14,733 research outputs found
Distribution of volumes and coordination number in jammed matter: mesoscopic ensemble
We investigate the distribution of the volume and coordination number
associated to each particle in a jammed packing of monodisperse hard sphere
using the mesoscopic ensemble developed in Nature 453, 606 (2008). Theory
predicts an exponential distribution of the orientational volumes for random
close packings and random loose packings. A comparison with computer generated
packings reveals deviations from the theoretical prediction in the volume
distribution, which can be better modeled by a compressed exponential function.
On the other hand, the average of the volumes is well reproduced by the theory
leading to good predictions of the limiting densities of RCP and RLP. We
discuss a more exact theory to capture the volume distribution in its entire
range. The available data suggests a plausible order/disorder transition
defining random close packings. Finally, we consider an extended ensemble to
calculate the coordination number distribution which is shown to be of an
exponential and inverse exponential form for coordinations larger and smaller
than the average, respectively, in reasonable agreement with the simulated
data.Comment: 20 pages, 6 figures, accepted by JSTA
New Candidates for Topological Insulators : Pb-based chalcogenide series
Here, we theoretically predict that the series of Pb-based layered
chalcogenides, PbBiSe and PbSbTe, are possible
new candidates for topological insulators. As increases, the phase
transition from a topological insulator to a band insulator is found to occur
between and 3 for both series. Significantly, among the new topological
insulators, we found a bulk band gap of 0.40eV in PbBiSe which is one
of the largest gap topological insulators, and that PbSbTe is
located in the immediate vicinity of the topological phase boundary, making its
topological phase easily tunable by changing external parameters such as
lattice constants. Due to the three-dimensional Dirac cone at the phase
boundary, massless Dirac fermions also may be easily accessible in
PbSbTe
Formations and dynamics of two-dimensional spinning asymmetric quantum droplets controlled by a PT-symmetric potential
In this paper, vortex solitons are produced for a variety of 2D spinning
quantum droplets (QDs) in a PT-symmetric potential, modeled by the amended
Gross-Pitaevskii equation with Lee-Huang-Yang corrections. In particular, exact
QD states are obtained under certain parameter constraints, providing a guide
to finding the respective generic family. In a parameter region of the unbroken
PT symmetry, different families of QDs originating from the linear modes are
obtained in the form of multipolar and vortex droplets at low and high values
of the norm, respectively, and their stability is investigated. In the spinning
regime, QDs become asymmetric above a critical rotation frequency, most of them
being stable. The effect of the PT -symmetric potential on the spinning and
nonspinning QDs is explored by varying the strength of the gain-loss
distribution. Generally, spinning QDs trapped in the PT -symmetric potential
exhibit asymmetry due to the energy flow affected by the interplay of the
gain-loss distribution and rotation. Finally, interactions between spinning or
nonspinning QDs are explored, exhibiting elastic collisions under certain
conditions.Comment: 18 pages, 10 figures (to be published in Chaos
Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epliayers Grown on Sapphire
Graded InGaN buffers are employed to relax the strain arising from the lattice and thermal mismatches between GaN/InGaN epilayers grown on sapphire. The formation of V-pits in linearly graded InGaN/GaN bulk epilayers is illustrated. The V-pits were sampled using Atomic Force Microscopy and Scanning Electron Microscopy to examine their variation from the theoretical geometry shape. We discovered that the size of the V-pit opening in linearly graded InGaN, with and without GaN cap layer, has a Gaussian distribution. As such, we deduce that the V-pits are produced at different rates, as the growth of the InGaN layer progresses. In Stage I, the V-pits form at a slow rate at the beginning and then accelerate in Stage II when a critical thickness is reached before decelerating in Stage III after arriving at a mean size. It is possible to fill the V-pits by growing a GaN cap layer. It turns out that the filling of the V-pits is more effective at lower growth temperature of the GaN cap layer and the size of the V-pits opening, which is continued in to GaN cap layer, is not dependent on the GaN cap layer thickness. Furthermore, graded InGaN/GaN layers display better strain relaxation as compared to conventionally grown bulk GaN. By employing a specially design configuration, the V-pits can be eliminated from the InGaN epilayer.Singapore-MIT Alliance (SMA
Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epilayers Grown on sapphire
Graded InGaN buffers were employed to relax the strain arising from the lattice and thermal mismatch in GaN/InGaN epilayers grown on sapphire. An enhanced strain relaxation was observed in GaN grown on a stack of five InGaN layers, each 200 nm thick with the In content increased in each layer, and with an intermediate thin GaN layer, 10 nm thick inserted between the InGaN layers, as compared to the conventional two-step growth of GaN epilayer on sapphire. The function of the intermediate layer is to progressively relax the strain and to annihilate the dislocations that build up in the InGaN layer. If the InGaN layers were graded too rapidly, more dislocations will be generated. This increases the probability of the dislocations getting entangled and thereby impeding the motion of the dislocations to relax the strain in the InGaN layer. The optimum growth conditions of the intermediate layer play a major role in promoting the suppression and filling of the V-pits in the GaN cap layer, and were empirically found to be a thin 10 nm GaN grown at 750 0°C and annealed at 1000 0°C.Singapore-MIT Alliance (SMA
