We develop an empirical potential for silicon which represents a considerable improvement over existing models in describing local bonding for bulk defects and disordered phases. The model consists of two- and three-body interactions with theoretically motivated functional forms that capture chemical and physical trends as explained in a companion paper. The numerical parameters in the functional form are obtained by fitting to a set of ab initio results from quantum-mechanical calculations based on density-functional theory in the local-density approximation, which include various bulk phases and defect structures. We test the potential by applying it to the relaxation of point defects, core properties of partial dislocations and the structure of disordered phases, none of which are included in the fitting procedure. For dislocations, our model makes predictions in excellent agreement with ab initio and tight-binding calculations. It is the only potential known to describe both the $30°$- and $90°$-partial dislocations in the glide set {111}. The structural and thermodynamic properties of the liquid and amorphous phases are also in good agreement with experimental and ab initio results. Our potential is capable of simulating a quench directly from the liquid to the amorphous phase, and the resulting amorphous structure is more realistic than with existing empirical preparation methods. These advances in transferability come with no extra computational cost, since force evaluation with our model is faster than with the popular potential of Stillinger-Weber, thus allowing reliable atomistic simulations of very large atomic systems.

We use recent theoretical advances to develop a functional form for interatomic forces in bulk silicon. The theoretical results underlying the model include an analysis of elastic properties for the diamond and graphitic structures and inversions of ab initio cohesive energy curves. The interaction model includes two-body and three-body terms which depend on the local atomic environment through an effective coordination number. This formulation is able to capture successfully (i) the energetics and elastic properties of the ground-state diamond lattice, (ii) the covalent rehybridization of undercoordinated atoms, and (iii) a smooth transition to metallic bonding for overcoordinated atoms. Because the essential features of chemical bonding in the bulk are built into the functional form, this model promises to be useful for describing interatomic forces in silicon bulk phases and defects. Although this functional form is remarkably realistic by the usual standards, it contains a small number of fitting parameters and requires computational effort comparable to the most efficient existing models. In a companion paper, a complete parametrization of the model is given, and excellent performance for condensed phases and bulk defects is demonstrated.

The thermoelastic properties of ferropericlase Mg _{1− x} Fe _x O ( x = 0.1875) throughout the iron high-to-low spin cross-over have been investigated by first principles at Earth's lower mantle conditions. This cross-over has important consequences for elasticity such as an anomalous bulk modulus ( K _S ) reduction. At room temperature the anomaly is somewhat sharp in pressure but broadens with increasing temperature. Along a typical geotherm it occurs across most of the lower mantle with a more significant K _S reduction at ≈1,400–1,600 km depth. This anomaly might also cause a reduction in the effective activation energy for diffusion creep and lead to a viscosity minimum in the mid-lower mantle, in apparent agreement with results from inversion of data related with mantle convection and postglacial rebound.

This letter presents an Exponential Slot Edge Antipodal Vivaldi Antenna (ESE-AVA), with improved radiative features as compared to the conventional Antipodal Vivaldi Antenna (AVA) design. It extends the low-end bandwidth limitation, mitigates the side and back lobe levels, corrects squint effect, and increases its main lobe gain. In order to confirm those features, a comparative study among the ESE-AVA, the low directivity conventional AVA and two popular modifications, regular slot edge (RSE) and the tapered slot edge (TSE) AVA is performed. A comparison between AVA and the proposed ESE-AVA at 6 GHz shows an improved gain of 8.3 dB, -15.5 dB of Side Lobe Level (SLL), and 0 degrees of main lobe squint (MLS), in contrast with 5 dB of gain, - 5 dB of SLL, and 5 degrees of MLS in the conventional AVA. By comparing the ESE-AVA with RSE-AVA and TSE-AVA, it was observed that its notches in exponential shape, similar to open the main radiator, besides mitigating the SLL also directs the E-fields distributions towards the main lobe. It reflects into a main lobe gain improvement.

We developed an empirical potential for interactions between Si and N to describe silicon nitride systems using the Tersoff functional form. The fitting parameters were found using a set of ab initio and experimental results of the crystalline phase. Using this empirical model, we explored the structural properties of amorphous silicon nitride through Monte Carlo simulations, and compared them to available experimental data. The good description of the $a-{\mathrm{SiN}}_x$ system for a wide range of nitrogen contents $\mathrm{}(0<x<\mathrm{}1.5\mathrm{})\mathrm{}$ shows the reliability of this model.