compounds. X-ray diffraction analysis of relevant crystal structures have been performed and comparatively discussed. Vapor sorption behavior of the compounds as solid receptor films coated on a quartz crystal microbalance considering a variety of solvent vapors has been scrutinized, indicating potential application as mass sensitive materials
Optimal Packings of Superballs
Dense hard-particle packings are intimately related to the structure of
low-temperature phases of matter and are useful models of heterogeneous
materials and granular media. Most studies of the densest packings in three
dimensions have considered spherical shapes, and it is only more recently that
nonspherical shapes (e.g., ellipsoids) have been investigated. Superballs
(whose shapes are defined by |x1|^2p + |x2|^2p + |x3|^2p <= 1) provide a
versatile family of convex particles (p >= 0.5) with both cubic- and
octahedral-like shapes as well as concave particles (0 < p < 0.5) with
octahedral-like shapes. In this paper, we provide analytical constructions for
the densest known superball packings for all convex and concave cases. The
candidate maximally dense packings are certain families of Bravais lattice
packings. The maximal packing density as a function of p is nonanalytic at the
sphere-point (p = 1) and increases dramatically as p moves away from unity. The
packing characteristics determined by the broken rotational symmetry of
superballs are similar to but richer than their two-dimensional "superdisk"
counterparts, and are distinctly different from that of ellipsoid packings. Our
candidate optimal superball packings provide a starting point to quantify the
equilibrium phase behavior of superball systems, which should deepen our
understanding of the statistical thermodynamics of nonspherical-particle
systems.Comment: 28 pages, 16 figure
Herringbone ordering and lattice distortions in a planar-molecule model for Langmuir monolayers
A model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out
Surface gravity waves in deep fluid at vertical shear flows
Special features of surface gravity waves in deep fluid flow with constant
vertical shear of velocity is studied. It is found that the mean flow velocity
shear leads to non-trivial modification of surface gravity wave modes
dispersive characteristics. Moreover, the shear induces generation of surface
gravity waves by internal vortex mode perturbations. The performed analytical
and numerical study provides, that surface gravity waves are effectively
generated by the internal perturbations at high shear rates. The generation is
different for the waves propagating in the different directions. Generation of
surface gravity waves propagating along the main flow considerably exceeds the
generation of surface gravity waves in the opposite direction for relatively
small shear rates, whereas the later wave is generated more effectively for the
high shear rates. From the mathematical point of view the wave generation is
caused by non self-adjointness of the linear operators that describe the shear
flow.Comment: JETP, accepte
Two-Dimensional 1,3,5-Tris(4-carboxyphenyl)benzene Self-Assembly at the 1-Phenyloctane/Graphite Interface Revisited
International audienceTwo-dimensional (2D) self-assembly of star-shaped 1,3,5-tris(4-carboxyphenyl)benzene molecules is investigated. Scanning tunneling microscopy reveals that this molecule can form three hydrogen-bonded networks at the 1-phenyloctane/graphite interface. One of these structures is close-packed and the two other ones are porous structures, with hexagonal and rectangular cavities. The network with rectangular cavities appears to be the most stable structure
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