637 research outputs found
Hard sphere crystallization gets rarer with increasing dimension
We recently found that crystallization of monodisperse hard spheres from the
bulk fluid faces a much higher free energy barrier in four than in three
dimensions at equivalent supersaturation, due to the increased geometrical
frustration between the simplex-based fluid order and the crystal [J.A. van
Meel, D. Frenkel, and P. Charbonneau, Phys. Rev. E 79, 030201(R) (2009)]. Here,
we analyze the microscopic contributions to the fluid-crystal interfacial free
energy to understand how the barrier to crystallization changes with dimension.
We find the barrier to grow with dimension and we identify the role of
polydispersity in preventing crystal formation. The increased fluid stability
allows us to study the jamming behavior in four, five, and six dimensions and
compare our observations with two recent theories [C. Song, P. Wang, and H. A.
Makse, Nature 453, 629 (2008); G. Parisi and F. Zamponi, Rev. Mod. Phys, in
press (2009)].Comment: 15 pages, 5 figure
Geometrical Frustration: A Study of 4d Hard Spheres
The smallest maximum kissing-number Voronoi polyhedron of 3d spheres is the
icosahedron and the tetrahedron is the smallest volume that can show up in
Delaunay tessalation. No periodic lattice is consistent with either and hence
these dense packings are geometrically frustrated. Because icosahedra can be
assembled from almost perfect tetrahedra, the terms "icosahedral" and
"polytetrahedral" packing are often used interchangeably, which leaves the true
origin of geometric frustration unclear. Here we report a computational study
of freezing of 4d hard spheres, where the densest Voronoi cluster is compatible
with the symmetry of the densest crystal, while polytetrahedral order is not.
We observe that, under otherwise comparable conditions, crystal nucleation in
4d is less facile than in 3d. This suggest that it is the geometrical
frustration of polytetrahedral structures that inhibits crystallization.Comment: 4 pages, 3 figures; revised interpretatio
Master-equation approach to the study of phase-change processes in data storage media
We study the dynamics of crystallization in phase-change materials using a master-equation approach in which the state of the crystallizing material is described by a cluster size distribution function. A model is developed using the thermodynamics of the processes involved and representing the clusters of size two and greater as a continuum but clusters of size one (monomers) as a separate equation. We present some partial analytical results for the isothermal case and for large cluster sizes, but principally we use numerical simulations to investigate the model. We obtain results that are in good agreement with experimental data and the model appears to be useful for the fast simulation of reading and writing processes in phase-change optical and electrical memories
Static Friction between Elastic Solids due to Random Asperities
Several workers have established that the Larkin domains for two three
dimensional nonmetallic elastic solids in contact with each other at a
disordered interface are enormously large. This implies that there should be
negligible static friction per unit area in the macroscopic solid limit.
The present work argues that the fluctuations in the heights of the random
asperities at the interface that occur in the Greenwood-Williamson model can
account for static friction.Comment: Contains some improvements in the treatment of the subjec
Orientation dependence of heterogeneous nucleation at the Cu–Pb solid-liquid interface
In this work, we examine the effect of surface structure on the heterogeneous nucleation of Pb crystals from the melt at a Cu substrate using molecular-dynamics (MD) simulation. In a previous work [Palafox-Hernandez et al., Acta Mater. 59, 3137 (2011)] studying the Cu/Pb solid-liquid interface with MD simulation, we observed that the structure of the Cu(111) and Cu(100) interfaces was significantly different at 625 K, just above the Pb melting temperature (618 K for the model). The Cu(100) interface exhibited significant surface alloying in the crystal plane in contact with the melt. In contrast, no surface alloying was seen at the Cu(111) interface; however, a prefreezing layer of crystalline Pb, 2-3 atomic planes thick and slightly compressed relative to bulk Pb crystal, was observed to form at the interface. We observe that at the Cu(111) interface the prefreezing layer is no longer present at 750 K, but surface alloying in the Cu(100) interface persists. In a series of undercooling MD simulations, heterogeneous nucleation of fcc Pb is observed at the Cu(111) interface within the simulation time (5 