1,339 research outputs found
Study of the one-dimensional off-lattice hot-monomer reaction model
Hot monomers are particles having a transient mobility (a ballistic flight)
prior to being definitely absorbed on a surface. After arriving at a surface,
the excess energy coming from the kinetic energy in the gas phase is dissipated
through degrees of freedom parallel to the surface plane. In this paper we
study the hot monomer-monomer adsorption-reaction process on a continuum
(off-lattice) one-dimensional space by means of Monte Carlo simulations. The
system exhibits second-order irreversible phase transition between a reactive
and saturated (absorbing) phases which belong to the directed percolation (DP)
universality class. This result is interpreted by means of a coarse-grained
Langevin description which allows as to extend the DP conjecture to transitions
occurring in continuous media.Comment: 13 pages, 5 figures, final version to appear in J. Phys.
Short-Time Critical Dynamics of Damage Spreading in the Two-Dimensional Ising Model
The short-time critical dynamics of propagation of damage in the Ising
ferromagnet in two dimensions is studied by means of Monte Carlo simulations.
Starting with equilibrium configurations at and magnetization
, an initial damage is created by flipping a small amount of spins in one
of the two replicas studied. In this way, the initial damage is proportional to
the initial magnetization in one of the configurations upon quenching the
system at , the Onsager critical temperature of the
ferromagnetic-paramagnetic transition. It is found that, at short times, the
damage increases with an exponent , which is much larger
than the exponent characteristic of the initial increase of the
magnetization . Also, an epidemic study was performed. It is found that
the average distance from the origin of the epidemic ()
grows with an exponent , which is the same,
within error bars, as the exponent . However, the survival
probability of the epidemics reaches a plateau so that . On the other
hand, by quenching the system to lower temperatures one observes the critical
spreading of the damage at , where all the measured
observables exhibit power laws with exponents , , and .Comment: 11 pages, 9 figures (included). Phys. Rev. E (2010), in press
Microbiological quality of Portuguese yogurts
The microbiological quality of four brands of natural yogurts and two probiotic yogurts available in the Portuguese
market, was evaluated during the shelf-life period. Although the specific flora decreased during storage it was always
within the range of recommended values. No coliforms and an insignificant number of fungi were detected
Antibiotic resistance of Enterobacteriaceae isolated from the domestic food related environments
Background: Multidrug resistant Enterobacteriaceae which was confined to the hospital environments is now emerging in the domestic food related environments as well. The main objective of the present study was to investigate the prevalence of antibiotic re-sistant Enterobacteriaceae in the domestic food related environments. Methods: Resistance to ampicillin, chloramphenicol, ciprofloxacin, gentamicin, tetracy-cline, nalidixic acid, nitrofurantoin, and trimethoprim was evaluated in 125 isolates; col-lected in domestic food related environments using agar micro dilution method. Results: Results indicated that 49.6% of the isolates were resistant to at least one antibi-otic (32.8% to ampicillin, 6.4% to nitrofurantoin, 4% to tetracycline, 3.2% to nalidixic acid, 2.4% to chloramphenicol and 1.7% to trimethoprim). Resistance to multiple antibi-otics was observed in 6.4% of the isolates. Conclusion: This study implicates existence of antibiotic resistant Enterobactericeae in the domestic food related environments. This resistance phenomenon requires continual vigilance; and further studies are required to evaluate the role of domestic surfaces in the transmission of resistant pathogens and spread of infectious diseases.info:eu-repo/semantics/publishedVersio
Experimental evidence on the development of scale invariance in the internal structure of self-affine aggregates
It is shown that an alternative approach for the characterization of growing
branched patterns consists of the statistical analysis of frozen structures,
which cannot be modified by further growth, that arise due to competitive
processes among neighbor growing structures. Scaling relationships applied to
these structures provide a method to evaluate relevant exponents and to
characterize growing systems into universality classes. The analysis is applied
to quasi-two-dimensional electrochemically formed silver branched patterns
showing that the size distribution of frozen structures exhibits scale
invariance. The measured exponents, within the error bars, remind us those
predicted by the Kardar-Parisi-Zhang equation.Comment: 11 pages, 4 figure
Effect of Gravity and Confinement on Phase Equilibria: A Density Matrix Renormalization Approach
The phase diagram of the 2D Ising model confined between two infinite walls
and subject to opposing surface fields and to a bulk "gravitational" field is
calculated by means of density matrix renormalization methods. In absence of
gravity two phase coexistence is restricted to temperatures below the wetting
temperature. We find that gravity restores the two phase coexistence up to the
bulk critical temperature, in agreement with previous mean-field predictions.
We calculate the exponents governing the finite size scaling in the temperature
and in the gravitational field directions. The former is the exponent which
describes the shift of the critical temperature in capillary condensation. The
latter agrees, for large surface fields, with a scaling assumption of Van
Leeuwen and Sengers. Magnetization profiles in the two phase and in the single
phase region are calculated. The profiles in the single phase region, where an
interface is present, agree well with magnetization profiles calculated from a
simple solid-on-solid interface hamiltonian.Comment: 4 pages, RevTeX and 4 PostScript figures included. Final version as
published. To appear in Phys. Rev. Let
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