3,410 research outputs found
The kinetics of homogeneous melting beyond the limit of superheating
Molecular dynamics simulation is used to study the time-scales involved in
the homogeneous melting of a superheated crystal. The interaction model used is
an embedded-atom model for Fe developed in previous work, and the melting
process is simulated in the microcanonical ensemble. We study
periodically repeated systems containing from 96 to 7776 atoms, and the initial
system is always the perfect crystal without free surfaces or other defects.
For each chosen total energy and number of atoms , we perform several
hundred statistically independent simulations, with each simulation lasting for
between 500 ps and 10 ns, in order to gather statistics for the waiting time
before melting occurs. We find that the probability distribution
of is roughly exponential, and that the mean value depends strongly on the excess of the initial steady temperature of the
crystal above the superheating limit identified by other researchers. The mean
also depends strongly on system size in a way that we have
quantified. For very small systems of atoms, we observe a persistent
alternation between the solid and liquid states, and we explain why this
happens. Our results allow us to draw conclusions about the reliability of the
recently proposed Z method for determining the melting properties of simulated
materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure
DNA adducts in peripheral blood lymphocytes from aluminum production plant workers determined by 32P-postlabeling and enzyme-linked immunosorbent assay
32P-Postlabeling analysis and enzyme-linked immunosorbent assay (ELISA) have been used to detect DNA adducts in peripheral blood lymphocytes from primary aluminum production plant workers who were exposed occupationally to a mixture of polycyclic aromatic hydrocarbons (PAHs). Preliminary results reported here are from a comparative study being performed in two aluminum plants. The levels of aromatic DNA adducts have been determined by the 32P-postlabeling assay in samples collected on two occasions, 1 year apart. PAH-DNA adduct levels have also been determined by competitive ELISA in the second set of DNA samples. The results show the necessity of follow-up biomonitoring studies to detect possible alterations in biological effect induced by changing exposures. The comparison of the results obtained by 32P-postlabeling and ELISA may lead to a better understanding of the power and weaknesses of the two methods applied in these studies
Search for Sub-TeV Gamma Rays Coincident with BATSE Gamma Ray Bursts
Project GRAND is a 100m x 100m air shower array of proportional wire chambers
(PWCs). There are 64 stations each with eight 1.29 m^2 PWC planes arranged in
four orthogonal pairs placed vertically above one another to geometrically
measure the angles of charged secondaries. A steel plate above the bottom pair
of PWCs differentiates muons (which pass undeflected through the steel) from
non-penetrating particles. FLUKA Monte Carlo studies show that a TeV gamma ray
striking the atmosphere at normal incidence produces 0.23 muons which reach
ground level where their angles and identities are measured. Thus,
paradoxically, secondary muons are used as a signature for gamma ray primaries.
The data are examined for possible angular and time coincidences with eight
gamma ray bursts (GRBs) detected by BATSE. Seven of the GRBs were selected
because of their good acceptance by GRAND and high BATSE Fluence. The eighth
GRB was added due to its possible coincident detection by Milagrito. For each
of the eight candidate GRBs, the number of excess counts during the BATSE T90
time interval and within plus or minus five degrees of BATSE's direction was
obtained. The highest statistical significance reported in this paper (2.7
sigma) is for the event that was predicted to be the most likely to be observed
(GRB 971110).Comment: To be presented at the XXVIII International Cosmic Ray Conference,
Tsukuba, Japa
Shear-melting of a hexagonal columnar crystal by proliferation of dislocations
A hexagonal columnar crystal undergoes a shear-melting transition above a
critical shear rate or stress. We combine the analysis of the shear-thinning
regime below the melting with that of synchrotron X-ray scattering data under
shear and propose the melting to be due to a proliferation of dislocations,
whose density is determined by both techniques to vary as a power law of the
shear rate with a 2/3 exponent, as expected for a creep model of crystalline
solids. Moreover, our data suggest the existence under shear of a line hexatic
phase, between the columnar crystal and the liquid phase
Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)PF salts
We report a study of the 16.5 GHz dielectric function of hydrogenated and
deuterated organic salts (TMTTF)PF. The temperature behavior of the
dielectric function is consistent with short-range polar order whose relaxation
time decreases rapidly below the charge ordering temperature. If this
transition has more a relaxor character in the hydrogenated salt, charge
ordering is strengthened in the deuterated one where the transition temperature
has increased by more than thirty percent. Anomalies in the dielectric function
are also observed in the spin-Peierls ground state revealing some intricate
lattice effects in a temperature range where both phases coexist. The variation
of the spin-Peierls ordering temperature under magnetic field appears to follow
a mean-field prediction despite the presence of spin-Peierls fluctuations over
a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure
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