10,099 research outputs found
Isolamento e caracterização molecular do vírus de Influenza suína.
Projeto/Plano de Ação: 03.03.23.200
Modeling the momentum distributions of annihilating electron-positron pairs in solids
Measuring the Doppler broadening of the positron annihilation radiation or
the angular correlation between the two annihilation gamma quanta reflects the
momentum distribution of electrons seen by positrons in the
material.Vacancy-type defects in solids localize positrons and the measured
spectra are sensitive to the detailed chemical and geometric environments of
the defects. However, the measured information is indirect and when using it in
defect identification comparisons with theoretically predicted spectra is
indispensable. In this article we present a computational scheme for
calculating momentum distributions of electron-positron pairs annihilating in
solids. Valence electron states and their interaction with ion cores are
described using the all-electron projector augmented-wave method, and atomic
orbitals are used to describe the core states. We apply our numerical scheme to
selected systems and compare three different enhancement (electron-positron
correlation) schemes previously used in the calculation of momentum
distributions of annihilating electron-positron pairs within the
density-functional theory. We show that the use of a state-dependent
enhancement scheme leads to better results than a position-dependent
enhancement factor in the case of ratios of Doppler spectra between different
systems. Further, we demonstrate the applicability of our scheme for studying
vacancy-type defects in metals and semiconductors. Especially we study the
effect of forces due to a positron localized at a vacancy-type defect on the
ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised
manuscript submitted on November 14 200
Physics issues in simulations with dynamical overlap fermions
We discuss the impact of various improvements on simulations of dynamical
overlap fermions using the Hybrid Monte Carlo algorithm. We focus on the usage
of fat links and multiple pseudo-fermion fields.Comment: 14 pages, 10 figure
Abstract delta modelling
Article / Letter to editorLeiden Inst Advanced Computer Science
Avaliação da presença do vírus influenza em suínos no sul do Brasil.
bitstream/item/57440/1/CUsersPiazzonDocumentsColecao-DigitalBoletim-de-Pesquisa-10.pdfProjeto/Plano de Ação: 16.00.300.04-00
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Single-step method for β-galactosidase assays in Escherichia coli using a 96-well microplate reader
AbstractHistorically, the lacZ gene is one of the most universally used reporters of gene expression in molecular biology. Its activity can be quantified using an artificial substrate, o-nitrophenyl-ß-d-galactopyranoside (ONPG). However, the traditional method for measuring LacZ activity (first described by J. H. Miller in 1972) can be challenging for a large number of samples, is prone to variability, and involves hazardous compounds for lysis (e.g., chloroform, toluene).Here we describe a single-step assay using a 96-well microplate reader with a proven alternative cell permeabilization method. This modified protocol reduces handling time by 90%
UV excess measures of accretion onto young very low-mass stars and brown dwarfs
Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown
dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission
arising in the Balmer and Paschen continua yields mass accretion rates ranging
from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS
spectra of roughly solar-mass accretors with accretion rates that range from
2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A
leads to a higher contrast between the accretion and photospheric emission
relative to higher-mass counterparts. The mass accretion rates measured here
are systematically 4-7 times larger than those from H-alpha emission line
profiles, with a difference that is consistent with but unlikely to be
explained by the uncertainty in both methods. The accretion luminosity
correlates well with many line luminosities, including high Balmer and many He
I lines. Correlations of the accretion rate with H-alpha 10% width and line
fluxes show a large amount of scatter. Our results and previous accretion rate
measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for
accretors in the Taurus Molecular Cloud.Comment: 13 pages text, 15 tables, 14 figures. Accepted by Ap
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