Rio de Janeiro, Brazil

The Brazilian Center for Physics Research is a physics research center in the Urca neighborhood of Rio de Janeiro sponsored by the Brazilian National Council for Scientific and Technological Development , linked to the Ministry of Science and Technology. CBPF was founded in 1949 from a joint effort of Cesar Lattes, José Leite Lopes, and Jayme Tiomno. Throughout its existence, CBPF became an internationally renowned research institution, organizing several international meetings and hosting many renowned physicists, like Richard Feynman and J. Robert Oppenheimer. It was also the starting point of important Brazilian institutions, like the National Institute for Pure and Applied Mathematics , the National Laboratory for Scientific Computation and the National Laboratory of Synchrotron Light . Since its creation, CBPF has been one of the most important Physics research institutions in Brazil, and its graduate program ranks among the best in the country. Wikipedia.

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Bagchi D.,Brazilian Center for Research in Physics (CBPF)
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2017

We study the thermal transport properties of the one-dimensional Fermi-Pasta-Ulam model (β type) with long-range interactions. The strength of the long-range interaction decreases with the (shortest) distance between the lattice sites as distance-δ, where δ≥0. Two Langevin heat baths at unequal temperatures are connected to the ends of the one-dimensional lattice via short-range harmonic interactions that drive the system away from thermal equilibrium. In the nonequilibrium steady state the heat current, thermal conductivity, and temperature profiles are computed by solving the equations of motion numerically. It is found that the conductivity κ has an interesting nonmonotonic dependence with δ with a maximum at δ=2.0 for this model. Moreover, at δ=2.0,κ diverges almost linearly with system size N and the temperature profile has a negligible slope, as one expects in ballistic transport for an integrable system. We demonstrate that the nonmonotonic behavior of the conductivity and the nearly ballistic thermal transport at δ=2.0 obtained under nonequilibrium conditions can be explained consistently by studying the variation of largest Lyapunov exponent λmax with δ, and excess energy diffusion in the equilibrium microcanonical system. © 2017 American Physical Society.

Liu D.,Ningbo University | Reboucas M.J.,Brazilian Center for Research in Physics (CBPF)
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

In the standard approach to cosmological modeling in the framework of general relativity, the energy conditions play an important role in the understanding of several properties of the Universe, including singularity theorems, the current accelerating expansion phase, and the possible existence of the so-called phantom fields. Recently, the f(T) gravity has been invoked as an alternative approach for explaining the observed acceleration expansion of the Universe. If gravity is described by a f(T) theory instead of general relativity, there are a number of issues that ought to be reexamined in the framework of f(T) theories. In this work, to proceed further with the current investigation of the limits and potentialities of the f(T) gravity theories, we derive and discuss the bounds imposed by the energy conditions on a general f(T) functional form. The null and strong energy conditions in the framework of f(T) gravity are derived from first principles, namely the purely geometric Raychaudhuri equation along with the requirement that gravity is attractive. The weak and dominant energy conditions are then obtained in a direct approach via an effective energy-momentum tensor for f(T) gravity. Although similar, the energy condition inequalities are different from those of general relativity, but in the limit f(T)=T, the standard forms for the energy conditions in general relativity are recovered. As a concrete application of the derived energy conditions to locally homogeneous and isotropic f(T) cosmology, we use the recent estimated value of the Hubble parameter to set bounds from the weak energy condition on the parameters of two specific families of f(T) gravity theories. © 2012 American Physical Society.

Faci S.,Brazilian Center for Research in Physics (CBPF)
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We present an SO(2,4)-covariant quantization of the free electromagnetic field in conformally flat spaces (CFSs). A CFS is realized in a six-dimensional space as an intersection of the null cone with a given surface. The smooth move of the latter is equivalent to perform a Weyl rescaling. This allows to transport the SO(2,4)-invariant quantum structure of the Maxwell field from Minkowski space to any CFS. Calculations are simplified and the CFS Wightman two-point functions are given in terms of their Minkowskian counterparts. The difficulty due to gauge freedom is surpassed by introducing two auxiliary fields and using the Gupta-Bleuler quantization scheme. The quantum structure is given by a vacuum state and creators/annihilators acting on some Hilbert space. In practice, only the Hilbert space changes under Weyl rescalings. Also, the quantum SO(2,4)-invariant free Maxwell field does not distinguish between two CFSs. © 2013 American Physical Society.

