366 research outputs found
Quantitative LEED I-V and ab initio study of the Si(111)-3x2-Sm surface structure and the missing half order spots in the 3x1 diffraction pattern
We have used Low Energy Electron Diffraction (LEED) I-V analysis and ab
initio calculations to quantitatively determine the honeycomb chain model
structure for the Si(111)-3x2-Sm surface. This structure and a similar 3x1
recontruction have been observed for many Alkali-Earth and Rare-Earth metals on
the Si(111) surface. Our ab initio calculations show that there are two almost
degenerate sites for the Sm atom in the unit cell and the LEED I-V analysis
reveals that an admixture of the two in a ratio that slightly favours the site
with the lower energy is the best match to experiment. We show that the I-V
curves are insensitive to the presence of the Sm atom and that this results in
a very low intensity for the half order spots which might explain the
appearance of a 3x1 LEED pattern produced by all of the structures with a 3x2
unit cell.Comment: 10 pages, 13 figures. Preliminary work presented at the the APS March
meeting, Baltimore MD, 2006. To be published in Phys. Rev. B. April/May 200
Improved real-space genetic algorithm for crystal structure and polymorph prediction
Existing genetic algorithms for crystal structure and polymorph prediction can suffer from stagnation during evolution, with a consequent loss of efficiency and accuracy. An improved genetic algorithm is introduced herein which penalizes similar structures and so enhances structural diversity in the population at each generation. This is shown to improve the quality of results found for the theoretical prediction of simple model crystal structures. In particular, this method is demonstrated to find three new zero-temperature phases of the Dzugutov potential that have not been previously reported
STM and ab initio study of holmium nanowires on a Ge(111) Surface
A nanorod structure has been observed on the Ho/Ge(111) surface using
scanning tunneling microscopy (STM). The rods do not require patterning of the
surface or defects such as step edges in order to grow as is the case for
nanorods on Si(111). At low holmium coverage the nanorods exist as isolated
nanostructures while at high coverage they form a periodic 5x1 structure. We
propose a structural model for the 5x1 unit cell and show using an ab initio
calculation that the STM profile of our model structure compares favorably to
that obtained experimentally for both filled and empty states sampling. The
calculated local density of states shows that the nanorod is metallic in
character.Comment: 4 pages, 12 figures (inc. subfigures). Presented at the the APS March
meeting, Baltimore MD, 200
Atomistic Molecular Dynamics Simulations of Shock Compressed Quartz
Atomistic non-equilibrium molecular dynamics (NEMD) simulations of shock wave
compression of quartz have been performed using the so-called BKS
semi-empirical potential of van Beest, Kramer and van Santen to construct the
Hugoniot of quartz. Our scheme mimics the real world experimental set up by
using a flyer-plate impactor to initiate the shock wave and is the first shock
wave simulation that uses a geom- etry optimised system of a polar slab in a
3-dimensional system employing periodic boundary conditions. Our scheme also
includes the relaxation of the surface dipole in the polar quartz slab which is
an essential pre-requisite to a stable simulation. The original BKS potential
is unsuited to shock wave calculations and so we propose a simple modification.
