Showing papers in "Brazilian Journal of Physics in 2020"

Complex Networks: a Mini-review

Abstract: Network analysis is a powerful tool that provides us a fruitful framework to describe phenomena related to social, technological, and many other real-world complex systems. In this paper, we present a brief review about complex networks including fundamental quantities, examples of network models, and the essential role of network topology in the investigation of dynamical processes as epidemics, rumor spreading, and synchronization. A quite of advances have been provided in this field, and many other authors also review the main contributions in this area over the years. However, we show an overview from a different perspective. Our aim is to provide basic information to a broad audience and more detailed references for those who would like to learn deeper the topic.

Ultra-High Negative Dispersion Based Single Mode Highly Nonlinear Bored Core Photonic Crystal Fiber (HNL-BCPCF): Design and Numerical Analysis

Abstract: In this paper, two different models of highly nonlinear bored core photonic crystal fibers (HNL-BCPCF) are presented and compared for attaining an ultra-high negative dispersion coefficient and high nonlinearity. We achieved this dispersion by tailoring a defect into the solid core of the two proposed models and appropriately scaling down the diameter of the neighboring airholes of the core. To investigate the optical transmission properties in the fiber, simulations were carried out employing the finite element method (FEM) having a perfectly matched layer. The simulation results exhibited large negative dispersion coefficients of − 2218 ps/(nm-km) and − 2221 ps/(nm-km) for the two proposed models, respectively, when the wavelength had been tuned to 1550 nm and the corresponding nonlinear coefficients stand out to be 117.6 W−1 km−1 and 118.4 W−1 km−1. The fabrication process had been made much more feasible as the design consists of circular airholes. In our analysis, this geometry of photonic crystal fiber is noticeably more robust for its successful achievement of an ultra-high negative dispersion with high nonlinearity and facilitates optical back propagation applications and dispersion compensation for optical transmission systems.

Klein-Gordon Oscillator Under the Effects of the Cornell-Type Interaction in the Kaluza-Klein Theory

Abstract: We have investigated the interaction between the Klein-Gordon oscillator and the Cornell-type potential in a background characterized by the Kaluza-Klein theory, where it is governed by the manifestation of the extra dimension through the Aharonov-Bohm effect for bound states. Then, in the search for bound state solutions, we analytically determine the relativistic energy profile of the oscillator under the effects of Cornell-type interaction and for the particular cases of Coulomb-type and linear potentials, where in all cases, the frequency of the relativistic oscillator has restricted values determined by the quantum numbers of the system.

Investigations on the Synthesis and Decay Properties of Roentgenium

Abstract: This study focusses on identification and analysis of possible isotopes of the superheavy element Roentgenium (Z = 111) by studying competition between the decay modes of the 259-300Rg isotopes. It is also parameterized an equation for the fusion barrier height and position. After this we have also identified the possible projectile-target combinations to synthesize 268-279Rg by studying compound nucleus formation probability, survival probability, and evaporation residue cross sections. The selected projectile-target combinations to synthesize the superheavy element Z = 111 with the combinations are 58Ni+210Bi, 59,60Co+210Po,35,36Cl+236Pu, 35Cl+239-242Pu, 36Cl+238,242,244Pu. The present work is compared with the experimental values available in literature.

A Time-Dependent Harmonic Oscillator with Two Frequency Jumps: an Exact Algebraic Solution

Abstract: We consider a harmonic oscillator (HO) with a time-dependent frequency which undergoes two successive abrupt changes. By assumption, the HO starts in its fundamental state with frequency ω0, then, at t = 0, its frequency suddenly increases to ω1 and, after a finite time interval τ, it comes back to its original value ω0. Contrary to what one could naively think, this problem is quite a non-trivial one. Using algebraic methods, we obtain its exact analytical solution and show that at any time t > 0 the HO is in a vacuum squeezed state. We compute explicitly the corresponding squeezing parameter (SP) relative to the initial state at an arbitrary instant and show that, surprisingly, it exhibits oscillations after the first frequency jump (from ω0 to ω1), remaining constant after the second jump (from ω1 back to ω0). We also compute the time evolution of the variance of a quadrature. Last, but not least, we calculate the vacuum (fundamental state) persistence probability amplitude of the HO, as well as its transition probability amplitude for any excited state.

