Nearly defect-free dynamical models of disordered solids: The case of amorphous silicon

It is widely accepted in the materials modeling community that defect-free realistic networks of amorphous silicon cannot be prepared by quenching from a molten state of silicon using classical or ab initio molecular-dynamics (MD) simulations. In this work, we address this long-standing problem by producing nearly defect-free ultra-large models of amorphous silicon, consisting of up to half-a-million atoms, using classical molecular-dynamics simulations. The structural, topological, electronic, and vibrational properties of the models are presented and compared with experimental data. A comparison of the models with those obtained from using the modified Wooten-Winer-Weaire bond-switching algorithm shows that the models are on par with the latter, which were generated via event-based total-energy relaxations of atomistic networks in the configuration space. The MD models produced in this work represent the highest quality of amorphous-silicon networks so far reported in the literature using molecular-dynamics simulations.Comment: 8 pages, 8 figure

EuroCALL 2011 Courseware exhibition: the VoiceForum platform for spoken interaction.

Showcased in the courseware exhibition, at the EuroCALL 2011 conference, VoiceForum is a web-based software platform for asynchronous learner interaction in threaded discussions using voice and text. A dedicated space is provided for the tutor who can give feedback on a posted message and dialogue with the participants at a separate level from the main interactional activity. In this way, the pedagogical input is always based on the contextualized needs of the participants, can be accessed as often as required and even searched as a resource for future reference. The software features a built-in sound recorder/player, a rich text editor and management tools to enrol participants, and create forums. Although still in the development stage, this software can be downloaded freely but must be installed on a web server. VoiceForum has been used extensively with French university students of English since 2006

Species Diversity and Relative Abundance of Fisheries Resources Found in Beach Seine along the Central Coast of Ghana

The diversity and relative species abundance of fisheries resources were studied from Winneba to Cape Coast on the central coast of Ghana during December 2007 to May 2009. Samples of organisms were collected at random from beach seine landings during the study period. The fishes were counted and identified to the family and species levels. Ecological indices such as Shannon-Wiener diversity index, equitability and Sørenson’s similarity index were used to analyse the data. Specimens from Winneba, Saltpond and Cape Coast comprise 56 species belonging to 30 families. Carangidae, Haemulidae, Clupeidae and Sciaenidae were some of the families, where key species occurred during the study. The relative abundance of key organisms in the beach seine landings include Chloroscombrus chrysurus (26.0%) in 2007, Brachydeuterus auritus (22.8%) in 2008, Ilisha africana (14.7%) in 2008, Sardinella aurita (13.1%) in 2009 and Selene dorsalis (11.2%) in 2007. The organisms that were in low relative abundance were Acanthurus monroviae, Penaeus notialis, Galeoides decadactylus and Trichiurus lepturus. Shannon-Wiener diversity index, estimated in the study, ranged from 2.54 to 2.83. Species equitability range was 0.67–0.77, and the Sørenson’s similarity estimated ranged was 0.66–0.69. The estimations of fish species diversity and equitability were higher (H’ = 2.83; J’ = 0.77) during the 2009 study in the central coast of Ghana. The similarity indicators in the various paired periods during the study showed considerable similarity in the organisms that were exploited by the beach seine in the area. The study explains the linkage between the diversity and relative species abundance of the coastal fisheries resources and offshore marine resources in Ghana, and the need to regulate beach seine operations in order not to over exploit the juvenile stocks

Experimentally Constrained Molecular Relaxation: The case of hydrogenated amorphous silicon

We have extended our experimentally constrained molecular relaxation technique (P. Biswas {\it et al}, Phys. Rev. B {\bf 71} 54204 (2005)) to hydrogenated amorphous silicon: a 540-atom model with 7.4 % hydrogen and a 611-atom model with 22 % hydrogen were constructed. Starting from a random configuration, using physically relevant constraints, {\it ab initio} interactions and the experimental static structure factor, we construct realistic models of hydrogenated amorphous silicon. Our models confirm the presence of a high frequency localized band in the vibrational density of states due to Si-H vibration that has been observed in a recent vibrational transient grating measurements on plasma enhanced chemical vapor deposited films of hydrogenated amorphous silicon.Comment: 13 pages, 4 figure

