Showing papers in "Brazilian Journal of Physics in 2011"

Quantum Metrology for Noisy Systems

Abstract: The estimation of parameters characterizing dynamical processes is a central problem in science and technology. It concerns for instance the evaluation of the duration of some interaction, of the value of a coupling constant, or yet of a frequency in atomic spectroscopy. The estimation error changes with the number N of resources employed in the experiment (which could quantify, for instance, the number of probes or the probing energy). For independent probes, it scales as \(1/\sqrt{N}\)—the standard limit—a consequence of the central-limit theorem. Quantum strategies, involving for instance entangled or squeezed states, may improve the precision, for noiseless processes, by an extra factor \(1/\sqrt{N}\), leading to the so-called Heisenberg limit. For noisy processes, an important question is if and when this improvement can be achieved. Here, we review and detail our recent proposal of a general framework for obtaining attainable and useful lower bounds for the ultimate limit of precision in noisy systems. We apply this bound to lossy optical interferometry and show that, independently of the initial states of the probes, it captures the main features of the transition, as N grows, from the 1/N to the \(1/\sqrt{N}\) behavior.

Non-Hermitian Hamiltonians with Real Spectrum in Quantum Mechanics

Abstract: Examples are given of non-Hermitian Hamiltonian operators which have a real spectrum. Some of the investigated operators are expressed in terms of the generators of the Weyl-Heisenberg algebra. It is argued that the existence of an involu- tive operator ˆ J which renders the Hamiltonian ˆ J- Hermitian leads to the unambiguous definition of an associated positive definite norm allowing for the standard probabilistic interpretation of quantum mechanics. Non-Hermitian extensions of the Poeschl- Teller Hamiltonian are also considered. Hermitian counterparts obtained by similarity transformations are constructed.

Thermal and Spectrophotometric Analysis of Liquid Crystal 8CB/8OCB Mixtures

Abstract: The binary system of 4-octyl-4′-cyanobiphenyl (8CB) and 4-octyloxy-4′-cyanobiphenyl (8OCB) has been studied by means of differential scanning calorimetry (DSC) and ultraviolet absorption spectrophotometry (UV). The phase-transition temperatures, enthalpies, and entropies have been determined by using calorimetric methods on DSC. The results indicate clearly the existence of three-phase regions across the crystalline-to-smectic A, smectic A-to-nematic, and nematic-to-isotropic transitions in the 8CB/8OCB mixtures. The obtained phase-transition temperatures of the 8CB/8OCB mixtures are between the data for 8CB and 8OCB. A few of the phase transitions cannot be observed at high heating rates. The phase-transition temperatures of the 8CB/8OCB mixtures rise with the heating rate between 2°C/min and 15°C/min. The activation energies were calculated by the Ozawa method for the phase transitions of 25% 8CB and 75% 8OCB liquid crystal mixtures. UV experiments were carried out to characterize the absorptivity constants of liquid crystal and their mixtures. The molar absorptivity and maximum absorption wavelengths were measured in chloroform solution by UV spectrophotometry. The maximum absorption wavelength of the 8CB/8OCB mixtures increases with decreasing percent weight of 8CB in 8OCB, a result associated with the different lengths of the alkyl chain.

An Introduction to Quantum Plasmas

Abstract: Shielding effects in non-degenerate and degenerate plasmas are compared. A detailed derivation of the Wigner-Poisson system is provided for electrostatic quantum plasmas in which relativistic, spin, and collisional effects are not essential. A detailed derivation of a quantum hydrodynamic model starting from the Wigner-Poisson system is presented. The route for this derivation considers the eikonal decomposition of the one-body wavefunctions of the quantum statistical mixture. The merits and limitations of the resulting quantum hydrodynamic model are discussed.