Membiela F.A.,Brazilian Center for Research in Physics (CBPF)
Nuclear Physics B | Year: 2014

Although inflation is a natural candidate to generate the lengths of coherence of magnetic fields needed to explain current observations, it needs to break conformal invariance of electromagnetism to obtain significant magnetic amplitudes. Of the simplest realizations are the kinetically-coupled theories f2(φ)Fμν Fμν (or IFF theories). However, these are known to suffer from electric fields backreaction or the strong coupling problem. In this work we shall confirm that such class of theories are problematic to support magnetogenesis during inflationary cosmology. On the contrary, we show that a bouncing cosmology with a contracting phase dominated by an equation of state with p > - ρ/3 can support magnetogenesis, evading the backreaction/strong-coupling problem. Finally, we study safe magnetogenesis in a particular bouncing model with an ekpyrotic-like contracting phase. In this case we found that f2(φ)F2-instabilities might arise during the final kinetic-driven expanding phase for steep ekpyrotic potentials. © 2014 The Author.

Vitenti S.D.P.,Brazilian Center for Research in Physics (CBPF) | Pinto-Neto N.,Brazilian Center for Research in Physics (CBPF)
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

It has been shown that a contracting universe with a dustlike (w>0) fluid may provide an almost scale invariant spectrum for the gravitational scalar perturbations. As the universe contracts, the amplitude of such perturbations are amplified. The gauge invariant variable Φ develops a growing mode which becomes much larger than the constant one around the bounce phase. The constant mode has its amplitude fixed by Cosmic Background Explorer (COBE) normalization, thus the amplitude of the growing mode can become much larger than 1. In this paper, we first show that this is a general feature of bouncing models, since we expect that general relativity should be valid in all scales away from the bounce. However, in the Newtonian gauge, the variable Φ gives the value of the metric perturbation, raising doubts on the validity of the linear perturbative regime at the bounce. In order to address this issue, we obtain a set of necessary conditions for the perturbative series to be valid along the whole history of the model, and we show that there is a gauge in which all these conditions are satisfied, for a set of models, if the constant mode is fixed by COBE normalization. As a by-product of this analysis, we point out that there are sets of solutions for the perturbation variables where some gauge-fixing conditions are not well defined, turning these gauges prohibited for those solutions. © 2012 American Physical Society.

Novello M.,Brazilian Center for Research in Physics (CBPF) | Bittencourt E.,Brazilian Center for Research in Physics (CBPF)
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We show that the Gordon metric belongs to a larger class of geometries, which are responsible to describe the paths of accelerated bodies in moving dielectrics as geodesics in a metric qμν different from the background one. This map depends only on the background metric and on the motion of the bodies under consideration. As a consequence, this method describes a more general property that concerns the elimination of any kind of force acting on bodies by a suitable change of the substratum metric. © 2012 American Physical Society.

Faci S.,Brazilian Center for Research in Physics (CBPF)
Classical and Quantum Gravity | Year: 2013

We present a simple, systematic and practical method to construct conformally invariant equations in arbitrary Riemann spaces. This method that we call 'Weyl-to-Riemann' is based on two features of the Weyl geometry. (i) Weyl space is defined by the metric tensor and the Weyl vector W; it is equivalent to the Riemann space when W is a gradient. (ii) Any homogeneous differential equation written in the Weyl space by means of the Weyl connection is conformally invariant. The Weyl-to-Riemann method selects those equations whose conformal invariance is preserved when reducing to the Riemann space. Applications to scalar, vector and spin-2 fields are presented, which demonstrate the efficiency of this method. In particular, a new conformally invariant spin-2 field equation is exhibited. This equation extends Grishchuk-Yudin's equation and fixes its limitations since it does not require the Lorenz gauge. Moreover, this equation reduces to the Drew-Gegenberg and Deser-Nepomechie equations in Minkowski and de Sitter spaces, respectively. © 2013 IOP Publishing Ltd.

