21,144 research outputs found
Emergence and spontaneous breaking of approximate O(4) symmetry at a weakly first-order deconfined phase transition
We investigate approximate emergent nonabelian symmetry in a class of weakly
first order `deconfined' phase transitions using Monte Carlo simulations and a
renormalization group analysis. We study a transition in a 3D classical loop
model that is analogous to a deconfined 2+1D quantum phase transition in a
magnet with reduced lattice symmetry. The transition is between the N\'eel
phase and a twofold degenerate valence bond solid (lattice-symmetry-breaking)
phase. The combined order parameter at the transition is effectively a
four-component superspin. It has been argued that in some weakly first order
`pseudocritical' deconfined phase transitions, the renormalization group flow
can take the system very close to the ordered fixed point of the symmetric
sigma model, where is the total number of `soft' order parameter
components, despite the fact that is not a microscopic symmetry. This
yields a first order transition with unconventional phenomenology. We argue
that this occurs in the present model, with . This means that there is a
regime of lengthscales in which the transition resembles a `spin-flop'
transition in the ordered sigma model. We give numerical evidence for
(i) the first order nature of the transition, (ii) the emergence of
symmetry to an accurate approximation, and (iii) the existence of a regime in
which the emergent is `spontaneously broken', with distinctive features
in the order parameter probability distribution. These results may be relevant
for other models studied in the literature, including 2+1D QED with two
flavours, the `easy-plane' deconfined critical point, and the N\'eel--VBS
transition on the rectangular lattice.Comment: 16 pages. v2: updated to journal versio
Hacia el modelado 3d de tumores cerebrales mediante endoneurosonografía y redes neuronales
Las cirugías mínimamente invasivas se han vuelto populares debido a que implican menos riesgos con respecto a las intervenciones tradicionales. En neurocirugía, las tendencias recientes sugieren el uso conjunto de la endoscopia y el ultrasonido, técnica llamada endoneurosonografía (ENS), para la virtualización 3D de las estructuras del cerebro en tiempo real. La información ENS se puede utilizar para generar modelos 3D de los tumores del cerebro durante la cirugía. En este trabajo, presentamos una metodología para el modelado 3D de tumores cerebrales con ENS y redes neuronales. Específicamente, se estudió el uso de mapas auto-organizados (SOM) y de redes neuronales tipo gas (NGN). En comparación con otras técnicas, el modelado 3D usando redes neuronales ofrece ventajas debido a que la morfología del tumor se codifica directamente sobre los pesos sinápticos de la red, no requiere ningún conocimiento a priori y la representación puede ser desarrollada en dos etapas: entrenamiento fuera de línea y adaptación en línea. Se realizan pruebas experimentales con maniquíes médicos de tumores cerebrales. Al final del documento, se presentan los resultados del modelado 3D a partir de una base de datos ENS.Minimally invasive surgeries have become popular because they reduce the typical risks of traditional interventions. In neurosurgery, recent trends suggest the combined use of endoscopy and ultrasound (endoneurosonography or ENS) for 3D virtualization of brain structures in real time. The ENS information can be used to generate 3D models of brain tumors during a surgery. This paper introduces a methodology for 3D modeling of brain tumors using ENS and unsupervised neural networks. The use of self-organizing maps (SOM) and neural gas networks (NGN) is particularly studied. Compared to other techniques, 3D modeling using neural networks offers advantages, since tumor morphology is directly encoded in synaptic weights of the network, no a priori knowledge is required, and the representation can be developed in two stages: off-line training and on-line adaptation. Experimental tests were performed using virtualized phantom brain tumors. At the end of the paper, the results of 3D modeling from an ENS database are presented
Randomized parallel approximations to max flow
The final publication is available at link.springer.comPeer ReviewedPostprint (author's final draft
Parallel algorithms for two processors precedence constraint scheduling
The final publication is available at link.springer.comPeer ReviewedPostprint (author's final draft
Bi-directional top hat D-Scan: single beam accurate characterization of nonlinear waveguides
The characterization of a third order nonlinear integrated waveguide is
reported for the first time by means of a top-hat Dispersive-Scan (D-Scan)
technique, a temporal analog of the top-hat Z-Scan. With a single laser beam,
and by carrying two counter-directional nonlinear transmissions to assess the
input and output coupling efficiencies, a novel procedure is described leading
to an accurate measurement of the TPA figure of merit, the effective Two-Photon
Absorption (TPA) and optical Kerr (including the sign) coefficients. The
technique is validated in a silicon strip waveguide for which the effective
nonlinear coefficients are measured with an accuracy of Comment: 5 pages, 4 figure
Upper limit to in scalar-tensor gravity theories
In a previous paper (Serna & Alimi 1996), we have pointed out the existence
of some particular scalar-tensor gravity theories able to relax the
nucleosynthesis constraint on the cosmic baryonic density. In this paper, we
present an exhaustive study of primordial nucleosynthesis in the framework of
such theories taking into account the currently adopted observational
constraints. We show that a wide class of them allows for a baryonic density
very close to that needed for the universe closure. This class of theories
converges soon enough towards General Relativity and, hence, is compatible with
all solar-system and binary pulsar gravitational tests. In other words, we show
that primordial nucleosynthesis does not always impose a very stringent bound
on the baryon contribution to the density parameter.Comment: uuencoded tar-file containing 16 pages, latex with 5 figures,
accepted for publication in Astrophysical Journal (Part 1
Charmed mesons at finite temperature and chemical potential
We compute the masses of the pseudoscalar mesons , and at
finite temperature and baryon chemical potential. The computations are based on
a symmetry- preserving Dyson-Schwinger equation treatment of a vector-vector
four quark contact interaction. The results found for the temperature
dependence of the meson masses are in qualitative agreement with lattice QCD
data and QCD sum rules calculations. The chemical potential dependence of the
masses provide a novel prediction of the present computation
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