Enhanced CV Entanglement Quantification in a CEL with Parametric Amplifier and Coupled to Squeezed Vacuum

Abstract: Externally induced entanglement amplification in a coherently pumped correlated emission laser (CEL) with parametric amplifier and coupled to a two-mode squeezed vacuum reservoir is presented. The combination of the master equation and stochastic differential equation is employed to investigate the entanglement of the two-mode light generated by the quantum system. The resulting solutions of the correlation properties of noise forces associated with the normal ordering are used to find the mean photon number of the cross-correlated mode and separate cavity modes, quadrature fluctuations, smallest eigenvalue of the symplectic matrix, and photon number correlation function. It is found that pumping atoms from the lower energy state to excited state results in a robust entanglement that remains in its maximum strength over a wide range of the strong classical driving radiation. The introduction of the nonlinear crystal into the linear cavity, and coupling the system to the two-mode squeezed vacuum environment lead to a significant enhancement of entanglement of the cavity light. The enhanced entanglement is quantified employing logarithmic negativity, Cauchy–Schwarz inequality, and Duan et al. criteria. It has been observed that an intense light can be produced where the entanglement is strong.

A Unique Jerk System with Abundant Dynamics: Symmetric or Asymmetric Bistability, Tristability, and Coexisting Bubbles

Abstract: Finding and revealing new features and behaviors of simple chaotic systems have always been an important and attractive research topic. This paper aims to introduce a new 3D autonomous jerk chaotic system and explore its rich dynamical behaviors including period-doubling bifurcation and reverse period-doubling bifurcation routes to chaos, crisis and internal crisis, multiple symmetric coexisting attractors, and antimonotonicity. Especially, the phenomena of asymmetric bistability (e.g., coexistence of a point attractor and chaotic attractor or coexistence of a point attractor and period-5 limit cycle), tristability (e.g., coexistence of a point attractor and a pair of symmetric chaotic or periodic attractors), and coexisting bubbles are found, which have been rarely reported before. By using standard nonlinear analysis methods such as bifurcation diagrams, the largest Lyapunov exponent, phase portraits, Poincare sections, 0–1 test chart and the basin of attraction for an attractor, and the complex dynamical behaviors of the system are investigated in detail. Furthermore, a corresponding hardware electronic circuit is designed to verify the numerical simulations.

Shock Wave Driven Out by a Piston in a Mixture of a Non-ideal Gas and Small Solid Particles Under the Influence of Azimuthal or Axial Magnetic Field

Abstract: One-dimensional self-similar unsteady flows behind a spherical or cylindrical shock wave driven out by a piston moving with time according to a power law in a dusty gas is investigated The medium is assumed to be the mixture of small solid particles of micro-size and a non-ideal gas The solid particles are uniformly distributed in the mixture, and the shock wave is assumed to be driven by the inner surface (piston) It is assumed that the equilibrium flow-conditions are maintained and the moving piston continuously supplies the variable energy input Similarity solutions exist only when the surrounding medium is of constant density Solutions are obtained, in both cases, when the flow between the shock and the piston is isothermal or adiabatic The shock waves in non-ideal dusty gas can be important for the description of shocks in supernova explosions, in the study of a flare produced shock in the solar wind, the central part of starburst galaxies, nuclear explosion, rupture of the pressurized vessel, in the analysis of data from exploding wire experiments, and cylindrically symmetric hypersonic flow problems associated with meteors or re-entry vehicles, etc The findings of the present work provided a clear picture of whether and how the non-idealness of the gas and the presence of the magnetic field affect the propagation of shock and the flow behind it It is interesting to note that in the presence of azimuthal magnetic field, the pressure and density vanish at the piston and hence a vacuum is formed at the center of symmetry for both the isothermal and adiabatic flows, which is in excellent agreement with the laboratory condition to produce the shock wave

