12,601 research outputs found
Chiral properties of two-flavor QCD in small volume and at large lattice spacing
We present results from simulations of two flavors of dynamical overlap
fermions on 8^4 lattices at three values of the sea quark mass and a lattice
spacing of about 0.16 fm. We measure the topological susceptibility and the
chiral condensate. A comparison of the low-lying spectrum of the overlap
operator with predictions from random matrix theory is made. To demonstrate the
effect of the dynamical fermions, we compare meson two-point functions with
quenched results. Algorithmic improvements over a previous publication and the
performance of the algorithm are discussed.Comment: 16 pages, 12 figure
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
Quark condensate in one-flavor QCD
We compute the condensate in QCD with a single quark flavor using numerical
simulations with the overlap formulation of lattice fermions. The condensate is
extracted by fitting the distribution of low lying eigenvalues of the Dirac
operator in sectors of fixed topological charge to the predictions of Random
Matrix Theory. Our results are in excellent agreement with estimates from the
orientifold large-N_c expansion.Comment: 12 pages, 4 figures, REVTeX4, v2: Small changes, extended
introduction, published versio
Field-enhancement calculations for a field-distortion triggered spark gap
We present the results of numerical field calculation which supplement a recent article in which we described a new design concept for field-distortion triggered spark gaps. The calculations verify the shielding and field enhancement assumptions made in the article, and they provide insight into the interaction of the design tradeoffs associated with simultaneously maximizing the holdoff voltage and the triggering capability of the gap
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
COMPARISON OF ORTHODOX WITH FIBREOPTIC OROTRACHEAL INTUBATION UNDER TOTAL I.V. ANAESTHESIA
Fibreoptic orotracheal intubation was compared with orthodox laryngoscopy and tracheal intubation using a total i.v. technique with propofol in 60 ASA I and II patients. There was no significant difference between the two techniques in haemodynamic profile (before, during and following the intubation procedure) and incidence of postoperative sore throat. Minimal oxygen saturation was 96% during the study; maximal end-tidal PCO2 after intubation was 5.4 kPa. Intubation time was faster (P < 0.01) in the orthodox group (30.7 (SEM 2.3) s) than in the fibreoptic group (52.7 (4.8) s
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