Angular rigidity in tetrahedral network glasses

A set of oxide and chalcogenide tetrahedral glasses are investigated using molecular dynamics simulations. It is shown that unlike stoichiometric selenides such as GeSe$_2$ and SiSe$_2$, germania and silica display large standard deviations in the associated bond angle distributions. Within bond-bending constraints theory, this pattern can be interpreted as a manifestation of {\it {broken}} (i.e. ineffective) oxygen bond-bending constraints. The same analysis reveals that the changes in the Ge composition affects mostly bending around germanium in binary Ge-Se systems, leaving Se-centred bending almost unchanged. In contrast, the corresponding Se twisting (quantified by the dihedral angle) depends on the Ge composition and is reduced when the system becomes rigid. Our results establishes the atomic-scale foundations of the phenomelogical rigidity theory, thereby profoundly extending its significance and impact on the structural description of network glasses.Comment: 5 pages, 4 figure

Molecular-dynamics study of collision, implantation, and fragmentation of Ag7 on Pd(100)

By using embedded-atom-method interatomic potentials and molecular-dynamics simulations, we study the collision of a single Ag/sub 7/ cluster on the Pd(100) substrate, at impact energies in the direction perpendicular to the (100) surface E/sub 1/=20 eV (2.86 eV/atom) and E/sub 2/=95 eV (13.6 eV/atom). Our results indicate that implantation occurs at both impact energies, but it is more important at E/sub 2/. As opposed to what is shown experimentally, little fragmentation takes place for E/sub 1/, while the calculated cluster fragmentation is in good agreement with the experimental data for E/sub 2/

Note: Accounting for pressure effects on the calculated equilibrium structure of glassy GeSe2:

First-principles molecular dynamics (FPMD) data on the structural properties of glassy GeSe2 under ambient conditions are produced by carefully considering and minimizing the effect of a residual pressure on the periodic system. When compared to previous FPMD results, this strategy leads to an improved agreement between theory and neutron diffraction experiments

Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations

Photoelectron (PES) spectra from aluminum cluster anions (from 12 to 15 atoms) at various temperature regimes, were studied using ab-initio molecular dynamics simulations and experimentally. The calculated PES spectra, obtained via shifting of the simulated electronic densities of states by the self-consistently determined values of the asymptotic exchange-correlation potential, agree well with the measured ones, allowing reliable structural assignments and theoretical estimation of the clusters' temperatures.Comment: RevTex, 3 gif figures. Scheduled for Oct 15, 1999, issue of Phys. Rev. B as Rapid Communicatio

Preliminary study on the use of the 137 Cs method for soil erosion investigation in the pampean region of Argentina

Soil erosion is the most important degradation process in A rgentine. According to the estimation of 4.9 millon ha in Pampa Ondulada Region, 1.600.000 ha (36% of agricultural soils) are affected by the erosion. Field measurements of soil erosion and sediment deposition using classical techniques are difficult, time consuming, and expensive but indispensable to feed the prediction models for conservation practices design and fa rm planning. Many authors have reported that the measurement of fallout nuclides is useful tool to characterize geomorphical processes. Walling and He proposes models for conve rting 137Cs depletion/enrichment amounts to net soil loss/deposition. These models are based in the comparison between a reference 137Cs profile in a long term undisturbed site (control site) and the 137Cs profiles in the suspected eroded or deposited sites in the landscape. The aim of this study is to provide a complete and well representative set of data on the erosion intensity in topographical conditions for the Pampa Ondulada Region in A rgentine by using a tracer technique. The study area is a small watershed (about 300 ha), located in Arroyo del Tala medium basin, within Partido of San Pedro in Buenos Aires Province, Argentine. This paper presents a group of results from a detailed investigation of erosion and sediment delivery, within a 49 ha cultivated field study site in this watershed. The base of sampling strategy is the grid approach. A reference inventory, representing the local fallout input, was searched for at a site experiencing neither erosion nor deposition. Radiocaesium analyses were made at the Nuclear Regulatory Authority Laboratory by a GE Hp detector. To make an interpretation of 137Cs distribution of soil losses and sedimentation, the Mass Balance Model 2 was used (Walling and He 1997). The erosion/deposition rates from Mass Balance Model 2 are in the range of 0 to -30 t·ha-1·y-1 for erosion, and 0 to 19 t·ha-1·y-1 for deposition, and these values matched well, with the rates of erosion obtained by classical methods. The 137Cs spatial and depth distribution are showed in a map, and enabled to study the relationship of the erosion to the topography, and a good discrimination in subclasses within moderate erosion class and sedimentation class

Investigation of size effects on the structure of liquid GeSe2 calculated via first-principles molecular dynamics

The structural properties of liquid GeSe2 have been calculated by first-principles molecular dynamics by using a periodic simulation box containing N = 480 atoms. This has allowed a comparison with previous results obtained on a smaller system size (N = 120) [M. Micoulaut, R. Vuilleumier, and C. Massobrio, Phys. Rev. B 79, 214205 (2009)]. In the domain of first-principles molecular dynamics, we obtain an assessment of system size effects of unprecedented quality. Overall, no drastic differences are found between the two sets of results, confirming that N = 120 is a suitable size to achieve a realistic description of this prototypical disordered network. However, for N = 480, short range properties are characterized by an increase of chemical order, the number of Ge tetrahedra coordinated to four Se atoms being larger. At the intermediate range order level, size effect mostly modify the low wavevector region (k similar to 1 angstrom(-1)) in the concentration-concentration partial structure factor

Distribución de poros en una ladera de pampa ondulada cultivada con siembra directa

243-248In order to select the most appropriate management techniques for eroded soils it is essential to first of all identify those soil attributes modified by this degradation process. The aim of this study was to determine the effect of different degrees of water erosion in a Vertic Argiudoll under direct sowing on the distribution of pores at two depths (0-5 cm and 5-10 cm). Sampling was carried out in three parallel transects in the slope direction. The volume occupied by different poresize classes, total porosity and depth to the argilic horizon were measured at 8 m-intervals in each transect. Soil texture was also measured at the two depths in different sections of the study area. The depth of the argilic horizon ranged between 32 and 19 cm and clay content varied between 22.6 (0-5 cm) and 28.0 percent (5-10 cm). However, no significant differences were found in the volume occupied by different pore-size classes between soils with varying degrees of erosion. Grouping the findings in terms of total porosity shows that greater porosity in each stratum is associated with a higher total volume (P less than 0.05) of pores larger than 51 um

Distribución de poros en una ladera de pampa ondulada cultivada con siembra directa

243-248In order to select the most appropriate management techniques for eroded soils it is essential to first of all identify those soil attributes modified by this degradation process. The aim of this study was to determine the effect of different degrees of water erosion in a Vertic Argiudoll under direct sowing on the distribution of pores at two depths (0-5 cm and 5-10 cm). Sampling was carried out in three parallel transects in the slope direction. The volume occupied by different poresize classes, total porosity and depth to the argilic horizon were measured at 8 m-intervals in each transect. Soil texture was also measured at the two depths in different sections of the study area. The depth of the argilic horizon ranged between 32 and 19 cm and clay content varied between 22.6 (0-5 cm) and 28.0 percent (5-10 cm). However, no significant differences were found in the volume occupied by different pore-size classes between soils with varying degrees of erosion. Grouping the findings in terms of total porosity shows that greater porosity in each stratum is associated with a higher total volume (P less than 0.05) of pores larger than 51 um