Repositório RCAAP

In this work we show that the Layzer theory for atomic calculations provides a theoretical framework and also a powerful computational approach if correct rules for the calculation of the screening parameters are given. Using the virial as a model for potential energy and splitting of two-body operators as sum of onebody operators, a neat definition of screening is given, satisfying diverse physically indispensable properties. Many different experimental and theoretical results are reproduced with high accuracy, with no fitting procedure involving energy levels or numerical potentials. A C++ code and an executable file are available upon request.

Making use of the quantum correlators associated with the Maxwell field vacuum distorted by the presence of plane parallel material surfaces we rederive the Casimir-Polder interaction in the presence of plane parallel conducting walls. For a configuration consisting of a conducting wall and a magnetically permeable one new results for the Casimir-Polder interaction potential are obtained.

Computer simulations of partial differential equations of mathematical physics typically lead to some kind of high-dimensional dynamical system. When there is chaotic behavior we are faced with fundamental dynamical difficulties. We choose as a paradigm of such high-dimensional system a kicked double rotor. This system is investigated for parameter values at which it is strongly non-hyperbolic through a mechanism called unstable dimension variability, through which there are periodic orbits embedded in a chaotic attractor with different numbers of unstable directions. Our numerical investigation is primarily based on the analysis of the finite-time Lyapunov exponents, which gives us useful hints about the onset and evolution of unstable dimension variability for the double rotor map, as a system parameter (the forcing amplitude) is varied.

Hypersonic flows past Brazilian satellite SARA at zero angle of attack in chemical and thermal nonequilibrium are investigated using an axisymmetric Navier-Stokes solver. The numerical solutions were carried out for freestream conditions equivalent to a typically re-entry trajectory with a range of Mach numbers from 10 to 25. The gas was chemically composed by seven air species O;N;NO;O2, N2;NO+; e- with 24 steps chemical reactions scheme and thermically characterized by a multi-temperature model. Comparisons have been made between the present computation and the distribution of pressure coefficient and the heat transfer obtained recently with Direct Simulation Monte Carlo Method[1]. The study also points out the influence of nonequilibrium phenomena like ionization, vibrational and electronic excitation on aerothermodynamic flow parameters.

A vortex line, shaped by a zigzag of pinning centers, is described here through a three-dimensional unit cell containing two pinning centers positioned symmetrically with respect to its center. The unit cell is a cube of side L = 12xi, the pinning centers are insulating spheres of radius R, taken within the range 0.2xi to 3.0xi, xi being the coherence length. We calculate the free energy density of these systems in the framework of the Ginzburg-Landau theory.

We use symbolic dynamics to follow the evolution of the Matsumoto-Chua circuit in the chaotic regime. We consider the evolution of the whole population of unstable periodic orbits and of the associated trajectories, in four chaotic attractors generated by the circuit. Symbolic planes and first return maps are built for different values of the control parameter. The bifurcation mechanism suggests the possibility of the existence of a homoclinic orbit.

In the present paper the emission of intact insulin quasi-molecular ion [M+H]+ during laser ablation (MALDI) is studied. It was observed in particular that the insulin TOF molecular peak increases as the laser power increases. The DE-Vestal method for the initial velocity measurements was improved theoretically including the distance (d0) until the free expansion regime can be considered. According to the present analysis, the v0 parameter given by the DE-Vestal method is interpreted as the initial velocity that the desorbed ion would have if no collision occurs in the plasma. The improved method interprets v0 as the "final" initial velocity, i.e., as the velocity that the desorbed ions have when the plasma free expansion starts and, effectively, collisions no longer occur. The new method allows also the determination of d0, the distance to the solid when the free expansion starts. The data fitting shows that the distance (d0) has a linear dependence on the laser's intensity. Extrapolation of these values gives I = 0.69 G W cm-2 as the minimum energy density necessary to produce high density plasma during the insulin ions desorption when using alphaCHCA matrix.

The overcompleteness of the coherent states basis leads to a multiplicity of representations of Feynman's path integral. These different representations, although equivalent quantum mechanically, lead to different semiclassical limits. Two such semiclassical formulas were derived in [1] for the two corresponding path integral forms suggested by Klauder and Skagerstan in [2]. Each of these formulas involve trajectories governed by a different classical representation of the Hamiltonian operator: the P representation in one case and the Q representation in other. In this paper we construct a third representation of the path integral whose semiclassical limit involves directly the Weyl representation of the Hamiltonian operator, i.e., the classical Hamiltonian itself.

Magnetic nanoparticles from magnetic colloidal suspensions were incorporated in the urethane/urea elastomer (PU/PBDO) by swelling fully crosslinked elastomer samples with a toluene and ferrofluid mixture. It is shown that ferrofluid grains can be efficiently incorporated into the matrix of elastomers. The dependence of the birefringence of both the pure and ferrofluid-doped elastomer samples on strain is linear. The ratio of birefringence to strain of the ferrofluid-doped samples is greater than that of the pure elastomer samples, indicating that ferrofluid grains are oriented by the strained polymer network. We propose that this strain-induced orientation is due to the shape anisotropy of the nanoparticles.