With this modification, we find that our calculated Hugoniot is in good
agreement with experimental shock wave data up to 25 GPa, but significantly
diverges beyond this point. We conclude that our modified BKS potential is
suitable for quartz under representative pressure conditions of the Earth core,
but unsuitable for high-pressure shock wave simulations. We also find that the
BKS potential incorrectly prefers the {\beta}-quartz phase over the
{\alpha}-quartz phase at zero-temperature, and that there is a {\beta}
\rightarrow {\alpha} phase-transition at 6 GPa.Comment: 19 pages, 13 figures, Accepted for publication in Journal of Chemical
Physic
Simulation of growth and development of diverse legume species in APSIM
This paper describes the physiological basis and validation of a generic legume model as it applies to 4 species: chickpea (Cicer arietinum L.), mungbean (Vigna radiata (L.) Wilczek), peanut (Arachis hypogaeaL.), and lucerne (Medicago sativa L.). For each species, the key physiological parameters were derived from the literature and our own experimentation. The model was tested on an independent set of experiments, predominantly from the tropics and subtropics of Australia, varying in cultivar, sowing date, water regime (irrigated or dryland), row spacing, and plant population density. The model is an attempt to simulate crop growth and development with satisfactory comprehensiveness, without the necessity of defining a large number of parameters. A generic approach was adopted in recognition of the common underlying physiology and simulation approaches for many legume species. Simulation of grain yield explained 77, 81, and 70% of the variance (RMSD = 31, 98, and 46 g/m2) for mungbean (n = 40, observed mean = 123 g/m2), peanut (n = 30, 421 g/m2), and chickpea (n = 31, 196 g/m2), respectively. Biomass at maturity was simulated less accurately, explaining 64, 76, and 71% of the variance (RMSD = 134, 236, and 125 g/m2) for mungbean, peanut, and chickpea, respectively. RMSD for biomass in lucerne (n = 24) was 85 g/m2 with an R2 of 0.55. Simulation accuracy is similar to that achieved by single-crop models and suggests that the generic approach offers promise for simulating diverse legume species without loss of accuracy or physiological rigour
Effect of disorder on the vortex-lattice melting transition
We use a three dimensional stacked triangular network of Josephson junctions
as a model for the study of vortex structure in the mixed state of high Tc
superconductors. We show that the addition of disorder destroys the first order
melting transition occurring for clean samples. The melting transition splits
in two different (continuous) transitions, ocurring at temperatures Ti and Tp
(>Ti). At Ti the perpendicular-to-field superconductivity is lost, and at Tp
the parallel-to-field superconductivity is lost. These results agree well with
recent experiments in YBaCuO.Comment: 4 pages + 2 figure
Pseudo-cryptic speciation in coccolithophores
Coccolithophores are a group of calcifying unicellular algae that constitute a major fraction of oceanic primary productivity, play an important role in the global carbon cycle, and are key biostratigraphic marker fossils. Their taxonomy is primarily based on the morphology of the minute calcite plates, or coccoliths, covering the cell. These are diverse and include widespread fine scale variation, of which the biological/taxonomic significance is unknown. Do they represent phenotypic plasticity, genetic polymorphisms, or species-specific characters? Our research on five commonly occurring coccolithophores supports the hypothesis that such variation represents pseudocryptic speciation events, occurring between 0.3 and 12.9 million years ago from a molecular clock estimation. This finding suggests strong stabilizing selection acting on coccolithophorid phenotypes. Our results also provide strong support for the use of fine scale morphological characters of coccoliths in the fossil record to improve biostratigraphic resolution and paleoceanographic data retrieval
The phase diagram of high-Tc's: Influence of anisotropy and disorder
We propose a phase diagram for the vortex structure of high temperature
superconductors which incorporates the effects of anisotropy and disorder. It
is based on numerical simulations using the three-dimensional Josephson
junction array model. We support the results with an estimation of the internal
energy and configurational entropy of the system. Our results give a unified
picture of the behavior of the vortex lattice, covering from the very
anysotropic BiSrCaCuO to the less anisotropic YBaCuO, and from the first order
melting ocurring in clean samples to the continuous transitions observed in
samples with defects.Comment: 8 pages with 7 figure
Vortex structure and resistive transitions in high-Tc superconductors
The nature of the resistive transition for a current applied parallel to the
magnetic field in high-Tc materials is investigated by numerical simulation on
the three dimensional Josephson junction array model. It is shown by using
finite size scaling that for samples with disorder the critical temperature Tp
for the c axis resistivity corresponds to a percolation phase transition of
vortex lines perpendicularly to the applied field. The value of Tp is higher
than the critical temperature for j perpendicular to H, but decreases with the
thickness of the sample and with anisotropy. We predict that critical behavior
around Tp should reflect in experimentally accessible quantities, as the I-V
curves.Comment: 8 pages + 6 figure
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