5,603 research outputs found
Harmonic oscillators in the Nos\'e - Hoover thermostat
We study the dynamics of an ensemble of non-interacting harmonic oscillators
in a nonlinear dissipative environment described by the Nos\'e - Hoover model.
Using numerical simulation we find the histogram for total energy, which agrees
with the analysis of the Nos\'e - Hoover equations effected with the method of
averaging. The histogram does not correspond to Gibbs' canonical distribution.
We have found oscillations at frequency proportional to ,
the dissipative parameter of thermostat and the characteristic
mass of particle, about the stationary state corresponding to equilibrium. The
oscillations could have an important bearing upon the analysis of simulating
molecular dynamics in the Nos\'e - Hoover thermostat.Comment: 7 pages, 4 figure
Ground States for Exponential Random Graphs
We propose a perturbative method to estimate the normalization constant in
exponential random graph models as the weighting parameters approach infinity.
As an application, we give evidence of discontinuity in natural parametrization
along the critical directions of the edge-triangle model.Comment: 12 pages, 3 figures, 1 tabl
Nonequilibrium stationary states with ratchet effect
An ensemble of particles in thermal equilibrium at temperature , modeled
by Nos\`e-Hoover dynamics, moves on a triangular lattice of oriented semi-disk
elastic scatterers. Despite the scatterer asymmetry a directed transport is
clearly ruled out by the second law of thermodynamics. Introduction of a
polarized zero mean monochromatic field creates a directed stationary flow with
nontrivial dependence on temperature and field parameters. We give a
theoretical estimate of directed current induced by a microwave field in an
antidot superlattice in semiconductor heterostructures.Comment: 4 pages, 5 figures (small changes added
Evaluation of techniques for removal of spacecraft contaminants from activated carbon
Alternative techniques for the regeneration of carbon contaminated with various spacecraft contaminants were evaluated. Four different modes of regeneration were evaluated: (1) thermal desorption via vacuum, (2) thermal desorption via nitrogen purge, (3) in-situ catalytic oxidation of adsorbed contaminants, and (4) in-situ non-catalytic oxidation of adsorbed contaminants
The Extended Range X-Ray Telescope center director's discretionary fund report
An Extended Range X-Ray Telescope (ERXRT) of high sensitivity and spatial resolution capable of functioning over a broad region of the X-ray/XUV portion of the spectrum has been designed and analyzed. This system has been configured around the glancing-incidence Wolter Type I X-ray mirror system which was flown on the Skylab Apollo Telescope Mount as ATM Experiment S-056. Enhanced sensitivity over a vastly broader spectral range can be realized by the utilization of a thinned, back-illuminated, buried-channel Charge Coupled Device (CCD) as the X-ray/XUV detector rather than photographic film. However, to maintain the high spatial resolution inherent in the X-ray optics when a CCD of 30 micron pixel size is used, it is necessary to increase the telescope plate scale. This can be accomplished by use of a glancing-incidence X-ray microscope to enlarge and re-focus the primary image onto the focal surface of the CCD
Comment on the calculation of forces for multibody interatomic potentials
The system of particles interacting via multibody interatomic potential of
general form is considered. Possible variants of partition of the total force
acting on a single particle into pair contributions are discussed. Two
definitions for the force acting between a pair of particles are compared. The
forces coincide only if the particles interact via pair or embedded-atom
potentials. However in literature both definitions are used in order to
determine Cauchy stress tensor. A simplest example of the linear pure shear of
perfect square lattice is analyzed. It is shown that, Hardy's definition for
the stress tensor gives different results depending on the radius of
localization function. The differences strongly depend on the way of the force
definition.Comment: 9 pages, 2 figure
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding
Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review
The MeV spectra of gamma-ray bursts measured with COMPTEL
The past decade has produced a wealth of observational data on the energy spectra of prompt emission from gamma-ray bursts. Most of the data cover the energy range from a few to several hundred keV. One set of higher energy
observations comes from the Imaging Compton Telescope COMPTEL on the Compton Observatory, which measured in the energy range from 0.75 to 30MeV. We analyzed the full 9.2 years COMPTEL data to reveal the significant detection of 44 gamma-ray bursts. We present preliminary results obtained in the process of preparing a final catalog of the spectral analysis of these events. In addition, we compare
the COMPTEL spectra to simultaneous BATSE measurements for purposes of cross-calibration
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