Alternative calculation of the physical mass of the p-meson

We have derived an expression for the physical mass and width of the ρ-meson in vacuum from its spectral function, calculated in the vector meson dominance model when a ρ 0 meson couples to two virtual pions π +–π −. The propagator is computed after evaluating the ρ-meson self-energy. The real part of the ρ-meson self-energy is given by a divergent integral and needs to be regularized; the regularization is done by using a double subtracted dispersion relation. The result leads to a closed analytical expression which allow us to evaluate the spectral function in a closed way. The physical mass, defined as the magnitude of the four-momentum |k| for which the spectral function S(k 2 ) attains its maximum value, is obtained, and it gives a value of 770 MeV, which is in total agreement with the reported experimental value of the ρ-meson mas

Molecular dynamics of coalescence and collisions of silver nanoparticles

We study how different relative orientations and impact velocity on the collision of two silver nanoparticles affect the first stages of the formation of a new, larger nanoparticle. In order to do this, we implemented a set of molecular dynamics simulations on the NVE ensemble on pairs of silver icosahedral nanoparticles at several relative orientations, that allowed us to follow the dynamics of the first nanoseconds of the coalescence processes. Using bond angle analysis, we found that the initial relative orientation of the twin planes has a critical role on the final stability of the resulting particle, and on the details of the dynamics itself. When the original particles have their closest twins aligned to each other, the formed nanoparticle will likely stabilize its structure onto a particle with a defined center and a low surface-to-volume ratio, while nanoparticles with misaligned twins will promote the formation of highly defective particles with a high inner energy

Structural and vibrational study of PCBM

The 1-(3-methoxycarbonyl) propyl-1-phenyl-(6,6)C61 (also called PCBM) is a C60 derivative widely used as an electron-acceptor in organic solar cells. To date, all the infrared spectra reported are experimental, mainly because of the calculations needed to study these structures are highly time-consuming. In this report we address for the first time the infrared spectrum calculation of PCBM with Cs symmetry by using the PW91/dnp level as implemented in the Dmol3 code. In this calculation we have found two intense peaks in the IR spectrum, that agree fairly with the 1187 and 1787 cm-1 measured experimentally

Molecular dynamics of free and graphite-supported Pt-Pd nanoparticles

The thermal characteristics of bimetallic Pt-Pd nanoparticles, both free and graphite-supported, were investigated through molecular dynamics simulations using quantum Sutton-Chen many-body potentials for the metal-metal interac- tions. The graphite substrate was represented as layers of fixed carbons sites and modeled with the Lennard-Jones po- tential model. The melting temperatures for bimetallic nanoparticles were estimated based on variations in thermody- namic properties such as potential energy and heat capacity. Melting temperatures of the nanoparticles were found to be considerably lower than those of bulk Pt and Pd. The Pt-Pd clusters exhibited a two-stage melting, where surface melt- ing of the external atoms is followed by homogeneous melting of the internal atoms. The melting transition temperature was found to increase when the particle is on the graphite support, with an increase at least 180 K higher than that of the same-sized free nanoparticle. The results of the density distributions perpendicular to the surface indicate that the Pd atoms have a tendency to remain at the surface, and the Pd atoms wet the graphite surface more than the Pt atoms, while root mean squares suggest that surface melting starts from the cluster surface, and surface melting was seen in both free and graphite-supported nanoparticles. Structural changes accompanying the thermal evolution were studied by the bond-orientational order parameter method

Nanomaterial properties: size and shape dependencies

Nanoscience and nanotechnology are among the most widely used terms in the modern scientific and technological literature. The idea of nanotechnology appeared for the first time in the famous talk “There is plenty of room at the bottom” given by the physicist Richard Feynman at the American Physical Society meeting at Caltech on December 29, 1959. Feynman described a process by which the ability to manipulate individual atoms and molecules might be developed, using one set of precise tools to build and operate another proportionally smaller set and so on down to the needed scale. In the course of this, he noted, scaling issues would arise from the changing magnitude of various physical phenomena: gravity would become less important whereas surface effects would become increasingly more significan

On the structure and normal modes of hydrogenated Ti-fullerene compounds

When titanium covers a C60 core, the metal atoms may suppress the fullerene’s capacity of storing hydrogen, depending on the number of Ti atoms covering the C60 framework, the Ti–C binding energy, and diffusion barriers. In this article, we study the structural and vibrational properties of the C60TiHn (n = 2, 4, 6, and 8) and C60Ti6H48 compounds. The IR spectra of C60TiHn compounds have a maximum attributable to the Ti–H stretching mode, which shifts to lower values in the structures with n = 4, 8, while their Raman spectra show two peaks corresponding to the stretching modes of H2 molecules at apical and azimuthal positions. On the other hand, the IR spectrum of C60Ti6H48 shows an intense peak due to the Ti–H in-phase stretching mode, while its Raman spectrum has a maximum attributed to the pentagonal pinch of the C60 core. Finally, we have found that the presence of one apical H2 molecule enhances the pentagonal pinch mode, becoming the maximum in the Raman spectrum

Defect structure in nanoalloys

The defect structure of bimetallic nanoparticles differs from that of particles made of a single atomic species. Using high resolution TEM imaging along with molecular dynamics simulations, it is possible to investigate the nature and features of these defects. The definition of a local order parameter allows one to locate regions with different kinds of stacking on the simulated nanoparticles and thus to make a direct comparison with the experimental observations