Application of Edwards' statistical mechanics to high dimensional jammed sphere packings
The isostatic jamming limit of frictionless spherical particles from Edwards'
statistical mechanics [Song \emph{et al.}, Nature (London) {\bf 453}, 629
(2008)] is generalized to arbitrary dimension using a liquid-state
description. The asymptotic high-dimensional behavior of the self-consistent
relation is obtained by saddle-point evaluation and checked numerically. The
resulting random close packing density scaling is
consistent with that of other approaches, such as replica theory and density
functional theory. The validity of various structural approximations is
assessed by comparing with three- to six-dimensional isostatic packings
obtained from simulations. These numerical results support a growing accuracy
of the theoretical approach with dimension. The approach could thus serve as a
starting point to obtain a geometrical understanding of the higher-order
correlations present in jammed packings.Comment: 13 pages, 7 figure
High Indium Concentration InGaN/GaN Grown on Sapphire Substrate by MOCVD
The InGaN system provides the opportunity to fabricate light emitting devices over the whole visible and ultraviolet spectrum due to band-gap energies E[subscript g] varying between 3.42 eV for GaN and 1.89 eV for InN. However, high In content in InGaN layers will result in a significant degradation of the crystalline quality of the epitaxial layers. In addition, unlike other III-V compound semiconductors, the ratio of gallium to indium incorporated in InGaN is in general not a simple function of the metal atomic flux ratio, f[subscript Ga]/f[subscript In]. Instead, In incorporation is complicated by the tendency of gallium to incorporate preferentially and excess In to form metallic droplets on the growth surface. This phenomenon can definitely affect the In distribution in the InGaN system. Scanning electron microscopy, room temperature photoluminescence, and X-ray diffraction techniques have been used to characterize InGaN layer grown on InN and InGaN buffers. The growth was done on c-plane sapphire by MOCVD. Results showed that green emission was obtained which indicates a relatively high In incorporation.Singapore-MIT Alliance (SMA
Quark-Meson Coupling Model for a Nucleon
The quark-meson coupling model for a nucleon is considered. The model
describes a nucleon as an MIT bag, in which quarks are coupled to scalar and
vector mesons. A set of coupled equations for the quark and the meson fields
are obtained and are solved in a self-consistent way. It is shown that the mass
of a nucleon as a dressed MIT bag interacting with sigma- and omega-meson
fields significantly differs from the mass of a free MIT bag. A few sets of
model parameters are obtained so that the mass of a dressed MIT bag becomes the
nucleon mass. The results of our calculations imply that the self-energy of the
bag in the quark-meson coupling model is significant and needs to be considered
in doing the nuclear matter calculations.Comment: 3 figure
Platinum composite nanowires for ultrasensitive mass detection
Platinum (Pt) composite nanowires were grown on the tip of tungsten (W) microprobes by focused-electron-beam induced chemical vapor deposition (FEB-CVD). An electrical field was used to drive a transversal mechanical vibration of the nanowires. Such nanowire vibrations were found to display the first and second harmonic resonances with frequencies in the range of tens of MHz. The Young's modulus of the nanowires was estimated to be in the range of (1.4 ± 0.1) × 102 GPa to (4.7 ± 0.2) × 102 GPa, dependent on the wire size. A mass responsivity of 2.1 × 1021 Hz/kg Hz/kg was demonstrated with the minimum detectable mass of about 0.4 attogram. Our results indicated the potentials of FEB-CVD for the fabrication of nano-balances on any surface for ultra-sensitive mechanical applications
Jamming II: Edwards' statistical mechanics of random packings of hard spheres
The problem of finding the most efficient way to pack spheres has an
illustrious history, dating back to the crystalline arrays conjectured by
Kepler and the random geometries explored by Bernal in the 60's. This problem
finds applications spanning from the mathematician's pencil, the processing of
granular materials, the jamming and glass transitions, all the way to fruit
packing in every grocery. There are presently numerous experiments showing that
the loosest way to pack spheres gives a density of ~55% (RLP) while filling all
the loose voids results in a maximum density of ~63-64% (RCP). While those
values seem robustly true, to this date there is no physical explanation or
theoretical prediction for them. Here we show that random packings of
monodisperse hard spheres in 3d can pack between the densities 4/(4 + 2 \sqrt
3) or 53.6% and 6/(6 + 2 \sqrt 3) or 63.4%, defining RLP and RCP, respectively.
The reason for these limits arises from a statistical picture of jammed states
in which the RCP can be interpreted as the ground state of the ensemble of
jammed matter with zero compactivity, while the RLP arises in the infinite
compactivity limit. We combine an extended statistical mechanics approach 'a la
Edwards' (where the role traditionally played by the energy and temperature in
thermal systems is substituted by the volume and compactivity) with a
constraint on mechanical stability imposed by the isostatic condition.
Ultimately, our results lead to a phase diagram that provides a unifying view
of the disordered hard sphere packing problem.Comment: 55 pages, 19 figures, C. Song, P. Wang, H. A. Makse, A phase diagram
for jammed matter, Nature 453, 629-632 (2008
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