Vortices in Low-Dimensional Magnetic Systems

Abstract: Vortices are objects that are important to describe several physical phenomena. There are many examples of such objects in nature as in a large variety of physical situations like in fluid dynamics, superconductivity, magnetism, and biology. Historically, the interest in magnetic vortex-like excitations begun in the 1960s. That interest was mainly associated with an unusual phase-transition phenomenon in two-dimensional magnetic systems. More recently, direct experimental evidence for the existence of magnetic vortex states in nano-disks was found. The interest in such model was renewed due to the possibility of the use of magnetic nano-disks as bit elements in nano-scale memory devices. The goal of this study is to review some key points for the understanding of the vortex behavior and the progress that have been done in the study of vortices in low-dimensional magnetic systems.

Fitting Cosmological Data to the Function q(Z) from GR Theory: Modified Chaplygin Gas

Abstract: In the Friedmann cosmology, the deceleration of the expansion q plays a fundamental role. We derive the deceleration as a function of redshift q(z) in two scenarios: ΛCDM model and modified Chaplygin gas (MCG) model. The function for the MCG model is then fitted to the cosmological data in order to obtain the cosmological parameters that minimize χ 2. We use the Fisher matrix to construct the covariance matrix of our parameters and reconstruct the q(z) function. We use Supernovae Ia, WMAP5, and BAO measurements to obtain the observational constraints. We determined the present acceleration as q 0 = − 0.65 ±0.19 for the MCG model using the Union2 dataset of SNeIa, BAO, and CMB and q 0 = − 0.67 ±0.17 for the Constitution dataset, BAO and CMB. The transition redshift from deceleration to acceleration was found to be around 0.80 for both datasets. We have also determined the dark energy parameter for the MCG model: Ω X0 = 0.81 ±0.03 for the Union2 dataset and Ω X0 = 0.83 ±0.03 using the Constitution dataset.

Discrete Energy Spectrum for a Spin-1/2 Quantum Particle Under the Influence of a Constant Force Field due to the Presence of Topological Defects

Abstract: We study a spin-1/2 quantum particle under a constant force field in the presence of a uniform distribution of parallel screw dislocations. We solve the Schrondiger equation, exactly, and find a discrete energy spectrum.

The Dielectric Tensor for Magnetized Dusty Plasmas with Superthermal Plasma Populations and Dust Particles of Different Sizes

Abstract: We present general expressions for the components of the dielectric tensor of magnetized dusty plasmas, valid for arbitrary direction of propagation and for situations in which populations of dust particles of different sizes are present in the plasma These expressions are derived using a kinetic approach which takes into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions, and features the components of the dielectric tensor in terms of a finite and an infinite series, containing all effects of harmonics and Larmor radius, and is valid for the whole range of frequencies above the plasma frequency of the dust particles, which are assumed to be motionless The integrals in velocity space which appear in the dielectric tensor are solved assuming that the electron and ion populations are described by anisotropic non-thermal distributions characterized by parameters κ∥ and κ⊥, featuring the Maxwellian as a limiting case These integrals can be written in terms of generalized dispersion functions, which can be expressed in terms of hypergeometric functions The formulation therefore becomes specially suitable for numerical analysis

Preparation and Characterization of SnS:Bi Thin Films

Abstract: Thin films based on Sn-S compounds are currently of great interest because of their potential applications in optoelectronic devices including solar cells. In this work, SnS:Bi thin films are prepared using a novel procedure based on sulfurization of their metallic precursors, varying the Bi content. The effect of the synthesis conditions on the optical properties, phase, and chemical composition of the SnS:Bi thin films was studied through spectral transmittance, X-ray diffraction, and X-ray photoelectron spectroscopy. It was established from transmittance measurements that the optical gap of the deposited films varies between 1.27 and 1.37 eV depending on the Bi content. The analysis revealed that the SnS:Bi thin films grow with a mixture of several phases which include SnS, Sn2S3 SnS2, and Bi2S3, depending on the Bi concentration. The studies also revealed that the conductivity type of the SnS:Bi films depends on the Bi content in the SnS lattice.

