3,457 research outputs found
Quantum Communication in Spin Systems With Long-Range Interactions
We calculate the fidelity of transmission of a single qubit between distant
sites on semi-infinite and finite chains of spins coupled via the magnetic
dipole interaction. We show that such systems often perform better than their
Heisenberg nearest-neighbour coupled counterparts, and that fidelities closely
approaching unity can be attained between the ends of finite chains without any
special engineering of the system, although state transfer becomes slow in long
chains. We discuss possible optimization methods, and find that, for any
length, the best compromise between the quality and the speed of the
communication is obtained in a nearly uniform chain of 4 spins.Comment: 15 pages, 8 eps figures, updated references, corrected text and
corrected figs. 1, 4 and
Non-Perturbative Approach to the Landau Gauge Gluodynamics
We discuss a non-perturbative lattice calculation of the ghost and gluon
propagators in the pure Yang-Mills theory in Landau gauge. The ultraviolet
behaviour is checked up to NNNLO yielding the value
\Lambda^{n_f=0}_{\ms}=269(5)^{+12}_{-9}\text{MeV}, and we show that lattice
Green functions satisfy the complete Schwinger-Dyson equation for the ghost
propagator for all considered momenta. The study of the above propagators at
small momenta showed that the infrared divergence of the ghost propagator is
enhanced, whereas the gluon propagator seem to remain finite and non-zero. The
result for the ghost propagator is consistent with the analysis of the
Slavnov-Taylor identity, whereas, according to this analysis, the gluon
propagator should diverge in the infrared, a result at odds with other
approaches.Comment: To appear in the proceedings of the workshop "Hadron Structure and
QCD: from LOW to HIGH energies" (St. Petersburg, Russia, 20-24 September
2005
Is the QCD ghost dressing function finite at zero momentum ?
We show that a finite non-vanishing ghost dressing function at zero momentum
satisfies the scaling properties of the ghost propagator Schwinger-Dyson
equation. This kind of Schwinger-Dyson solutions may well agree with lattice
data and provides an interesting alternative to the widely spread claim that
the gluon dressing function behaves like the inverse squared ghost dressing
function, a claim which is at odds with lattice data. We demonstrate that, if
the ghost dressing function is less singular than any power of , it must be
finite non-vanishing at zero momentum: any logarithmic behaviour is for
instance excluded. We add some remarks about coupled Schwinger-Dyson analyses.Comment: 8 pages, 2 figure
The Infrared Behaviour of the Pure Yang-Mills Green Functions
We study the infrared behaviour of the pure Yang-Mills correlators using
relations that are well defined in the non-perturbative domain. These are the
Slavnov-Taylor identity for three-gluon vertex and the Schwinger-Dyson equation
for ghost propagator in the Landau gauge. We also use several inputs from
lattice simulations. We show that lattice data are in serious conflict with a
widely spread analytical relation between the gluon and ghost infrared critical
exponents. We conjecture that this is explained by a singular behaviour of the
ghost-ghost-gluon vertex function in the infrared. We show that, anyhow, this
discrepancy is not due to some lattice artefact since lattice Green functions
satisfy the ghost propagator Schwinger-Dyson equation. We also report on a
puzzle concerning the infrared gluon propagator: lattice data seem to favor a
constant non vanishing zero momentum gluon propagator, while the Slavnov-Taylor
identity (complemented with some regularity hypothesis of scalar functions)
implies that it should diverge.Comment: 25 pages, 7 figures; replaced version with some references adde and
an enlarged discussion of the non-renormalization theorem; second replacement
with improved figures and added reference
Asymptotic behavior of the ghost propagator in SU3 lattice gauge theory
We study the asymptotic behavior of the ghost propagator in the quenched
SU(3) lattice gauge theory with Wilson action. The study is performed on
lattices with a physical volume fixed around 1.6 fm and different lattice
spacings: 0.100 fm, 0.070 fm and 0.055 fm. We implement an efficient algorithm
for computing the Faddeev-Popov operator on the lattice. We are able to
extrapolate the lattice data for the ghost propagator towards the continuum and
to show that the extrapolated data on each lattice can be described up to
four-loop perturbation theory from 2.0 GeV to 6.0 GeV. The three-loop values
are consistent with those extracted from previous perturbative studies of the
gluon propagator. However the effective \Lambda_{\ms} scale which reproduces
the data does depend strongly upon the order of perturbation theory and on the
renormalization scheme used in the parametrization. We show how the truncation