ns) at 592 K—a 26 K undercooling. Nucleation and growth at Cu(111) proceeded layerwise with a nearly planar critical nucleus. Quantitative analysis yielded heterogeneous nucleation barriers that are more than two orders of magnitude smaller than the predicted homogeneous nucleation barriers from classical nucleation theory. Nucleation was considerably more difficult on the Cu(100) surface-alloyed substrate. An undercooling of approximately 170 K was necessary to observe nucleation at this interface within the simulation time. From qualitative observation, the critical nucleus showed a contact angle with the Cu(100) surface of over 90°, indicating poor wetting of the Cu(100) surface by the nucleating phase, which according to classical heterogeneous nucleation theory provides an explanation of the large undercooling necessary to nucleate on the Cu(100) surface, relative to Cu(111), whose surface is more similar to the nucleating phase due to the presence of the prefreezing layer
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature
We present a new fabrication method of graphene spin-valve devices which
yields enhanced spin and charge transport properties by improving both the
electrode-to-graphene and graphene-to-substrate interface. First, we prepare
Co/MgO spin injection electrodes onto Si/SiO. Thereafter, we
mechanically transfer a graphene-hBN heterostructure onto the prepatterned
electrodes. We show that room temperature spin transport in single-, bi- and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion
lengths reaching 10m combined with carrier mobilities exceeding 20,000
cm/Vs.Comment: 15 pages, 5 figure
Collective Behavior of Asperities in Dry Friction at Small Velocities
We investigate a simple model of dry friction based on extremal dynamics of
asperities. At small velocities, correlations develop between the asperities,
whose range becomes infinite in the limit of infinitely slow driving, where the
system is self-organized critical. This collective phenomenon leads to
effective aging of the asperities and results in velocity dependence of the
friction force in the form .Comment: 7 pages, 8 figures, revtex, submitted to Phys. Rev.
Simulation of fluid-solid coexistence in finite volumes: A method to study the properties of wall-attached crystalline nuclei
The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte
Carlo simulations at densities inside the two-phase coexistence region of fluid
and solid. Choosing a geometry where the system is confined between two flat
walls, and a wall-colloid potential that leads to incomplete wetting of the
crystal at the wall, conditions can be created where a single nanoscopic
wall-attached crystalline cluster coexists with fluid in the remainder of the
simulation box. Following related ideas that have been useful to study
heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we
estimate the contact angles from observations of the crystalline clusters in
thermal equilibrium. We find fair agreement with a prediction based on Young's
equation, using estimates of interface and wall tension from the study of flat
surfaces. It is shown that the pressure versus density curve of the finite
system exhibits a loop, but the pressure maximum signifies the "droplet
evaporation-condensation" transition and thus has nothing in common with a van
der Waals-like loop. Preparing systems where the packing fraction is deep
inside the two-phase coexistence region, the system spontaneously forms a "slab
state", with two wall-attached crystalline domains separated by (flat)
interfaces from liquid in full equilibrium with the crystal in between;
analysis of such states allows a precise estimation of the bulk equilibrium
properties at phase coexistence
Homogeneous nucleation of colloidal melts under the influence of shearing fields
We study the effect of shear flow on homogeneous crystal nucleation, using
Brownian Dynamics simulations in combination with an umbrella sampling like
technique. The symmetry breaking due to shear results in anisotropic radial
distribution functions. The homogeneous shear rate suppresses crystal
nucleation and leads to an increase of the size of the critical nucleus. These
observations can be described by a simple, phenomenological extension of
classical nucleation theory. In addition, we find that nuclei have a
preferential orientation with respect to the direction of shear. On average the
longest dimension of a nucleus is along the vorticity direction, while the
shortest dimension is preferably perpendicular to that and slightly tilted with
respect to the gradient direction.Comment: 10 pages, 8 figures, Submitted to J. Phys.: Condens. Matte
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