A Modified Generalized Chaplygin Gas as the Unified Dark Matter–Dark Energy Revisited

Abstract: A modified generalized Chaplygin gas (MGCG) is considered as the unified dark matter-dark energy revisited. The character of MGCG is endued with the dual role, which behaves as matter at early times and as a quiessence dark energy at late times. The equation of state for MGCG is p = -alpha rho/(1 + alpha) - nu(z)rho(-alpha)/(1 + alpha), where nu(z) = -[rho(0c)(1 + z)(3)]((1 + alpha))(1 - Omega(0B))(alpha) {alpha Omega(0DM) + Omega(0DE)[omega(DE) + alpha(1 + omega(DE))](1 + z)3(omega DE(1+alpha))}. Some cosmological quantities, such as the densities of different components of the universe Omega(i) (i, respectively, denotes baryons, dark matter, and dark energy) and the deceleration parameter q, are obtained. The present deceleration parameter q(0), the transition redshift z(T), and the redshift z(eq), which describes the epoch when the densities in dark matter and dark energy are equal, are also calculated. To distinguish MGCG from others, we then apply the Statefinder diagnostic. Later on, the parameters (alpha and omega(DE)) of MGCG are constrained by combination of the sound speed c(s)(2), the age of the universe t(0), the growth factor m, and the bias parameter b. It yields alpha = -3.07(-4.98)(+5.66) x 10(-2) and omega(DE) = -1.05(-0.11)(+0.06). Through the analysis of the growth of density perturbations for MGCG, it is found that the energy will transfer from dark matter to dark energy which reach equal at z(eq) similar to 0.48 and the density fluctuations start deviating from the linear behavior at z similar to 0.25 caused by the dominance of dark energy.

A Note on Classification of Cylindrically Symmetric Non-static Space-Times According to Their Teleparallel Killing Vector Fields in the Teleparallel Theory of Gravitation

Abstract: In this paper, we explored the conservation laws of cylindrically symmetric non-static space–times by using direct integration technique. This classification also covers non-static plane symmetric space–times, static cylindrically symmetric space–times and plane symmetric static space–times. In this paper, we will only present the results of non-static cylindrically symmetric and non-static plane symmetric space–times. The results of static cylindrically symmetric space–times and plane static space–times can be found in Shabbir and Khan (Mod Phys Lett A 25:525, 2010). It turns out that the non-static cylindrically symmetric space–times admit four, five, or seven conservation laws. It is important to note that the above space–times admit at least one or at the most four extra conservation laws.

Combination of Raman, Infrared, and X-Ray Energy-Dispersion Spectroscopies and X-Ray Diffraction to Study a Fossilization Process

Abstract: X-ray diffraction was combined with X-ray energy-dispersion, Fourier-transform infrared, and Raman spectroscopies to study the fossilization of a Cretaceous specimen of the plant Brachyphyllum castilhoi, a fossil from the Ipubi Formation, in the Araripe Sedimentary Basin, Northeastern Brazil. Among the possible fossilization processes, which could involve pyrite, silicon oxide, calcium oxide, or other minerals, we were able to single out pyritization as the central mechanism producing the fossil, more than 100 million years ago. In addition to expanding the knowledge of the Ipubi Formation, this study shows that, when combined with other experimental techniques, Raman spectroscopy is a valuable tool at the paleontologist’s disposal.

Electron-Beam-Induced Modifications in High-Density Polyethylene

Abstract: Post-irradiation studies have been carried out to elucidate the effects of electron beam irradiation on the structural, optical, dielectric, and thermal properties of high-density polyethylene (HDPE) films. The experimental results showed that both the optical band gap and activation energy of HDPE films decreases with an increase in the doses of electron radiation. The electrical measurements showed that dielectric constant and the ac conductivity of HDPE increases with an increase in the dose of electron radiation. The thermal analysis carried out using DSC and TGA revealed that the melting temperature, degree of crystallinity, and thermal stability of the HDPE films increased, obviously, due to the predominant cross-linking reaction following high doses of electron irradiation.