Pinto-Neto N.,Brazilian Center for Research in Physics (CBPF) | Fabris J.C.,Ferrari
Classical and Quantum Gravity | Year: 2013

We review the main results that have been obtained in quantum cosmology from the perspective of the de Broglie-Bohm quantum theory. As it is a dynamical theory of assumed objectively real trajectories in the configuration space of the physical system under investigation, this quantum theory is not essentially probabilistic and dispenses the collapse postulate, turning it suitable to be applied to cosmology. In the framework of minisuperspace models, we show how quantum cosmological effects in the de-Broglie-Bohm approach can avoid the initial singularity, and isotropize the Universe. We then extend minisuperspace in order to include linear cosmological perturbations. We present the main equations which govern the dynamics of quantum cosmological perturbations evolving in non-singular quantum cosmological backgrounds, and calculate some of their observational consequences. These results are not known how to be obtained in other approaches to quantum theory. In the general case of full superspace, we enumerate the possible structures of quantum space and time that emerge from the de Broglie-Bohm picture. Finally, we compare some of the results coming from the de Broglie-Bohm theory with other approaches, and discuss the physical reasons for some discrepancies that occur. © 2013 IOP Publishing Ltd.

Brazilian Center for Research in Physics (CBPF) | Date: 2013-05-16

The present invention relates to a magnetic system for attaching and removing medical devices and prostheses on human skin comprising external disks 4 and silicone implants 5; said system can increase, decrease or eliminate the magnetic attraction or repulsion force as a function of the rotation of the external disks 4 in relation to the silicone implants 5 such that the skin between the external disks 4 and the silicone implants 5 is subject to a pressure compatible with viable capillary microcirculation.

Aolita L.,Free University of Berlin | De Melo F.,Brazilian Center for Research in Physics (CBPF) | Davidovich L.,Federal University of Rio de Janeiro
Reports on Progress in Physics | Year: 2015

One of the greatest challenges in the fields of quantum information processing and quantum technologies is the detailed coherent control over each and every constituent of quantum systems with an ever increasing number of particles. Within this endeavor, harnessing of many-body entanglement against the detrimental effects of the environment is a major pressing issue. Besides being an important concept from a fundamental standpoint, entanglement has been recognized as a crucial resource for quantum speed-ups or performance enhancements over classical methods. Understanding and controlling many-body entanglement in open systems may have strong implications in quantum computing, quantum simulations of many-body systems, secure quantum communication or cryptography, quantum metrology, our understanding of the quantum-to-classical transition, and other important questions of quantum foundations. In this paper we present an overview of recent theoretical and experimental efforts to underpin the dynamics of entanglement under the influence of noise. Entanglement is thus taken as a dynamic quantity on its own, and we survey how it evolves due to the unavoidable interaction of the entangled system with its surroundings. We analyze several scenarios, corresponding to different families of states and environments, which render a very rich diversity of dynamical behaviors. In contrast to single-particle quantities, like populations and coherences, which typically vanish only asymptotically in time, entanglement may disappear at a finite time. In addition, important classes of entanglement display an exponential decay with the number of particles when subject to local noise, which poses yet another threat to the already-challenging scaling of quantum technologies. Other classes, however, turn out to be extremely robust against local noise. Theoretical results and recent experiments regarding the difference between local and global decoherence are summarized. Control and robustness-enhancement techniques, scaling laws, statistical and geometrical aspects of multipartite-entanglement decay are also reviewed; all in order to give a broad picture of entanglement dynamics in open quantum systems addressed to both theorists and experimentalists inside and outside the field of quantum information. © 2015 IOP Publishing Ltd.

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