Network insensitivity to parameter noise via adversarial regularization

Neuromorphic neural network processors, in the form of compute-in-memory crossbar arrays of memristors, or in the form of subthreshold analog and mixed-signal ASICs, promise enormous advantages in compute density and energy efficiency for NN-based ML tasks. However, these technologies are prone to computational non-idealities, due to process variation and intrinsic device physics. This degrades the task performance of networks deployed to the processor, by introducing parameter noise into the deployed model. While it is possible to calibrate each device, or train networks individually for each processor, these approaches are expensive and impractical for commercial deployment. Alternative methods are therefore needed to train networks that are inherently robust against parameter variation, as a consequence of network architecture and parameters. We present a new adversarial network optimisation algorithm that attacks network parameters during training, and promotes robust performance during inference in the face of parameter variation. Our approach introduces a regularization term penalising the susceptibility of a network to weight perturbation. We compare against previous approaches for producing parameter insensitivity such as dropout, weight smoothing and introducing parameter noise during training. We show that our approach produces models that are more robust to targeted parameter variation, and equally robust to random parameter variation. Our approach finds minima in flatter locations in the weight-loss landscape compared with other approaches, highlighting that the networks found by our technique are less sensitive to parameter perturbation. Our work provides an approach to deploy neural network architectures to inference devices that suffer from computational non-idealities, with minimal loss of performance. ..

A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorption on the Relaxed (0001) Surface of Double Hexagonal Close Packed Americium

Ab initio total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorption on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals method. Chemisorption energies were optimized with respect to the distance of the adatom from the relaxed surface for three adsorption sites, namely top, bridge, and hollow hcp sites, the adlayer structure corresponding to coverage of a 0.25 monolayer in all cases. Chemisorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The two-fold bridge adsorption site was found to be the most stable site for O at both the NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and 8.368 eV respectively, while the three-fold hollow hcp adsorption site was found to be the most stable site for H with chemisorption energies of 3.136 eV at the NSOC level and 3.217 eV at the SOC level. The respective distances of the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang. Overall our calculations indicate that chemisorption energies in cases with SOC are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range. The work functions and net magnetic moments respectively increased and decreased in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The partial charges inside the muffin-tins, difference charge density distributions, and the local density of states have been used to analyze the Am-adatom bond interactions in detail. The implications of chemisorption on Am 5f electron localization-delocalization are also discussed.Comment: 9 Tables, 5 figure

Assessment of Groundwater Quality and its Suitability for Domestic and Agricultural Purposes in parts of the Central Region, Ghana

Groundwater in parts of the Central Region of Ghana was assessed to determine its suitability for domestic use and irrigation activities. Stiff and Piper diagrams show that the predominant water type in the area is Na-Cl, which is characterized by relatively high salinities. This water type occurs in aquifers underlying six coastal districts, namely; Abura-Asebu-Kwamankese (AA), Mfantseman (MF), Gomoa East (GE), Awutu Senya (AwS), Effutu (EF) and Cape Coast (CC). Other water types are Ca-HCO3, Ca-SO4 and Ca-Cl, that occur in districts located further away from the coast. Silicate weathering and ion exchange are identified as the geochemical processes responsible for the various ions in the groundwater. Possible processes that could be responsible for the source of Na+ and Cl- are sea water intrusion and dissolution of minerals. The results also show that the groundwater is mainly supersaturated with respect to both calcite and gypsum, indicating high concentrations of both bicarbonate and sulfate ions in solution. This is associated with considerable enrichment of the groundwater with sulfate, bicarbonate, and calcium ions. On a Wilcox diagram, the groundwater samples, mainly from the districts coded as AA, TLD, AS and AN, which are located away from the coast, plot within the “excellent to good” category, suggesting that water from the area is of acceptable quality for irrigation purposes. Similarly, Sodium Adsorption Ratio (SAR) and conductivity values for the groundwater in the AA, TLD, AS and AN districts indicate low SAR and low to medium salinity respectively. Groundwater in aquifers underlying these districts is therefore suitable for irrigation. Generally, groundwater samples from locations along the coast have high salinity and are not suitable for domestic and irrigation purposes. However, about 72% of the water samples predominantly in districts located away from the sea is suitable for the purposes stated