Bianchi -V Space -Time with Anisotropic Dark Energy in General Relativity

Abstract: In a spatially homogeneous and anisotropic Bianchi type-V space-time the consequences of the presence of dynamically anisotropic dark energy and perfect fluid with heat-conduction are studied. We assume that dark energy is minimally interacting with matter and has an equation of state which is modified in a consistent way with the conservation of energy momentum tensor. Exact solutions of Einstein field equations are obtained by taking constant value of deceleration parameter. We find that this assumption is reasonable for the observation of the present day universe. The physical and geometrical properties of the models, the behavior of the anisotropy of dark energy and the thermodynamical relations that govern such solutions are discussed in detail.

Growth of ZnO Nanostructures on Porous Silicon and Oxidized Porous Silicon Substrates

Abstract: We have investigated an oxidation of substrate effect on structural morphology of zinc oxide (ZnO) rods. ZnO rods are grown on porous silicon (PS) and on thermally oxidized porous silicon substrates by carbothermal reduction of ZnO powder through chemical vapour transport and condensation. Porous silicon is fabricated by electrochemical etching of silicon in hydrofluoric acid solution. The effects of substrates on morphology and structure of ZnO nanostructures have been studied. The morphology of substrates is studied by atomic force microscopy in contact mode. The texture coefficient of each sample is calculated from X-ray diffraction data that demonstrate random orientation of ZnO rods on oxidized porous silicon substrate. The morphology of structures is investigated by scanning electron microscopy that confirms the surface roughness tends to increase the growth rate of ZnO rods on oxidized PS compared with porous silicon substrate. A green emission has been observed in ZnO structures grown on oxidized PS substrates by photoluminescence measurements.

Interplay of Quantum and Classical Fluctuations Near Quantum Critical Points

Abstract: For a system near a quantum critical point (QCP), above its lower critical dimension dL, there is in general a critical line of second-order phase transitions that separates the broken symmetry phase at finite temperatures from the disordered phase. The phase transitions along this line are governed by thermal critical exponents that are different from those associated with the quantum critical point. We point out that, if the effective dimension of the QCP, deff = d + z (d is the Euclidean dimension of the system and z the dynamic quantum critical exponent) is above its upper critical dimension \(d_{_{C}}\) there is an intermingle of classical (thermal) and quantum critical fluctuations near the QCP. This is due to the breakdown of the generalized scaling relation ψ = νz between the shift exponent ψ of the critical line and the crossover exponent νz, for \(d+z>d_{_{C}}\) by a dangerous irrelevant interaction. This phenomenon has clear experimental consequences, like the suppression of the amplitude of classical critical fluctuations near the line of finite temperature phase transitions as the critical temperature is reduced approaching the QCP.

Representative Gamma-ray Computed Tomography Calibration for Applications in Soil Physics

Abstract: Tomographic image quality in soil physics ap- plications is extremely dependent on calibration. Here, good calibrations of the system are necessary to avoid errors during soil evaluations by computed tomography (CT), which can hamper interpretations of physical pa- rameters of the soil. In order to analyze the relevance of a good calibration curve (CC) for measurements of soil physical properties, determinations of soil bulk density (ρb ) were obtained using four different CCs established for a homemade CT scanner dedicated especially to soil physics. The calibrations of the system were ob- tained through the relationship between tomographic units and corresponding linear attenuation coefficients (μ1) of different materials. Data show that different calibration curves produce distinct ρb values affecting the quality of results of this soil physical property when evaluated by CT. However, it was demonstrated that even using non-homogeneous materials for CT calibra- tion the results of ρb practically are of the same order of magnitude of the whole system error estimated in 0.05 g cm −3 (taking the water as reference).

New Asymptotic Formulae for the Point Coulomb Phase Shift

Abstract: We investigate the Coulomb phase shift, and derive and analyze new and more precise analytical formulae. We consider next to leading order terms to the Stirling’s approximation, and show that they are important at small values of the angular momentum l and other regimes. We employ the uniform approximation. The use of our expressions in low-energy scattering of charged particles is discussed and some comparisons are made with other approximation methods. Potential use of our results in the search for deviations from pure Mott scattering of sub-barrier scattering of very heavy ions is pointed out.

