69,355 research outputs found

### Homogeneous SPC/E water nucleation in large molecular dynamics simulations

We perform direct large molecular dynamics simulations of homogeneous SPC/E
water nucleation, using up to $\sim 4\cdot 10^6$ molecules. Our large system
sizes allow us to measure extremely low and accurate nucleation rates, down to
$\sim 10^{19}\,\textrm{cm}^{-3}\textrm{s}^{-1}$, helping close the gap between
experimentally measured rates $\sim 10^{17}\,\textrm{cm}^{-3}\textrm{s}^{-1}$.
We are also able to precisely measure size distributions, sticking
efficiencies, cluster temperatures, and cluster internal densities. We
introduce a new functional form to implement the Yasuoka-Matsumoto nucleation
rate measurement technique (threshold method). Comparison to nucleation models
shows that classical nucleation theory over-estimates nucleation rates by a few
orders of magnitude. The semi-phenomenological nucleation model does better,
under-predicting rates by at worst, a factor of 24. Unlike what has been
observed in Lennard-Jones simulations, post-critical clusters have temperatures
consistent with the run average temperature. Also, we observe that
post-critical clusters have densities very slightly higher, $\sim 5\%$, than
bulk liquid. We re-calibrate a Hale-type $J$ vs. $S$ scaling relation using
both experimental and simulation data, finding remarkable consistency in over
$30$ orders of magnitude in the nucleation rate range, and $180\,$K in the
temperature range.Comment: Accepted for publication in the Journal of Chemical Physic

### A study of the effect of forcing function characteristics on human operator dynamics in manual control

The effect of the spectrum of the forcing function on the human pilot dynamics in manual control was investigated. A simple compensatory tracking experiment was conducted, where the controlled element was of a second-order dynamics and the forcing function was a random noise having a dominant frequency. The dominant frequency and the power of the forcing function were two variable parameters during the experiment. The results show that the human pilot describing functions are dependent not only on the dynamics of the controlled element, but also on the characteristics of the forcing function. This suggests that the human pilot behavior should be expressed by the transfer function taking into consideration his ability to sense and predict the forcing function

### Diffractive heavy pseudoscalar-meson productions by weak neutral currents

A first theoretical study for neutrino-induced diffractive productions of
heavy pseudoscalar-mesons, \eta_c and \eta_b, off a nucleon is performed based
on factorization formalism in QCD. We evaluate the forward diffractive
production cross section in perturbative QCD in terms of the light-cone wave
functions of Z boson and \eta_{c,b} mesons, and the gluon distribution of the
nucleon. The diffractive production of \eta_c is governed by the axial vector
coupling of the longitudinally polarized Z boson to Q\bar{Q} pair, and the
resulting \eta_c production cross section is larger than the J/\psi one by one
order of magnitude. The bottomonium \eta_b production, which shows up for
higher beam energy, is also discussed.Comment: 5 pages with 3 embedded figures. Talk presented at the 15th
International Spin Physics Symposium, Spin 2002, Brookhaven National
Laboratory, September 9-14, 200

### The flares of August 1972

Analysis is made of observations of the August, 1972 flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms, and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impart and by heating by the energetic electrons and protons. The region showed twisted flux and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface

### Ratio of Quark Masses in Duality Theories

We consider N=2 SU(2) Seiberg-Witten duality theory for models with N_f=2 and
N_f=3 quark flavors. We investigate arbitrary large bare mass ratios between
the two or three quarks at the singular points. For N_f=2 we explore large bare
mass ratios corresponding to a singularity in the strong coupling region. For
N_f=3 we determine the location of both strong and weak coupling singularities
that produce specific large bare mass ratios.Comment: 12 pages. Standard Latex. Version appearing in Mod. Phys. Lett.

### Even-Odd and Super-Even Effects in the Attractive Hubbard Model

The canonical BCS wave function is tested for the attractive Hubbard model.
Results are presented for one dimension, and are compared with the exact
solutions by the Bethe ansatz and the results from the conventional grand
canonical BCS approximation, for various chain lengths, electron densities, and
coupling strengths. While the exact ground state energies are reproduced very
well both by the canonical and grand canonical BCS approximations, the
canonical method significantly improves the energy gaps for small systems and
weak coupling. The ``parity'' effect due to the number of electrons being even
or odd naturally emerges in our canonical results. Furthermore, we find a
``super-even'' effect: the energy gap oscillates as a function of even electron
number, depending on whether the number of electrons is $4 m$ or $4 m + 2$ (m
integer). Such oscillations as a function of electron number should be
observable with tunneling measurements in ultrasmall metallic grains.Comment: 20 pages, 9 figure

### Direct Simulations of Homogeneous Bubble Nucleation: Agreement with CNT and no Local Hot Spots

We present results from direct, large-scale molecular dynamics (MD)
simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations
contain half a billion Lennard-Jones (LJ) atoms and cover up to 56 million
time-steps. The unprecedented size of the simulated volumes allows us to
resolve the nucleation and growth of many bubbles per run in simple direct
micro-canonical (NVE) simulations while the ambient pressure and temperature
remain almost perfectly constant. We find bubble nucleation rates which are
lower than in most of the previous, smaller simulations. It is widely believed
that classical nucleation theory (CNT) generally underestimates bubble
nucleation rates by very large factors. However, our measured rates are within
two orders of magnitude of CNT predictions - only at very low temperatures does
CNT underestimate the nucleation rate significantly. Introducing a small,
positive Tolman length leads to very good agreement at all temperatures, as
found in our recent vapor-to-liquid nucleation simulations. The critical
bubbles sizes derived with the nucleation theorem agree well with the CNT
predictions at all temperatures. Local hot spots reported in the literature are
not seen: Regions where a bubble nucleation events will occur are not above the
average temperature, and no correlation of temperature fluctuations with
subsequent bubble formation is seen.Comment: 15 pages, 13 figures. Submitted to PRE. Simulation movies available
at http://www.ics.uzh.ch/~diemand/movies

### Simple improvements to classical bubble nucleation models

We revisit classical nucleation theory (CNT) for the homogeneous bubble
nucleation rate and improve the classical formula using a new prefactor in the
nucleation rate. Most of the previous theoretical studies have used the
constant prefactor determined by the bubble growth due to the evaporation
process from the bubble surface. However, the growth of bubbles is also
regulated by the thermal conduction, the viscosity, and the inertia of liquid
motion. These effects can decrease the prefactor significantly, especially when
the liquid pressure is much smaller than the equilibrium one. The deviation in
the nucleation rate between the improved formula and the CNT can be as large as
several orders of magnitude. Our improved, accurate prefactor and recent
advances in molecular dynamics simulations and laboratory experiments for argon
bubble nucleation enable us to precisely constrain the free energy barrier for
bubble nucleation. Assuming the correction to the CNT free energy is of the
functional form suggested by Tolman, the precise evaluations of the free energy
barriers suggest the Tolman length is $\simeq 0.3 \sigma$ independently of the
temperature for argon bubble nucleation, where $\sigma$ is the unit length of
the Lenard-Jones potential. With this Tolman correction and our new prefactor
one gets accurate bubble nucleation rate predictions in the parameter range
probed by current experiments and molecular dynamics simulations.Comment: 10pages, 6figures, Accepted for publication in Physical Review

- …