We present an ab initio study of the relaxations introduced in TiO (2) when a Cd impurity substitutes a Ti atom and an experimental test of this calculation by a perturbed-angular-correlation (PAC) measurement of the orientation of the electric-field gradient (EFG) tensor at the Cd site. The ab initio calculation predicts strong anisotropic relaxations of the nearest oxygen neighbors of the impurity and a change of the orientation of the largest EFG tensor component, V(33 ), from the [001] to the [110] direction upon substitution of a Ti atom by a Cd impurity. The last prediction is confirmed by the PAC experiment that shows that V(33 ) at the Cd site is parallel to either the [110] or the [1 1;0] crystal axis.

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 TiO2 when a Cd impurity replaces substitutionally 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 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 [P.R.L. 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 TiO2 even approximately.

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.PACS numbers: 61.25.Mv, 61.20.Ja, 71.22.+i Liquid metal surfaces have attracted much attention during the last years [1][2][3][4][5][6]. Their particular properties, very different from those of non-metals, have been investigated byÅngström-resolution experiments, and simulated by different approaches. One of their most interesting features is the atomic layering, a density oscillation that originates at the sharp liquid-vapor interface, and extends several atomic diameters into the bulk. Although some experiments supported an increased surface density at l-Hg [1], and some kind of atomic layering was predicted theoretically [7], its existence has been demonstrated unambiguously only recently by X-Ray reflectivity in Hg [3], Ga [4,8] and Ga-In alloys [9]. Most of the experiments have been done close to the low melting temperatures of these metals, but Regan et al [8] studied the Ga surface up to 170• C. They found that capilarity waves strongly decrease the reflectivity peak heights, but not the peak widths, suggesting that the decay length of local layering is temperature-independent, and that surface layering is still present quite above the melting point.Different explanations have been proposed to account for this effect [10]. Rice et al [5] have argued that the abrupt decay of the delocalized electron density forms a flat potential barrier against which the ions lay orderly, like hard spheres against a hard wall. Tosatti et al [6] have used the glue model of metallic cohesion to argue that surface atoms, trying to effectively recover their optimal coordination, alternatively increase and decrease their density. Surface layering effects, like surfaceenhanced smectic ordering, have also been observed in liquid crystals [11]. In this case, its origin is the tendency of the highly nonspherical molecules to present a particular orientation towards the surface.Liquid silicon (l-Si) is a rather peculiar system. Silicon transforms, at 1684 K, from a covalent semiconductor solid, with diamond structure and coordination 4, to a liquid metal. Experiments [12,13] and MD simulations [14,15] show that its coordination (∼ 6 − 7) is lower than that of typical liquids (∼ 12), due to the persistence of directional bonding in the liquid phase. In spite of the enormous literature on its solid surfaces, very little is known on the structure of the liquid silicon surface. Measurements ar...

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.