Geometrothermodynamics of a Charged Black Hole of String Theory

Abstract: The thermodynamics of the Gibbons–Maeda–Garfinkle–Horowitz–Strominger charged black hole from string theory is reformulated within the context of the recently developed formalism of geometrothermodynamics. The geometry of the space of equilibrium states is curved, but we show that the thermodynamic curvature does not diverge at the phase transition point expected when the black hole solution becomes a naked singularity. This provides a counterexample to the conventional notion that such a divergence signals the occurrence of a second-order phase transition.

Stochastic Processes and Mean Field Systems Defined by Nonlinear Markov Chains: An Illustration for a Model of Evolutionary Population Dynamics

Abstract: In physics, there is a growing interest in studying stochastic processes described by evolution equations such as nonlinear master equations and nonlinear Fokker–Planck equations that define the so-called nonlinear Markov processes and are nonlinear with respect to probability densities. In this context, however, relatively little is known about nonlinear Markov processes defined by nonlinear Markov chains. In the present work, we demonstrate explicitly how the nonlinear Markov chain approach can be carried out by addressing a model for evolutionary population dynamics. In line with the nonlinear Markov chain approach, we derive a measure that tells us how attractive it is for a biological entity to evolve towards a particular biological type. Likewise, a measure for the noise level of the evolutionary process is obtained. Both measures are found to be implicitly time dependent. Finally, a simulation scheme for the many-body system corresponding to the Markov chain model is discussed.

Analytical Formulation for ϕ 4 Field Potential Dynamics

Abstract: An analytical model for adding a space-dependent potential to the ϕ 4 field equation of motion is presented, by constructing a collective coordinate system for the solitary solutions of this model. The interaction of ϕ 4 solitons with a delta function potential barrier and also delta function potential well is investigated. Most of the characters of interaction are derived analytically while they are calculated by other models numerically. We will find that the behaviour of a solitary solution is like a point particle which is moved under the influence of a complicated effective potential. The effective potential is a function of the field initial conditions and also of parameters of the added potential.

Bounds on the Slope and Curvature of Isgur-Wise Function in a QCD-Inspired Quark Model

Abstract: The quantum chromodynamics-inspired potential model pursued by us earlier has been recently modified to incorporate an additional factor ‘c’ in the linear cum Coulomb potential. While it felicitates the inclusion of standard confinement parameter b = 0.183 GeV2 unlike in previous work, it still falls short of explaining the Isgur-Wise function for the B mesons without ad hoc adjustment of the strong coupling constant. In this work, we determine the factor ‘c’ from the experimental values of decay constants and masses and show that the reality constraint on ‘c’ yields bounds on the strong coupling constant as well as on slope and curvature of Isgur-Wise function allowing more flexibility to the model.

Comparison of Digital Imaging Systems for Neutron Radiography

Abstract: The characteristics of three digital imaging systems for neutron radiography purposes have been compared. Two of them make use of films, CR-39 and Kodak AA, and the third makes use of a LiF scintillator, for image registration. The irradiations were performed in the neutron radiography facility installed at the IEA-R1 nuclear research reactor of IPEN-CNEN/SP. According to the obtained results, the system based on CR-39 is the slowest to obtain an image, and the best in terms of resolution but the worse in terms of contrast. The system based on Kodak AA is faster than the prior, exhibits good resolution and contrast. The system based on the scintillator is the fastest to obtain an image, and best in terms of contrast but the worse in terms of resolution.

News and Views: Perspectives for Nuclear Energy in Brazil After Fukushima

Abstract: More than two decades after the Chernobyl accident, the world was experiencing a nuclear renaissance when an earthquake followed by a tsunami, both of uncommon proportions, led to major releases of radiation at the Fukushima Dai-ichi nuclear central. Many countries are now reevaluating decisions to expand their nuclear parks, a change of course motivated by a number of considerations. Combined with the same premises, lessons learned from the history of its nuclear program compel Brazil to turn to the renewable sources of energy at its disposal.

