3,193 research outputs found
Melting and Rippling Phenomenan in Two Dimensional Crystals with localized bonding
We calculate Root Mean Square (RMS) deviations from equilibrium for atoms in
a two dimensional crystal with local (e.g. covalent) bonding between close
neighbors. Large scale Monte Carlo calculations are in good agreement with
analytical results obtained in the harmonic approximation. When motion is
restricted to the plane, we find a slow (logarithmic) increase in fluctuations
of the atoms about their equilibrium positions as the crystals are made larger
and larger. We take into account fluctuations perpendicular to the lattice
plane, manifest as undulating ripples, by examining dual layer systems with
coupling between the layers to impart local rigidly (i.e. as in sheets of
graphene made stiff by their finite thickness). Surprisingly, we find a rapid
divergence with increasing system size in the vertical mean square deviations,
independent of the strength of the interplanar coupling. We consider an
attractive coupling to a flat substrate, finding that even a weak attraction
significantly limits the amplitude and average wavelength of the ripples. We
verify our results are generic by examining a variety of distinct geometries,
obtaining the same phenomena in each case.Comment: 17 pages, 28 figure
Percolation of Immobile Domains in Supercooled Thin Polymeric Films
We present an analysis of heterogeneous dynamics in molecular dynamics
simulations of a thin polymeric film, supported by an absorbing structured
surface. Near the glass transition "immobile" domains occur throughout the
film, yet the probability of their occurrence decreasing with larger distance
from the surface. Still, enough immobile domains are located near the free
surface to cause them to percolate in the direction perpendicular to surface,
at a temperature near the glass transition temperature. This result is in
agreement with a recent theoretical model of glass transition
Free energies, vacancy concentrations and density distribution anisotropies in hard--sphere crystals: A combined density functional and simulation study
We perform a comparative study of the free energies and the density
distributions in hard sphere crystals using Monte Carlo simulations and density
functional theory (employing Fundamental Measure functionals). Using a recently
introduced technique (Schilling and Schmid, J. Chem. Phys 131, 231102 (2009))
we obtain crystal free energies to a high precision. The free energies from
Fundamental Measure theory are in good agreement with the simulation results
and demonstrate the applicability of these functionals to the treatment of
other problems involving crystallization. The agreement between FMT and
simulations on the level of the free energies is also reflected in the density
distributions around single lattice sites. Overall, the peak widths and
anisotropy signs for different lattice directions agree, however, it is found
that Fundamental Measure theory gives slightly narrower peaks with more
anisotropy than seen in the simulations. Among the three types of Fundamental
Measure functionals studied, only the White Bear II functional (Hansen-Goos and
Roth, J. Phys.: Condens. Matter 18, 8413 (2006)) exhibits sensible results for
the equilibrium vacancy concentration and a physical behavior of the chemical
potential in crystals constrained by a fixed vacancy concentration.Comment: 17 pages, submitted to Phys. Rev.
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part II: The intrinsic electronic midgap states
We propose a structural model that treats in a unified fashion both the
atomic motions and electronic excitations in quenched melts of pnictide and
chalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we argued
these quenched melts represent aperiodic -networks that are highly
stable and, at the same time, structurally degenerate. These networks are
characterized by a continuous range of coordination. Here we present a
systematic way to classify these types of coordination in terms of discrete
coordination defects in a parent structure defined on a simple cubic lattice.
We identify the lowest energy coordination defects with the intrinsic midgap
electronic states in semiconductor glasses, which were argued earlier to cause
many of the unique optoelectronic anomalies in these materials. In addition,
these coordination defects are mobile and correspond to the transition state
configurations during the activated transport above the glass transition. The
presence of the coordination defects may account for the puzzling discrepancy
between the kinetic and thermodynamic fragility in chalcogenides. Finally, the
proposed model recovers as limiting cases several popular types of bonding
patterns proposed earlier, including: valence-alternation pairs, hypervalent
configurations, and homopolar bonds in heteropolar compounds.Comment: 17 pages, 15 figures, revised version, final version to appear in J.
Chem. Phy
Parallel J-W Monte Carlo Simulations of Thermal Phase Changes in Finite-size Systems
The thermodynamic properties of 59 TeF6 clusters that undergo
temperature-driven phase transitions have been calculated with a canonical
J-walking Monte Carlo technique. A parallel code for simulations has been
developed and optimized on SUN3500 and CRAY-T3E computers. The Lindemann
criterion shows that the clusters transform from liquid to solid and then from
one solid structure to another in the temperature region 60-130 K.Comment: 4 pages, 5 figures; presented at the conference on computational
physics, Aachen (2001) accepted for publication in Comp.Phys.Com
Potts-Percolation-Gauss Model of a Solid
We study a statistical mechanics model of a solid. Neighboring atoms are
connected by Hookian springs. If the energy is larger than a threshold the
"spring" is more likely to fail, while if the energy is lower than the
threshold the spring is more likely to be alive. The phase diagram and
thermodynamic quantities, such as free energy, numbers of bonds and clusters,
and their fluctuations, are determined using renormalization-group and
Monte-Carlo techniques.Comment: 10 pages, 12 figure
Effect of a thin AlO_x layer on transition-edge sensor properties
We have studied the physics of transition-edge sensor (TES) devices with an
insulating AlOx layer on top of the device to allow implementation of more
complex detector geometries. By comparing devices with and without the
insulating film, we have observed significant additional noise apparently
caused by the insulator layer. In addition, AlOx was found to be a relatively
good thermal conductor. This adds an unforeseen internal thermal feature to the
system.Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conferenc
Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
refereein
Historical origins of nasa's launch operations center to july 1, 1962
Historical origins of NASA Launch Operations Center to July 1, 196
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