Dynamics of a Nondegenerate Three-Level Laser with Parametric Amplifier and Coupled to a Two-Mode Squeezed Vacuum Reservoir

Abstract: In this paper, the effect of a squeezed parameter and parametric amplifier on squeezing and entanglement of a correlated emission laser is thoroughly analyzed. The combination of the master equation and stochastic differential equation is presented to study the quantum features of the light. Moreover, with the aid of the resulting solutions together with the correlation properties of noise operators, the quadrature squeezing and entanglement and the mean number of photon pairs of the cavity light are determined. It is found that a large amplitude of the classical driving radiation induces a strong correlation between the top and bottom states of three-level atoms to produce a high degree of squeezing and entanglement. Furthermore, the presence of parametric amplifier and squeezed parameter is found to enhance the degree of squeezing and entanglement of the cavity light.

A Comparison of Time-Frequency Methods for Nonlinear Dynamics and Chaos Analysis in an Energy Harvesting Model

Abstract: In this paper, an energy harvesting model based on a portal frame structure, modeled as a duffing system, with a non-ideal excitation force, DC motor with an unbalanced mass, is presented and the piezoelectric coupling is designed to exhibit nonlinear characteristics. Nonlinearity included provides higher power output over a wide frequency range. This analysis was carried out by numerical simulation of the proposed mathematical formulation. Thus, the bifurcation diagram and the largest Lyapunov exponents are plotted to investigate the dynamic behavior by ranging the voltage applied to the DC motor. In this way, power harvesting is analyzed for two different dynamic responses: periodic and chaotic behavior. Furthermore, this work exhibits an application of frequency-domain techniques, such as short-time Fourier transform, continuous wavelet transform, synchrosqueezed wavelet transform, and Wigner-Ville distribution methods. These methods are often used to analyze non-stationary signals, allowing the verification of dynamic behavior and power harvesting. Therefore, this paper aims to apply the time-frequency methods, mentioned previously, to analyze the mechanical system response in different behaviors.

A Conjecture on the Relationship Between Critical Residual Entropy and Finite Temperature Pseudo-transitions of One-dimensional Models

Abstract: Recently pseudo-critical temperature clues were observed in one-dimensional spin models, such as the Ising-Heisenberg spin models, among others. Here we report a relationship between the zero-temperature phase boundary residual entropy (critical residual entropy) and pseudo-transition. Usually, the residual entropy increases in the phase boundary, which means the system becomes more degenerate at the phase boundary compared with its adjacent states. However, this is not always the case; at zero temperature, there are some phase boundaries where the entropy holds the largest residual entropy of the adjacent states. Therefore, we can propose the following conjecture: If residual entropy at zero temperature is a continuous function at least from the one-sided limit at a critical point, then pseudo-transition evidence will appear at finite temperature near the critical point. We expect that this argument would apply to study more realistic models. Only by analyzing the residual entropy at zero temperature, one could identify a priori whether the system will exhibit the pseudo-transition at finite temperature. To strengthen our conjecture, we use two examples of Ising-Heisenberg models, which exhibit pseudo-transition behavior: one frustrated coupled tetrahedral chain and another unfrustrated diamond chain.

Improved tanΦ(ξ)2$\tan \left (\frac {\Phi (\xi )}{2}\right )$ -Expansion Approach for Burgers Equation in Nonlinear Dynamical Model of Ion Acoustic Waves

Abstract: The main purpose of this research is to find new and more exact traveling wave solutions of nonlinear dynamical model of ion acoustic waves in plasma with the help of famous reductive perturbation technique. A well-known nonlinear evolution equation, namely Burgers equation, is derived to investigate ion acoustic waves in relativistic plasma containing electrons and positrons. The traveling wave solution of Burgers equation is carried out by an affective integration tool, namely improved $\tan \left (\frac {\Phi (\xi )}{2}\right )$ expansion method. As a result, different types of solutions such as periodic, kink, rational, exponential function, and hyperbolic function, are obtained. The wave profiles of solitary wave solutions are graphically depicted. The algorithm of the suggested technique is also outlined. It is admitted that the used method is efficient and influential tool for constructing solitary wave solutions of nonlinear evolution equations.

