922 research outputs found
Infra-Red Finite Charge Propagation
The Coulomb gauge has a long history and many uses. It is especially useful
in bound state applications. An important feature of this gauge is that the
matter fields have an infra-red finite propagator in an on-shell
renormalisation scheme. This is, however, only the case if the renormalisation
point is chosen to be the static point on the mass shell, p = (m, 0, 0, 0). In
this letter we show how to extend this key property of the Coulomb gauge to an
arbitrary relativistic renormalisation point. This is achieved through the
introduction of a new class of gauges of which the Coulomb gauge is a limiting
case. A physical explanation for this result is given.Comment: 8 pages, plain TeX, to appear in Modern Physics Letters
D-branes and Discrete Torsion II
We derive D-brane gauge theories for C^3/Z_n x Z_n orbifolds with discrete
torsion and study the moduli space of a D-brane at a point. We show that, as
suggested in previous work, closed string moduli do not fully resolve the
singularity, but the resulting space -- containing n-1 conifold singularities
-- is somewhat surprising. Fractional branes also have unusual properties.
We also define an index which is the CFT analog of the intersection form in
geometric compactification, and use this to show that the elementary D6-brane
wrapped about T^6/Z_n x Z_n must have U(n) world-volume gauge symmetry.Comment: harvmac, 25 p
Defect CFTs and holographic multiverse
We investigate some aspects of a recent proposal for a holographic
description of the multiverse. Specifically, we focus on the implications on
the suggested duality of the fluctuations of a bubble separating two universes
with different cosmological constants. We do so by considering a similar
problem in a 2+1 CFT with a codimension one defect, obtained by an M5-brane
probe embedding in AdS_4x S^7, and studying its spectrum of fluctuations. Our
results suggest that the kind of behavior required by the spectrum of bubble
fluctuations is not likely to take place in defect CFTs with an AdS dual,
although it might be possible if the defect supports a non-unitary theory.Comment: 19 pages; v2: typos fixed, minor changes
Regular quantum graphs
We introduce the concept of regular quantum graphs and construct connected
quantum graphs with discrete symmetries. The method is based on a decomposition
of the quantum propagator in terms of permutation matrices which control the
way incoming and outgoing channels at vertex scattering processes are
connected. Symmetry properties of the quantum graph as well as its spectral
statistics depend on the particular choice of permutation matrices, also called
connectivity matrices, and can now be easily controlled. The method may find
applications in the study of quantum random walks networks and may also prove
to be useful in analysing universality in spectral statistics.Comment: 12 pages, 3 figure
Mejoramiento de la refractariedad de arcillas por lavado o intercambio iónico
Se estudió el mejoramiento de la refractariedad da arcillas que, a pesar de tener un elevado contenido de alúmina, presentaban un bajo cono pirométrico equivalente. El tratamiento se realizó por lavado con agua y por intercambio iónico.
Como se esperaba, después de los tratamientos mencionados, se logró mejorar el C.P.E. de las arcillas tratadas, tanto por lavado como por intercambio iónico con soluciones acuosas de ácido clorhídrico y acetato de amonio.
Se indican en el texto las condiciones experimentales y se proporcionan los resultados obtenidos.The improvement of the refractoriness of clays having a high content of aluminium oxide and a very low P.C.E. was studied after treating them by water and by ionic exchange.
As it was expected the P.C.E. of the clays under treatment was improved not only by ionic exchange with aqueous solutions of hydrochloric acid and ammonium acetate, but also washing them with water* The experimental conditions and the results obtained were presented in this paper
A fast and easy approach to the simulation of binary mixtures sorption kinetics
Diffusivity of a component in a binary mixture is affected by the presence of a second component. The knowledge of the influence on each other component diffusivity is very useful for the prediction of sorption kinetics of binary mixtures. In this work kinetic studies of Cr(VI) and Cu(II) binary mixtures sorption onto exhausted coffee encapsulated in calcium alginate beads were carried out in both bench and pilot scale experiments. The spectroscopic analysis evidenced the complexity of the process since different mechanisms such as adsorption, redox reaction and ion exchange are involved. Experimental data were fitted to the Linear Adsorption Model (LAM). An empirical quadratic model was developed to estimate LAM parameters (De) and α = Cf/(Ci − Cf) as a function of the initial concentration of metals in the mixture. The empirical model developed enables to estimate the LAM parameters (De and α) of metal ions binary mixtures provided that the initial concentration of the metal ions is included in the range of concentrations studied. The estimated parameters introduced in LAM equation allow simulating the corresponding binary mixtures sorption kinetics. This study constitutes a fast and easy approach to the modelling of sorption kinetics of complex systems in which different processes take place simultaneously
Pediatric Left Posteroseptal Accessory Pathway Ablation from Giant Coronary Sinus with Persistent Left Superior Cava
We report a pediatric patient with persistent left superior vena cava and a D-transposition of great arteries, which is an uncommon relation. It is crucial to know the anatomy of the persistent left superior vena cava and the dilated coronary sinus to plan the mapping techniques in cases of posterior accessory pathways
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Broadening of mode-locking pulses in quantum-dot semiconductor lasers : simulation, analysis and experiments
We consider a mode-locked quantum-dot edge-emitting semiconductor laser
consisting of a reverse biased saturable absorber and a forward biased
amplifying section. To describe the dynamics of this laser we use the
traveling wave model taking into account carrier exchange processes between a
reservoir and the quantum dots. A comprehensive parameter study is presented
and an analysis of mode-locking pulse broadening with an increase of
injection current is performed. The results of our theoretical analysis are
supported by experimental data demonstrating a strong pulse asymmetry in a
monolithic two section quantum dot mode-locked lase
Broadening of mode-locking pulses in quantum-dot semiconductor lasers: Simulation, analysis and experiments
We consider a mode-locked quantum-dot edge-emitting semiconductor laser consisting of a reverse biased saturable absorber and a forward biased amplifying section. To describe the dynamics of this laser we use the traveling wave model taking into account carrier exchange processes between a reservoir and the quantum dots. A comprehensive parameter study is presented and an analysis of mode-locking pulse broadening with an increase of injection current is performed. The results of our theoretical analysis are supported by experimental data demonstrating a strong pulse asymmetry in a monolithic two section quantum dot mode-locked laser
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