210 research outputs found
Backward correlations and dynamic heterogeneities: a computer study of ion dynamics
We analyse the correlated back and forth dynamics and dynamic
heterogeneities, i.e. the presence of fast and slow ions, for a lithium
metasilicate system via computer simulations. For this purpose we define, in
analogy to previous work in the field of glass transition, appropriate
three-time correlation functions. They contain information about the dynamics
during two successive time intervals. First we apply them to simple model
systems in order to clarify their information content. Afterwards we use this
formalism to analyse the lithium trajectories. A strong back-dragging effect is
observed, which also fulfills the time-temperature superposition principle.
Furthermore, it turns out that the back-dragging effect is long-ranged and
exceeds the nearest neighbor position. In contrast, the strength of the dynamic
heterogeneities does not fulfill the time-temperature superposition principle.
The lower the temperature, the stronger the mobility difference between fast
and slow ions. The results are then compared with the simple model systems
considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure
The limits of life and the biosphere in earth’s interior
Fifty years of scientific ocean drilling have shown that microorganisms are widespread deep inside the ocean floor. Microbial populations exist in both organic-matter-rich and nutrient-poor sediments (Kallmeyer et al., 2012; D’Hondt et al., 2015), in sediments that are millions of years old and are buried to over a kilometer depth (Roussel et al., 2008; Ciobanu et al., 2014; Inagaki et al., 2015), and deep inside the basaltic oceanic crust (Orcutt et al., 2011; Lever et al., 2013). In these varied environments, metabolic activity is extraordinarily low (D’Hondt et al., 2009; Hoehler and Jørgensen 2013; Lever et al. 2015a), but microbial cells remain physiologically active (Morono et al., 2011) or survive in their dormant phases (Lomstein et al., 2012). The total amount of sub-surface biomass is still being debated (Hinrichs and Inagaki, 2012; Kallmeyer et al., 2012; Parkes et al., 2014) and the factors posing ultimate limits to deep life and the habitability of Earth remain to be resolved
Comparison of Theoretical and Experimental Bending and Torsional Moments of Endodontic Files and Reamers
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67347/2/10.1177_00220345670460051101.pd
Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions
Molecular dynamics simulations are performed to study the lithium jumps in
LiPO3 glass. In particular, we calculate higher-order correlation functions
that probe the positions of single lithium ions at several times. Three-time
correlation functions show that the non-exponential relaxation of the lithium
ions results from both correlated back-and-forth jumps and the existence of
dynamical heterogeneities, i.e., the presence of a broad distribution of jump
rates. A quantitative analysis yields that the contribution of the dynamical
heterogeneities to the non-exponential depopulation of the lithium sites
increases upon cooling. Further, correlated back-and-forth jumps between
neighboring sites are observed for the fast ions of the distribution, but not
for the slow ions and, hence, the back-jump probability depends on the
dynamical state. Four-time correlation functions indicate that an exchange
between fast and slow ions takes place on the timescale of the jumps
themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites
featuring fast and slow lithium dynamics, respectively, are intimately mixed.
In addition, a backward correlation beyond the first neighbor shell for highly
mobile ions and the presence of long-range dynamical heterogeneities suggest
that fast ion migration occurs along preferential pathways in the glassy
matrix. In the melt, we find no evidence for correlated back-and-forth motions
and dynamical heterogeneities on the length scale of the next-neighbor
distance.Comment: 12 pages, 13 figure
Evidence for softening of first-order transition in 3D by quenched disorder
We study by extensive Monte Carlo simulations the effect of random bond
dilution on the phase transition of the three-dimensional 4-state Potts model
which is known to exhibit a strong first-order transition in the pure case. The
phase diagram in the dilution-temperature plane is determined from the peaks of
the susceptibility for sufficiently large system sizes. In the strongly
disordered regime, numerical evidence for softening to a second-order
transition induced by randomness is given. Here a large-scale finite-size
scaling analysis, made difficult due to strong crossover effects presumably
caused by the percolation fixed point, is performed.Comment: LaTeX file with Revtex, 4 pages, 4 eps figure
Ising model on 3D random lattices: A Monte Carlo study
We report single-cluster Monte Carlo simulations of the Ising model on
three-dimensional Poissonian random lattices with up to 128,000 approx. 503
sites which are linked together according to the Voronoi/Delaunay prescription.
