220 research outputs found
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
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
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
Small-Angle X-Ray Scattering in Amorphous Silicon: A Computational Study
We present a computational study of small-angle x-ray scattering (SAXS) in amorphous silicon (α-Si) with particular emphasis on the morphology and microstructure of voids. The relationship between the scattering intensity in SAXS and the three-dimensional structure of nanoscale inhomogeneities or voids is addressed by generating large high-quality (α-Si networks with 0.1%–0.3% volume concentration of voids, as observed in experiments using SAXS and positron annihilation spectroscopy. A systematic study of the variation of the scattering intensity in the small-angle scattering region with the size, shape, number density, and the spatial distribution of the voids in the networks is presented. Our results suggest that the scattering intensity in the small-angle region is particularly sensitive to the size and the total volume fraction of the voids, but the effect of the geometry or shape of the voids is less pronounced in the intensity profiles. A comparison of the average size of the voids obtained from the simulated values of the intensity, using the Guinier approximation and Kratky plots, with that of the same from the spatial distribution of the atoms in the vicinity of void surfaces is presented
Electronic structure and magnetic state of transuranium metals under pressure
Electronic structure of bcc Np, fcc Pu, Am, and Cm pure metals under pressure
has been investigated employing the LDA+U method with spin-orbit coupling
(LDA+U+SO). Magnetic state of the actinide ions was analyzed in both LS and jj
coupling schemes to reveal the applicability of corresponding coupling bases.
It was demonstrated that whereas Pu and Am are well described within the jj
coupling scheme, Np and Cm can be described appropriately neither in {m-sigma},
nor in {jmj} basis, due to intermediate coupling scheme realizing in these
metals that requires some finer treatment. The LDA+U+SO results for the
considered transuranium metals reveal bands broadening and gradual 5f electron
delocalization under pressure.Comment: 5 pages, 5 figure
Information-Driven Inverse Approach to Disordered Solids: Applications to Amorphous Silicon
© Physical Review Materials
Publisher\u27s Versio
Atomistic modeling of amorphous silicon carbide: An approximate first-principles study in constrained solution space
Localized basis ab initio molecular dynamics simulation within the density
functional framework has been used to generate realistic configurations of
amorphous silicon carbide (a-SiC). Our approach consists of constructing a set
of smart initial configurations that conform essential geometrical and
structural aspects of the materials obtained from experimental data, which is
subsequently driven via first-principles force-field to obtain the best
solution in a reduced solution space. A combination of a priori information
(primarily structural and topological) along with the ab-initio optimization of
the total energy makes it possible to model large system size (1000 atoms)
without compromising the quantum mechanical accuracy of the force-field to
describe the complex bonding chemistry of Si and C. The structural, electronic
and the vibrational properties of the models have been studied and compared to
existing theoretical models and available data from experiments. We demonstrate
that the approach is capable of producing large, realistic configurations of
a-SiC from first-principles simulation that display excellent structural and
electronic properties of a-SiC. Our study reveals the presence of predominant
short-range order in the material originating from heteronuclear Si-C bonds
with coordination defect concentration as small as 5% and the chemical disorder
parameter of about 8%.Comment: 16 pages, 7 figure
Density functional study of the actinide nitrides
The full potential all electron linearized augmented plane wave plus local
orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K,
has been used to systematically investigate the structural, electronic, and
magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu,
Am). The theoretical formalism used is the generalized gradient approximation
to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional. Each compound has been studied at six levels
of theory: non-magnetic (NM), non-magnetic with spin-orbit coupling (NM+SOC),
ferromagnetic (FM), ferromagnetic with spin-orbit coupling (FM+SOC),
anti-ferromagnetic (AFM), and anti-ferromagnetic with spin-orbit coupling
(AFM+SOC). The structural parameters, bulk moduli, densities of states, and
charge distributions have been computed and compared to available experimental
data and other theoretical calculations published in the literature. The total
energy calculations indicate that the lowest energy structures of AcN, ThN, and
PaN are degenerate at the NM+SOC, FM+SOC, and AFM+SOC levels of theory with
vanishing total magnetic moments in the FM+SOC and AFM+SOC cases, making the
ground states essentially non-magnetic with spin-orbit interaction. The ground
states of UN, NpN, PuN, and AmN are found to be FM+SOC at the level of theory
used in the present computations. The nature of the interactions between the
actinide metals and nitrogen atom, and the implications on 5f electron
delocalization and localization are discussed in detail.Comment: 5 tables, 12 figure
Footballers' Image Rights in the New Media Age
Image rights, broadly defined as the commercial appropriation of someone's personality, including indices of their image, voice, name and signature, have become increasingly important in the political economy of media sport. A range of legal, economic and political arguments have developed in the UK as to what image rights actually are, their legal efficacy and their potential impact on developments in the long-standing relationship between sport and the media. This paper focuses on the problematic definition of the term in the UK context and how it relates to certain economic and commercial transformations in British football. Using the English Premier League and the ‘celebrity footballer’ David Beckham as its primary focus, the paper traces the rise of image rights clauses in player contracts. This process is analysed in the context of rapid and dramatic change in the media coverage of the sport. The paper focuses on the growing legal complexities of protecting star images in relation to the Internet and the wider issues of football, fandom and popular culture
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