Gabriel Fabricius

Metal impurities in an oxide: Ab initiostudy of electronic and structural properties of Cd in rutileTiO2

In this work we undertake the problem of a transition metal impurity in an oxide. We present an ab initio study of the relaxations introduced in

Modelling pertussis transmission to evaluate the effectiveness of an adolescent booster in Argentina.

{\mathrm{TiO}}_{2}

Atomic layering at the liquid silicon surface: A first-principles simulation

when a Cd impurity substitutionally replaces a Ti atom. Using the full-potential linearized-augmented-plane-wave method, we obtain relaxed structures for different charge states of the impurity and computed the electric-field gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are dependent on the charge state of the impurity. This dependence is very remarkable in the case of the EFG and is explained by analyzing the electronic structure of the studied system. We predict fairly anisotropic relaxations for the nearest oxygen neighbors of the Cd impurity. The experimental confirmation of this prediction and a brief report of these calculations have recently been presented [Phys. Rev. Lett. 89, 55503 (2002)]. Our results for relaxations and EFGs are in clear contradiction with previous studies of this system that assumed isotropic relaxations, and point out that no simple model is viable to describe relaxations and the EFG at Cd in

Fickian crossover and length scales from two point functions in supercooled liquids

{\mathrm{TiO}}_{2}

Distance between inherent structures and the influence of saddles on approaching the mode coupling transition in a simple glass former

even approximately.

Modelling the effect of changes in vaccine effectiveness and transmission contact rates on pertussis epidemiology.

Due to the current epidemiological situation of pertussis, several countries have implemented vaccination strategies that include a booster dose for adolescents. Since there is still no evidence showing that the adolescent booster has a positive effect on the most vulnerable group represented by infants, it is difficult to universalize the recommendation to include such reinforcement. In this work we present an age-structured compartmental deterministic model that considers the outstanding epidemiological features of the disease in order to assess the impact of the booster dose at age 11 years (Tdap booster) to infants. To this end, we performed different parameterizations of the model that represent distinct possible epidemiological scenarios. The results obtained show that the inclusion of a single Tdap dose at age 11 years significantly reduces the incidence of the disease within this age group, but has a very low impact on the risk group (0-1 year). An effort to improve the coverage of the first dose would have a much greater impact on infants. These results hold in the 18 scenarios considered, which demonstrates the robustness of these conclusions.

Critical behavior of an Ising system on the Sierpinski carpet: a short-time dynamics study.

We simulate the liquid silicon surface with first-principles molecular dynamics in a slab geometry. We find that the atom-density profile presents a pronounced layering, similar to those observed in low-temperature liquid metals like Ga and Hg. The depth-dependent pair correlation function shows that the effect originates from directional bonding of Si atoms at the surface, and propagates into the bulk. The layering has no major effects in the electronic and dynamical properties of the system, that are very similar to those of bulk liquid Si. To our knowledge, this is the first study of a liquid surface by first-principles molecular dynamics.

FLAPW Study of the EFG Tensor at Cd Impurities in In2O3

Particle motion of a Lennard-Jones supercooled liquid near the glass transition is studied by molecular dynamics simulations. We analyze the wave vector dependence of relaxation times in the incoherent self-scattering function and show that at least three different regimes can be identified and its scaling properties determined. The transition from one regime to another happens at characteristic length scales. The length scale associated with the onset of Fickian diffusion corresponds to the maximum size of heterogeneities in the system, and the characteristic time scale is several times larger than the alpha relaxation time. A second crossover length scale is observed, which corresponds to the typical time and length of heterogeneities, in agreement with results from four point functions. The different regimes can be traced back to the behavior of the van Hove distribution of displacements, which shows a characteristic exponential regime in the heterogeneous region before the crossover to Gaussian diffusion and should be observable in experiments. Our results show that it is possible to obtain characteristic length scales of heterogeneities through the computation of two point functions at different times.

Time correlation functions between inherent structures: a connection between landscape topology and the dynamics of glassy systems

We analyze through molecular dynamics simulations of a Lennard-Jones (LJ) binary mixture the statistics of the distances between inherent structures sampled at temperatures above the mode coupling transition temperature T(MCT). After equilibrating at T>T(MCT) we take equilibrated configurations and randomly perturb the coordinates of a given number of particles. After that we find the nearby inherent structures (IS) of both the original and perturbed configurations and evaluate the distance between them. This distance presents an inflection point at T(li) approximately 1 with a strong decrease below this temperature which goes to a small but nonzero value on approaching T(MCT). In the low-temperature region we study the statistics of events which give zero distance, i.e., dominated by minima, and find evidence that the number of saddles decreases exponentially near T(MCT). This implies that saddles continue to exist even at T<or=T(MCT). As at T(MCT) the diffusivity goes to zero, our results imply that there are saddles associated with nondiffusive events at T

Discrete scale invariance effects in the nonequilibrium critical behavior of the Ising magnet on a fractal substrate.

The incidence of the highly infectious respiratory disease named pertussis or whooping cough has been increasing for the past two decades in different countries, as in much of the highly vaccinated world. A decrease in vaccine effectiveness over time, especially when acellular vaccines were used for primary doses and boosters, and pathogen adaptation to the immunity conferred by vaccines have been proposed as possible causes of the resurgence. The contributions of these factors are not expected to be the same in different communities, and this could lead to different epidemiological trends. In fact, differences in the magnitude and dynamics of pertussis outbreaks as well as in the distribution of notified cases by age have been reported in various regions. Using an age-structured mathematical model designed by us, we evaluated how the changes in some of the parameters that could be related to the above proposed causes of disease resurgence - vaccine effectiveness and effective transmission rates - may impact on pertussis transmission. When a linear decrease in vaccine effectiveness (VE) was assayed, a sustained increase in pertussis incidence was detected mainly in infants and children. On the other hand, when changes in effective transmission rates (βij) were made, a dynamic effect evidenced by the presence of large peaks followed by deep valleys was detected. In this case, greater incidence in adolescents than in children was observed. These different trends in the disease dynamics due to modifications in VE or βij were verified in 18 possible scenarios that represent different epidemiological situations. Interestingly we found that both incidence trends produced by the model and their age distribution resemble the profiles obtained from data reported in several regions. The implications of these correlations are discussed.