264 research outputs found
A Geometrical Interpretation of Hyperscaling Breaking in the Ising Model
In random percolation one finds that the mean field regime above the upper
critical dimension can simply be explained through the coexistence of infinite
percolating clusters at the critical point. Because of the mapping between
percolation and critical behaviour in the Ising model, one might check whether
the breakdown of hyperscaling in the Ising model can also be intepreted as due
to an infinite multiplicity of percolating Fortuin-Kasteleyn clusters at the
critical temperature T_c. Preliminary results suggest that the scenario is much
more involved than expected due to the fact that the percolation variables
behave differently on the two sides of T_c.Comment: Lattice2002(spin
Intra-household allocation with shared expenditure choices. Experimental evidence from Filipino migrants
Sharing information concerning expenditure choices between a migrant and the recipient affects the migrantâs allocation patterns. In a lab-in-the-field experiment, Filipino migrants are asked to earmark an in-kind budget to be delivered to their most closely connected household (MCCH). When the MCCH is fully aware of the migrantâs decisions (i.e., symmetric information), we observe that the migrant raises the portion for consumption goods in the range of 10.0â10.5% with respect to the case when the migrantâs choices are not disclosed (i.e., asymmetric information). Moreover, when sharing information, the migrant relies on more involvement of the recipient household and lowers by 7â9% the allocation to expenses she could monitor ex-post more strictly. The former result is consistent with the signaling motive, whereas the latter supports the presence of strategic behavior by the migrant remitter. Education allocations are significantly higher in intra- rather than inter-household transfers and this provides insights for conditional cash transfer policies
Estimating the generation interval from the incidence rate, the optimal quarantine duration and the efficiency of fast switching periodic protocols for COVIDâ19
The transmissibility of an infectious disease is usually quantified in terms of the reproduction
number Rt representing, at a given time, the average number of secondary cases caused by an
infected individual. Recent studies have enlightened the central role played by w(z), the distribution
of generation times z, namely the time between successive infections in a transmission chain. In
standard approaches this quantity is usually substituted by the distribution of serial intervals, which
is obtained by contact tracing after measuring the time between onset of symptoms in successive
cases. Unfortunately, this substitution can cause important biases in the estimate of Rt . Here we
present a novel method which allows us to simultaneously obtain the optimal functional form of
w(z) together with the daily evolution of Rt , over the course of an epidemic. The method uses, as
unique information, the daily series of incidence rate and thus overcomes biases present in standard
approaches. We apply our method to one year of data from COVID-19 officially reported cases in the
21 Italian regions, since the first confirmed case on February 2020. We find that w(z) has mean value
z â 6 days with a standard deviation a â 1 day, for all Italian regions, and these values are stable
even if one considers only the first 10 days of data recording. This indicates that an estimate of the
most relevant transmission parameters can be already available in the early stage of a pandemic. We
use this information to obtain the optimal quarantine duration and to demonstrate that, in the case
of COVID-19, post-lockdown mitigation policies, such as fast periodic switching and/or alternating
quarantine, can be very efficient
Fracture in Three-Dimensional Fuse Networks
We report on large scale numerical simulations of fracture surfaces using
random fuse networks for two very different disorders. There are some
properties and exponents that are different for the two distributions, but
others, notably the roughness exponents, seem universal. For the universal
roughness exponent we found a value of zeta = 0.62 +/- 0.05. In contrast to
what is observed in two dimensions, this value is lower than that reported in
experimental studies of brittle fractures, and rules out the minimal energy
surface exponent, 0.41 +/- 0.01.Comment: 4 pages, RevTeX, 5 figures, Postscrip
Crack roughness and avalanche precursors in the random fuse model
We analyze the scaling of the crack roughness and of avalanche precursors in
the two dimensional random fuse model by numerical simulations, employing large
system sizes and extensive sample averaging. We find that the crack roughness
exhibits anomalous scaling, as recently observed in experiments. The roughness
exponents (, ) and the global width distributions are found
to be universal with respect to the lattice geometry. Failure is preceded by
avalanche precursors whose distribution follows a power law up to a cutoff
size. While the characteristic avalanche size scales as , with a
universal fractal dimension , the distribution exponent differs
slightly for triangular and diamond lattices and, in both cases, it is larger
than the mean-field (fiber bundle) value
Tolerance and Sensitivity in the Fuse Network
We show that depending on the disorder, a small noise added to the threshold
distribution of the fuse network may or may not completely change the
subsequent breakdown process. When the threshold distribution has a lower
cutoff at a finite value and a power law dependence towards large thresholds
with an exponent which is less than , the network is not sensitive
to the added noise, otherwise it is. The transition between sensitivity or not
appears to be second order, and is related to a localization-delocalization
transition earlier observed in such systems.Comment: 12 pages, 3 figures available upon request, plain Te
Reentrant phase diagram and pH effects in cross-linked gelatin gels
Experimental results have shown that the kinetics of bond formation in
chemical crosslinking of gelatin solutions is strongly affected not only by
gelatin and reactant concentrations but also by the solution pH. We present an
extended numerical investigation of the phase diagram and of the kinetics of
bond formation as a function of the pH, via Monte Carlo simulations of a
lattice model for gelatin chains and reactant agent in solution. We find a
reentrant phase diagram, namely gelation can be hindered either by loop
formation, at low reactant concentrations, or by saturation of active sites of
the chains via formation of single bonds with crosslinkers, at high reactant
concentrations. The ratio of the characteristic times for the formation of the
first and of the second bond between the crosslinker and an active site of a
chain is found to depend on the reactant reactivity, in good agreement with
experimental data.Comment: 8 pages, 8 figure
Dynamic critical exponents of the Ising model with multispin interactions
We revisit the short-time dynamics of 2D Ising model with three spin
interactions in one direction and estimate the critical exponents
and . Taking properly into account the symmetry of the
Hamiltonian we obtain results completely different from those obtained by Wang
et al.. For the dynamic exponent our result coincides with that of the
4-state Potts model in two dimensions. In addition, results for the static
exponents and agree with previous estimates obtained from finite
size scaling combined with conformal invariance. Finally, for the new dynamic
exponent we find a negative and close to zero value, a result also
expected for the 4-state Potts model according to Okano et al.Comment: 12 pages, 9 figures, corrected Abstract mistypes, corrected equation
on page 4 (Parameter Q
Kinetics of bond formation in crosslinked gelatin gels
In chemical crosslinking of gelatin solutions, two different time scales
affect the kinetics of the gel formation in the experiments. We complement the
experimental study with Monte Carlo numerical simulations of a lattice model.
This approach shows that the two characteristic time scales are related to the
formation of single bonds crosslinker-chain and of bridges between chains. In
particular their ratio turns out to control the kinetics of the gel formation.
We discuss the effect of the concentration of chains. Finally our results
suggest that, by varying the probability of forming bridges as an independent
parameter, one can finely tune the kinetics of the gelation via the ratio of
the two characteristic times.Comment: 8 pages, 9 figures, revised versio
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
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