Casimir Force Between Two Spatially Dispersive Dielectric Parallel Slabs

Abstract: We investigate the Casimir force F between two parallel spatially dispersive semiconductor slabs, whose dielectric response function accounts for nonlocal effects. Nonlocal effects are induced by the presence of excitonic transitions in the semiconductor slabs for which our studies consider the A n = 1 exciton in CdS and the Z31s one in CuCl. In order to explore the nonlocal effects in the Casimir force, we first calculate the S and P polarized reflection coefficients of the excitonic slabs then we use them in the functional form of F. The slabs are considered as homogeneous, and nonhomogeneous media where the latter is a periodic system having a unit cell with period d. We present numerical calculations of F as a function of the vacuum gap of width L between the slabs, different slab thicknesses, and periods. Comparisons between numerical results obtained by using a nonlocal and a local theory show that the nonlocal effects are more significant at short separations of the slabs. F suffers a small decrease as a consequence of the energy absorption induced by the excitons.

Thermodynamics of a Peyrard-Bishop One-Dimensional Lattice with On-site "Hump" Potential

Abstract: This paper presents a thermodynamic study of DNA through a Peyrard–Bishop one-dimensional lattice with an on-site “hump” potential. The transfer integral operator method was used to obtain the thermodynamic properties of the system and the solution of the Schrodinger-type equation that emerges from this formalism was determined by the variational method. With the parameters of the potential, commonly used in literature, the value obtained for the denaturation temperature was extremely high. This work suggests different parameters to describe the thermodynamics of DNA macromolecule.

Two- and Four-Level Systems in Magnetic Fields Restricted in Time

Abstract: We describe some new exact solutions for two- and four-level systems. In all the cases, external fields have a restricted behavior in time. First, we consider a method to construct new solutions for one-spin equation and give some explicit examples: One of them is in a external magnetic field that acts during a finite time interval. Then we show how these solutions can be used to solve the two-spin equation problem. A solution for two interacting spins is analyzed in the case when the field difference between the external fields in each spin varies adiabatically, vanishing on the time infinity. The latter system can be identified with a quantum gate realized by two coupled quantum dots. The probability of the Swap operation for such a gate can be explicitly expressed in terms of special functions. Using the obtained expressions, we construct plots for the Swap operation for some parameters of the external magnetic field and interaction function.

On Special Requantization of a Black Hole

Abstract: Quantized expressions for the gravitational energy and momentum are derived from a linearized theory of teleparallel gravity The derivation relies on a second-quantization procedure that constructs annihilation and creation operators for the graviton The resulting gravitational field is a collection of gravitons, each of which has precise energy and momentum On the basis of the weak-field approximation of Schwarzschild’s solution, a new form for the quantization of the mass of a black hole is derived

Dynamical Behavior of an Epidemic Model

Abstract: It is known that natural systems are undeni- ably subject to random fluctuations, arising from either environmental variability or internal effects. In this paper, a spatial version of an epidemic model which contains some important factors, such as noise on the infective and diffusion processes on both the suscepti- ble and infective, is investigated. From the numerical results, we know that noise can induce instability and enhance the oscillation of the species density and the cooperation between noise and diffusion gives rise to the appearance of a rich transport phenomenology. Our results show that noise can play a prominent role in the spatial epidemic model.

Cluster Model Analysis of Pion Elastic and Inelastic Scattering from 12C

Abstract: Angular distributions of differential cross sections for the 12C(π ±, π ±)12C and 12C(π ±, π ±)12C* reactions at pion kinetic energy ranging from 50 to 260 MeV have been analyzed with the 3α-particle model of 12C. The model provides good fits to a wide range of data. Differential cross sections for inelastic transitions to the (2 + ; 4.44 MeV) and (3 − ; 9.64 MeV) states in 12C are computed and the deformation lengths δ 2 and δ 3 are extracted. It is found that the extracted deformation lengths are sensitive to the nuclear model used and similar to the corresponding values found with other probes and nuclear models.