521 research outputs found
Collective Correlations of Brodmann Areas fMRI Study with RMT-Denoising
We study collective behavior of Brodmann regions of human cerebral cortex
using functional Magnetic Resonance Imaging (fMRI) and Random Matrix Theory
(RMT). The raw fMRI data is mapped onto the cortex regions corresponding to the
Brodmann areas with the aid of the Talairach coordinates. Principal Component
Analysis (PCA) of the Pearson correlation matrix for 41 different Brodmann
regions is carried out to determine their collective activity in the idle state
and in the active state stimulated by tapping. The collective brain activity is
identified through the statistical analysis of the eigenvectors to the largest
eigenvalues of the Pearson correlation matrix. The leading eigenvectors have a
large participation ratio. This indicates that several Broadmann regions
collectively give rise to the brain activity associated with these
eigenvectors. We apply random matrix theory to interpret the underlying
multivariate data
Ecological Studies of Blue Grama Grass (Bouteloua gracilis)
Blue grama grass (Bouteloua gracilis) was selected for detailed description here because of its wide distribution and importance throughout the Great Plains Region in the United States.https://scholars.fhsu.edu/fort_hays_studies_series/1058/thumbnail.jp
Electronic Structure of Dangling Bonds in Amorphous Silicon Studied via a Density-Matrix Functional Method
A structural model of hydrogenated amorphous silicon containing an isolated
dangling bond is used to investigate the effects of electron interactions on
the electronic level splittings, localization of charge and spin, and
fluctuations in charge and spin. These properties are calculated with a
recently developed density-matrix correlation-energy functional applied to a
generalized Anderson Hamiltonian, consisting of tight-binding one-electron
terms parametrizing hydrogenated amorphous silicon plus a local interaction
term. The energy level splittings approach an asymptotic value for large values
of the electron-interaction parameter U, and for physically relevant values of
U are in the range 0.3-0.5 eV. The electron spin is highly localized on the
central orbital of the dangling bond while the charge is spread over a larger
region surrounding the dangling bond site. These results are consistent with
known experimental data and previous density-functional calculations. The spin
fluctuations are quite different from those obtained with unrestricted
Hartree-Fock theory.Comment: 6 pages, 6 figures, 1 tabl
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
Surfactant effect in heteroepitaxial growth. The Pb - Co/Cu(111) case
A MonteCarlo simulations study has been performed in order to study the
effect of Pb as surfactant on the initial growth stage of Co/Cu(111). The main
characteristics of Co growing over Cu(111) face, i.e. the decorated double
layer steps, the multiple layer islands and the pools of vacancies, disappear
with the pre-evaporation of a Pb monolayer. Through MC simulations, a full
picture of these complex processes is obtained. Co quickly diffuses through the
Pb monolayer exchanging place with Cu atoms at the substrate. The exchange
process diffusion inhibits the formation of pure Co islands, reducing the
surface stress and then the formation of multilayer islands and the pools of
vacancies. On the other hand, the random exchange also suppress the nucleation
preferential sites generated by Co atoms at Cu steps, responsible of the step
decoration.Comment: 4 pages, latex, 2 figures embedded in the tex
Bandgap Change of Carbon Nanotubes: Effect of Small Tensile and Torsional Strain
We use a simple picture based on the electron approximation to study
the bandgap variation of carbon nanotubes with uniaxial and torsional strain.
We find (i) that the magnitude of slope of bandgap versus strain has an almost
universal behaviour that depends on the chiral angle, (ii) that the sign of
slope depends on the value of and (iii) a novel change in sign
of the slope of bandgap versus uniaxial strain arising from a change in the
value of the quantum number corresponding to the minimum bandgap. Four orbital
calculations are also presented to show that the orbital results are
valid.Comment: Revised. Method explained in detai
Integrated method for quantitative morphometry and oxygen transport modelling in striated muscle
Identifying structural limitations in O2 transport is primarily restricted by current methods employed to characterise the nature of physiological remodelling. Inadequate resolution or breadth of available data has impaired development of routine diagnostic protocols and effective therapeutic strategies. Understanding O2 transport within striated muscle faces major challenges, most notably in quantifying how well individual fibres are supplied by the microcirculation, which has necessitated exploring tissue O2 supply using theoretical modelling of diffusive exchange. Having identified capillary domains as a suitable model for the description of local O2 supply, and requiring less computation than numerically calculating the trapping regions that are supplied by each capillary via biophysical transport models, we sought to design a high throughput method for histological analysis. We present an integrated package that identifies optimal protocols for identification of important input elements, processing of digitised images with semi-automated routines, and incorporation of these data into a mathematical modelling framework with computed output visualised as the tissue partial pressure of O2 (PO2) distribution across a biopsy sample. Worked examples are provided using muscle samples from experiments involving rats and humans
Thyroid Hormone Promotes Remodeling of Coronary Resistance Vessels
Low thyroid hormone (TH) function has been linked to impaired coronary blood flow, reduced density of small arterioles, and heart failure. Nonetheless, little is known about the mechanisms by which THs regulate coronary microvascular remodeling. The current study examined the initial cellular events associated with coronary remodeling induced by triiodothyronine (T3) in hypothyroid rats. Rats with established hypothyroidism, eight weeks after surgical thyroidectomy (TX), were treated with T3 for 36 or 72 hours. The early effects of T3 treatment on coronary microvasculature were examined morphometrically. Gene expression changes in the heart were assessed by quantitative PCR Array. Hypothyroidism resulted in arteriolar atrophy in the left ventricle. T3 treatment rapidly induced small arteriolar muscularization and, within 72 hours, restored arteriolar density to control levels. Total length of the capillary network was not affected by TX or T3 treatment. T3 treatment resulted in the coordinate regulation of Angiopoietin 1 and 2 expression. The response of Angiopoietins was consistent with vessel enlargement. In addition to the well known effects of THs on vasoreactivity, these results suggest that THs may affect function of small resistance arteries by phenotypic remodeling of vascular smooth muscle cells (VSMC)
Ab initio molecular dynamics using density based energy functionals: application to ground state geometries of some small clusters
The ground state geometries of some small clusters have been obtained via ab
initio molecular dynamical simulations by employing density based energy
functionals. The approximate kinetic energy functionals that have been employed
are the standard Thomas-Fermi along with the Weizsacker correction
and a combination . It is shown that the functional
involving gives superior charge densities and bondlengths over the
standard functional. Apart from dimers and trimers of Na, Mg, Al, Li, Si,
equilibrium geometries for and clusters have also
been reported. For all the clusters investigated, the method yields the ground
state geometries with the correct symmetries with bondlengths within 5\% when
compared with the corresponding results obtained via full orbital based
Kohn-Sham method. The method is fast and a promising one to study the ground
state geometries of large clusters.Comment: 15 pages, 3 PS figure
Persistent currents in carbon nanotubes based rings
Persistent currents in rings constructed from carbon nanotubes are
investigated theoretically. After studying the contribution of finite
temperature or quenched disorder on covalent rings, the complexity due to the
bundle packing is addressed. The case of interacting nanotori and
self-interacting coiled nanotubes are analyzed in details in relation with
experiments.Comment: 7 sections, 9 figure
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