Chaos and Coexisting Bifurcations in a Novel 3D Autonomous System with a Non-Hyperbolic Fixed Point: Theoretical Analysis and Electronic Circuit Implementation

Abstract: A 3D autonomous chaotic system with the distinguishing feature of having a couple of fixed points, one of which is non-hyperbolic, is proposed. Interestingly, these fixed points become all hyperbolic in other parameters’ regions yielding diverse modes of oscillations in the system. The stability of the equilibria is discussed based on the Routh–Hurwitz stability criterion. The complex dynamics of the proposed system is numerically explored by using phase space trajectory plots, bifurcation diagrams, graphs of Lyapunov exponents, and basins of attraction. It is found that the system experiences period-doubling bifurcation, coexisting bifurcations, periodic windows, and chaos when monitoring its parameters. When the system is tuned to develop non-hyperbolic chaos, the basin of attraction of the strange attractor (coexisting with one of the fixed points) intersects with neighborhood of equilibria which is typical of self-excited oscillations. The coexistence between periodic and chaotic behaviors is found for specific parameter values. The analysis of the basins of attraction for the coexisting attractors reveals extremely complex structures. PSpice simulations based on a suitably designed electronic analog of the system confirm the results of theoretical analysis. The model proposed in this work shows “elegant” mathematical simplicity (i.e., only quadratic nonlinearities) and extremely rich modes of oscillations, and thus may be regarded as a prototypal member of the recently discovered and very restricted class of nonlinear systems developing non-hyperbolic chaos.

The Hidden Ramsauer-Townsend Effect in Positron Scattering by Rare Gas Atoms

Abstract: The aim of this work is to present and discuss the Ramsauer-Townsend effect in the context of low-energy positron scattering by rare gas atoms. Opposed to what happens in electron-atom scattering, the competition between the static repulsive and polarization attractive potentials makes the phenomenon improbable in positron case. Nonetheless, we show using a semiempirical potential formulation that, while the effect is explicitly observed in the total cross sections for He and Ne, for the heavier noble gases it becomes “hidden” or implicit. We show that as the atomic polarizability increases, the suppression of s-wave goes to higher energies due to the variation of the scattering length with the polarizability. No specific signature for the effect is found in the shape of the differential cross sections but curiously, while the effect is implicit in the total cross section for Ar, it generates a minimum structure in the momentum transfer cross section. The theme is presented for non-specialist audience with emphasis in basic atomic and scattering theories along with practical results, the main objective being to boost the traditional discussions with a new view on the subject.

Design of Micro-ring Resonator Based 4 × 4 Optical Router for Photonic Network Applications

Abstract: A 4 × 4 optical router which is composed of only four micro-ring resonator based switching elements is designed without any need of optoelectronic conversion for use in integrated optical networks. The geometrical structure including the number of rings and waveguide junctions are optimized through numerical simulation, which minimizes the crosstalk and the transmission loss and improves the loss of different paths of the optical router. The design can route four optical single-wavelength input signals to any of the four output ports and provided extinction ratio larger than 14 dB for all available optical paths. The average switching power consumption is about 1.2 mW per ring. The routing functionality and optical signal reliability are confirmed through numerical simulation.

Stochastic approach to epidemic spreading

Abstract: We analyze four models of epidemic spreading using a stochastic approach in which the primary stochastic variables are the numbers of individuals in each class. The stochastic approach is described by a master equation and the transition rates for each process such as infection or recovery are set up by using the law of mass action. We perform numerical simulations as well as numerical integration of the evolution equations for the average number of each class of individuals. The onset of the epidemic spreading is obtained by a linear analysis of the disease free state, from which follows the initial exponential increase of the infected and the frequency of new cases. The order parameter and the variance in the number of individuals are also obtained characterizing the onset of epidemic spreading as a critical phase transition.