For each lattice size quenched averages are performed over 96 realizations. By
using reweighting techniques and finite-size scaling analyses we investigate
the critical properties of the model in the close vicinity of the phase
transition point. Our random lattice data provide strong evidence that, for the
available system sizes, the resulting effective critical exponents are
indistinguishable from recent high-precision estimates obtained in Monte Carlo
studies of the Ising model and \phi^4 field theory on three-dimensional regular
cubic lattices.Comment: 35 pages, LaTex, 8 tables, 8 postscript figure
Dynamic structure factor of the Ising model with purely relaxational dynamics
We compute the dynamic structure factor for the Ising model with a purely
relaxational dynamics (model A). We perform a perturbative calculation in the
expansion, at two loops in the high-temperature phase and at one
loop in the temperature magnetic-field plane, and a Monte Carlo simulation in
the high-temperature phase. We find that the dynamic structure factor is very
well approximated by its mean-field Gaussian form up to moderately large values
of the frequency and momentum . In the region we can investigate,
, , where is the correlation
length and the zero-momentum autocorrelation time, deviations are at
most of a few percent.Comment: 21 pages, 3 figure
Critical behavior of weakly-disordered anisotropic systems in two dimensions
The critical behavior of two-dimensional (2D) anisotropic systems with weak
quenched disorder described by the so-called generalized Ashkin-Teller model
(GATM) is studied. In the critical region this model is shown to be described
by a multifermion field theory similar to the Gross-Neveu model with a few
independent quartic coupling constants. Renormalization group calculations are
used to obtain the temperature dependence near the critical point of some
thermodynamic quantities and the large distance behavior of the two-spin
correlation function. The equation of state at criticality is also obtained in
this framework. We find that random models described by the GATM belong to the
same universality class as that of the two-dimensional Ising model. The
critical exponent of the correlation length for the 3- and 4-state
random-bond Potts models is also calculated in a 3-loop approximation. We show
that this exponent is given by an apparently convergent series in
(with the central charge of the Potts model) and
that the numerical values of are very close to that of the 2D Ising
model. This work therefore supports the conjecture (valid only approximately
for the 3- and 4-state Potts models) of a superuniversality for the 2D
disordered models with discrete symmetries.Comment: REVTeX, 24 pages, to appear in Phys.Rev.
Measurement of the partial widths of the Z into up- and down-type quarks
Using the entire OPAL LEP1 on-peak Z hadronic decay sample, Z -> qbarq gamma
decays were selected by tagging hadronic final states with isolated photon
candidates in the electromagnetic calorimeter. Combining the measured rates of
Z -> qbarq gamma decays with the total rate of hadronic Z decays permits the
simultaneous determination of the widths of the Z into up- and down-type
quarks. The values obtained, with total errors, were Gamma u = 300 ^{+19}_{-18}
MeV and Gamma d = 381 ^{+12}_{-12} MeV. The results are in good agreement with
the Standard Model expectation.Comment: 22 pages, 5 figures, Submitted to Phys. Letts.
Search for R-Parity Violating Decays of Scalar Fermions at LEP
A search for pair-produced scalar fermions under the assumption that R-parity
is not conserved has been performed using data collected with the OPAL detector
at LEP. The data samples analysed correspond to an integrated luminosity of
about 610 pb-1 collected at centre-of-mass energies of sqrt(s) 189-209 GeV. An
important consequence of R-parity violation is that the lightest supersymmetric
particle is expected to be unstable. Searches of R-parity violating decays of
charged sleptons, sneutrinos and squarks have been performed under the
assumptions that the lightest supersymmetric particle decays promptly and that
only one of the R-parity violating couplings is dominant for each of the decay
modes considered. Such processes would yield final states consisting of
leptons, jets, or both with or without missing energy. No significant
single-like excess of events has been observed with respect to the Standard
Model expectations. Limits on the production cross- section of scalar fermions
in R-parity violating scenarios are obtained. Constraints on the supersymmetric
particle masses are also presented in an R-parity violating framework analogous
to the Constrained Minimal Supersymmetric Standard Model.Comment: 51 pages, 24 figures, Submitted to Eur. Phys. J.
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