of the perturbative series can account for the magnitude of the dependency in
this energy range. The contribution of non-perturbative corrections will be
discussed elsewhere.Comment: 26 pages, 7 figure
Dimensionamento de fundações de estacas pré-moldadas em concreto protendido carregadas lateralmente
TCC (graduação) - Universidade Federal de Santa Catarina. Centro TecnolĂłgico. Engenharia Civil.Neste trabalho Ă© proposto o dimensionamento de fundações profundas em estacas prĂ©-moldadas em concreto protendido, com carregamento axial de compressĂŁo e carregamento horizontal, para uma residĂŞncia unifamiliar a ser construĂda no bairro Lagoa da Conceição, FlorianĂłpolis, SC, Brasil. Para tal, foram analisados trĂŞs boletins de sondagem do tipo SPT do local e realizados os cálculos de capacidade de carga vertical de acordo com os mĂ©todos consagrados de Aoki-Velloso, DĂ©court- Quaresma, fazendo-se uma análise comparativa entre os resultados, optando-se pelo que apresenta a maior capacidade de carga do solo quando comparado com a carga máxima de projeto para uma unidade de estaca. Para capacidade de resistĂŞncia lateral do solo foi empregado o MĂ©todo de Broms, aplicado em estacas longas de cabeça engastada, cravadas em solo arenoso. A análise estrutural da peça prĂ©-moldada foi feita com base nos critĂ©rios e especificações fornecidas por uma empresa da regiĂŁo da Grande FlorianĂłpolis, conceituada na área de prĂ©-moldados. Ao fim, Ă© apresentada uma distribuição das estacas de forma a manter a mĂnima seção exigida e a planta de locação das fundações
Disentangling trophic interactions inside a Caribbean marine reserve
Author Posting. © Ecological Society of America, 2010. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 20 (2010): 1979–1992, doi:10.1890/09-1217.1.Recent empirical studies have demonstrated that human activities such as fishing can strongly affect the natural capital and services provided by tropical seascapes. However, policies to mitigate anthropogenic impacts can also alter food web structure and interactions, regardless of whether the regulations are aimed at single or multiple species, with possible unexpected consequences for the ecosystems and their associated services. Complex community response to management interventions have been highlighted in the Caribbean, where, contrary to predictions from linear food chain models, a reduction in fishing intensity through the establishment of a marine reserve has led to greater biomass of herbivorous fish inside the reserve, despite an increased abundance of large predatory piscivores. This positive multi-trophic response, where both predators and prey benefit from protection, highlights the need to take an integrated approach that considers how numerous factors control species coexistence in both fished and unfished systems. In order to understand these complex relationships, we developed a general model to examine the trade-offs between fishing pressure and trophic control on reef fish communities, including an exploration of top-down and bottom-up effects. We then validated the general model predictions by parameterizing the model for a reef system in the Bahamas in order to tease apart the wide range of species responses to reserves in the Caribbean. Combining the development of general theory and site-specific models parameterized with field data reveals the underlying driving forces in these communities and enables us to make better predictions about possible population and community responses to different management schemes.This work was
supported by funding from the Bahamas Biocomplexity Project
(U.S. NSF Biocomplexity grant OCE-0119976) and U.S. EPA
Science to Achieve Results (R832223)
Constraints on the IR behaviour of gluon and ghost propagator from Ward-Slavnov-Taylor identities
We consider the constraints of the Slavnov-Taylor identity of the IR
behaviour of gluon and ghost propagators and their compatibility with solutions
of the ghost Dyson-Schwinger equation and with the lattice picture.Comment: 5 pages, 2 figure
Divergent IR gluon propagator from Ward-Slavnov-Taylor identities?
We exploit the Ward-Slavnov-Taylor identity relating the 3-gluons to the
ghost-gluon vertices to conclude either that the ghost dressing function is
finite and non vanishing at zero momentum while the gluon propagator diverges
(although it may do so weakly enough not to be in contradiction with current
lattice data) or that the 3-gluons vertex is non-regular when one momentum goes
to zero. We stress that those results should be kept in mind when one studies
the Infrared properties of the ghost and gluon propagators, for example by
means of Dyson-Schwinger equations.Comment: 6 pages, bibte
Imperio de Satanás abatido por la triunfante riqueza, solenizada en la corte celestial de Dios eterno, por el bien de la limosna : asistiendo al triunfo los santos padres antiguos y modernos...
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