Study of Particle Creation with Quadratic Equation of State in Higher Derivative Theory

Abstract: We investigate the effect of particle creation in flat Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime under the higher derivative theory composition. The exact solution of the field equation in higher derivative theory is obtained by considering the quadratic equation of state, p = ωρ2 − ρ and different forms of deceleration parameter. The physical acceptance and stability of the models are explored by discussing energy conditions and squared sound speed. In addition to that, the statefinder diagnostic pair {r,s} is also scrutinized to measure the deviation of considered models from Λ cold dark matter (ΛCDM) model.

Quantum Simulation of FMO Complex Using One-Parameter Semigroup of Generators

Abstract: The application of open quantum systems in biological processes such as photosynthetic complexes has recently received renewed attention. In this paper, we introduce a quantum algorithm for simulation of Markovian dynamics of the Fenna-Matthew-Olson (FMO) complex that exists in photosynthesis using a “universal set” of a one-parameter semigroup of generators. We investigate the details of each generator that has been obtained from spectral decomposition of the Gorini-Kossakowski-Sudarshan (GKS) matrix by using linear combination and unitary conjugation. Also, we present a simple quantum circuit for the implementation of these generators.

Life-Cycle Evaluation of Anisotropic Particle-Based Magnetorheological Fluid in MR Brake Performance

Abstract: The work reports the life-cycle test of flake-shaped particle-based magnetorheological (MR) fluid using the MR brake system and evaluated the performance of brake. The choice of the application is based on the understanding that normal force in this case is zero and the same is not true for other applications. Thus, it will exhibit only shear-induced deformation/degradation of MR fluid properties. The test was performed under a constant magnetic field for 105 cycles at 300 rpm. The MR fluid collected after cyclic operation exhibits no change in surface morphology as well as MR properties. Thus, the torque transmission even after this cycle remains the same. This is not the same when commercially available spherical particle-based MR fluid is used. There is a decrement in the torque transmission of MR brake by ~ 30%. The results are discussed on the basis of MR effect and the change in surface morphology of particles. The results confirm that the use of flake-shaped-based MR fluid gives better performance when used for long-term application.

First-Order Gauge-Invariant Generalization of the Quantum Rigid Rotor

Abstract: A first-order gauge-invariant formulation for the two-dimensional quantum rigid rotor is long known in the theoretical physics community as an isolated peculiar model. Parallel to that fact, the longstanding constraints abelianization problem, aiming at the conversion from second- to first-class systems for quantization purposes, has been approached a number of times in the literature with a handful of different forms and techniques and still continues to be a source of lively and interesting discussions. Connecting these two points, we develop a new systematic method for converting second-class systems to first-class ones, valid for a family of systems encompassing the quantum rigid rotor as a special case. In particular, the gauge invariance of the quantum rigid rotor is fully clarified and generalized in the context of arbitrary translations along the radial momentum direction. Our method differs substantially from previous ones as it does not rely neither on the introduction of new auxiliary variables nor on the a priori interpretation of the second-class constraints as coming from a gauge-fixing process.

On the Dilution Refrigerator Thermal Connection for the SCHENBERG Gravitational Wave Detector

Abstract: The resonant-mass gravitational wave detector SCHENBERG was designed by the Brazilian group Graviton to be sensitive to a central frequency nearing 3200 Hz and a bandwidth of 200 Hz. It has a spherical antenna weighing 1150 kg that is connected to the outer environment by a suspension system designed to attenuate local noise due to seism as well as other sources. Should a gravitational wave pass by the detector, the antenna is expected to vibrate. This motion will be monitored by six parametric transducers whose output signals will be digitally analyzed. In order to improve the sensitivity of the detector, it must be cooled down to the lowest possible temperature, and for this purpose a dilution refrigerator is planned to be implemented in the detector. It is known that such device produces vibration when operational, consequently introducing noise in the system. Using the finite elements method, this work investigates thermal connections between the dilution refrigerator and the sphere suspension that allow the detector to operate within its projected sensitivity. The finite elements method showed an attenuation of 240dB in the best-valued thermal connection.