The Walecka model contains essentially two parameters that are associated with the Lorentz scalar (S) and vector (V) interactions. These parameters are related to a two-body interaction consisting of S and V, imposing the condition that the two-body binding energy is fixed. We have obtained a set of different values for the nuclear matter binding energies (B N) at equilibrium densities (rhoo). We investigated the existence of a linear correlation between B N and rhoo, claimed to be universal for nonrelativistic systems and usually known as the Coester line, and found an approximate linear correlation only if V - S remains constant. It is shown that the relativistic content of the model, which is related to the strength of V - S, is responsible for the shift of the Coester line to the empirical region of nuclear matter saturation.

The concept that heavy fermions are close to a quantum critical point and that this proximity determines their physical behavior has opened new perspectives in the study of these systems. It has provided a new paradigm for understanding and probing the properties of these strongly correlated materials. Scaling ideas were important to establish this approach. We give below a brief and personal account of the genesis of some of these ideas 15 years ago, their implications and the future prospects for this exciting field.

The main steps in the development of the ideas on decoherence are briefly reviewed, together with their present achievements. Unsolved problems are also pointed out.

The problem of the linearity of the Schrödinger equation is described from a historical perspective. It is argued that the Schrödinger picture on which this equation is based cannot be retained in relativistic quantum theory. A closer analysis of realistic experiments might offer a clue how to modify the evolution equation for the state vectors in quantum field theory.

Quantum experiments with complex objects are of fundamental interest as they allow to quantitatively trace the quantum-to-classical transition under the influence of various interactions between the quantum object and its environment. We briefly review the present status of matter wave interferometry and decoherence studies with large molecules and focus in particular on the challenges for novel beam methods for molecular quantum optics with clusters, macromolecules or nanocrystals.

Some recent theoretical results on the stability of quantum dynamics against perturbations of the Hamiltonian - the so-called Loschmidt echoes and their relation to various decoherence measures are reviewed. We show that the representation of Loschmidt echoes in terms of the Wigner function can explain some seemingly paradoxical behavior of the quantum-classical correspondence.

We discuss the stability of quantum motion under system's perturbations in the light of the corresponding classical behavior. In particular we focus our attention on the so called "fidelity" or Loschmidt echo, its relation with the decay of correlations, and discuss the quantum-classical correspondence. We then report on the numerical simulation of the double-slit experiment, where the initial wave-packet is bounded inside a billiard domain with perfectly reflecting walls. If the shape of the billiard is such that the classical ray dynamics is regular, we obtain interference fringes whose visibility can be controlled by changing the parameters of the initial state. However, if we modify the shape of the billiard thus rendering classical (ray) dynamics fully chaotic, the interference fringes disappear and the intensity on the screen becomes the (classical) sum of intensities for the two corresponding one-slit experiments. Thus we show a clear and fundamental example in which transition to chaotic motion in a deterministic classical system, in absence of any external noise, leads to a profound modification in the quantum behavior.

We study two broad classes of physically dissimilar problems, each corresponding to stochastically driven escape from a potential well. The first class, often used to model noise-induced order parameter reversal, comprises Ginzburg-Landau-type field theories defined on finite intervals, perturbed by thermal or other classical spatiotemporal noise. The second class comprises systems in which a single degree of freedom is perturbed by both thermal and quantum noise. Each class possesses a transition in its escape behavior, at a critical value of interval length and temperature, respectively. It is shown that there exists a mapping from one class of problems to the other, and that their respective transitions can be understood within a unified theoretical context. We consider two applications within the first class: thermally induced breakup of monovalent metallic nanowires, and stochastic reversal of magnetization in thin ferromagnetic annuli. Finally, we explore the depth of the analogy between the two classes of problems, and discuss to what extent each case exhibits the characteristic signs of critical behavior at a sharp second-order phase transition.

Feature binding denotes how a large collection of coupled neurons combines external signals with internal memories into new coherent patterns of meaning. An external stimulus spreads over an assembly of coupled neurons, building up a corresponding collective state. Thus, the synchronization of spike trains of many individual neurons is the basis of a coherent perception. Homoclinic chaos has been proposed as the most suitable way to code information in time by trains of equal spikes occurring at apparently erratic times; a new quantitative indicator, called propensity, is introduced to select the most appropriate neuron model. In order to classify the set of different perceptions, the percept space is given a metric structure. The distance in percept space is conjugate to the duration of the perception in the sense that an uncertainty relation in percept space is associated with time limited perceptions. Thus coding of different percepts by synchronized spike trains entails fundamental quantum features with a quantum constant related to the details of the perceptual chain and very different from Planck's action.

Four models of energy decoherence are discussed from the common perspective of intrinsic time-uncertainty. The four authors - Milburn, Adler, Penrose, and myself - have four different approaches. The present work concentrates on their common divisors at the level of the proposed equations rather than at the level of the interpretations. General relationships between time-uncertainty and energy-decoherence are presented in both global and local sense. Global and local master equations are derived. (The local concept is favored.)

A recently introduced discrete formalism allows to solve the problem of time in quantum gravity in a relational manner. Quantum mechanics formulated with a relational time is not exactly unitary and implies a fundamental mechanism for decoherence of quantum states. The mechanism is strong enough to render the black hole information puzzle unobservable.