Compensation Temperature Behavior in a Nanotube Core-Shell Structure with RKKY Interactions: Monte Carlo Simulations

Abstract: In this paper, we have studied the magnetic properties of the nanotube core-shell structure with RKKY (Ruderman-Kittel-Kasuya-Yosida) interactions, using Monte Carlo simulations (MCS). The system consists of hexagonal core-shell nanotube structure with mixed spins σ = ± 3/2, ± 1/2 (of the core) and S = ±1, 0 (of the shell), separated by non-magnetic nanotubes. Initially, we start this study by discussing for zero temperature, the ground-state phase diagrams in different planes. Moreover, we investigate for the positive temperature values, the effect of the RKKY interactions on the thermal magnetization and magnetic susceptibility of the system. Additionally, we study the effects of the exchange coupling interactions of the core and of the shell on the compensation and transition temperatures. Finally, we explore the behavior of the magnetic hysteresis cycles as a function of the non-magnetic nanotubes number, the temperature, and the exchange coupling parameters.

α-decay Chains of Superheavy Nuclei Nh Using a Finite-Range NN Interaction

Abstract: The α-decay half-lives of nihonium, 279 − 284Nh, isotopes and their α-decay products are studied by employing the density-dependent cluster model. The Wentzel-Kramers-Brillouin semiclassical approximation including the Bohr-Sommerfeld quantization condition is adopted in the evaluation of the α-decay widths. The double-folding α-daughter potential was determined using the effective M3Y-Paris nucleon-nucleon (NN) interaction. Both spherical and deformed shapes for the daughter nuclei are considered with the zero-range exchange contribution of the NN interaction. Investigations of the effects of nonlocality on α-decay half-lives have been studied through the finite-range exchange part of the NN interaction. We found that the inclusion of the finite-range exchange part decreases α-decay half-lives as compared with those values using the zero-range calculations. The inclusion of deformation degrees of freedom in the daughter nuclei reduces the α-decay half-lives as compared with spherical shape. The calculated α-decay half-lives are compared with other theoretical models and they are in good agreement. The competition between α-decay and spontaneous fission is studied and the possible decay modes are presented for unknown nuclei.

A Novel Covariant Approach to Gravito-Electromagnetism

Abstract: This paper describes general relativity at the gravito-electromagnetic precision level as a constrained field theory. In this novel formulation, the gravity field comprises two auxiliary fields, a static matter field and a moving matter field. Equations of motion, continuity equation, energy conservation, field tensor, energy-momentum tensor, constraints, and Lagrangian formulation are presented as a simple and unified formulation that can be useful for future research.

Experimental and DFT Studies on Thermochromism Induced Binary HBLC Mixture

Abstract: Novel linear double hydrogen-bonded liquid crystals (HBLCs) were derived from diglycolic acid (DGA, a non-mesogenic compound) and 4-n-alkoxybenzoic acids (nOBA, n = 7 and 8 mesogenic compounds). Hydrogen bond (H bond) formation and its vibrational stretching frequencies had been calculated by experimental and theoretical IR spectroscopy. The calculated band gap energy (4.96 eV) using a UV-Vis spectrum clearly reveals the coincidence of highest occupied molecular orbital-lowest unoccupied molecular orbital band energy of the present HBLC mixture. Further, X-ray diffraction (XRD) studies at room temperature confirm the monoclinic nature of the HBLC mixture. Mesophases and their transition temperature were studied by a polarized optical microscope (POM) and differential scanning calorimetry (DSC). The order of the phase transition was evaluated by the thermal analysis. Due to the rotary motion of molecules, nematic phase (threaded texture) with thermochromic effect was observed. The induced thermochromism in the present HBLC and its possible color recording applications were discussed. Molecular descriptors (using computational density functional theory (DFT)) of the present mixture indicate the hardness and softness of the HBLC mixture. Natural bond orbital (NBO) studies revealed the O–H…O stabilization energy in the present HBLC mixture. Also, the lone pair (LP)-to-π* transition confirms the existence of intermolecular hydrogen bonding in the HBLC mixture. The calculated band gap energy of the DGA + nOBA HBLC mixture is a more useful parameter to identify suitable hydrogen-bonded liquid crystal material for photonic applications. Mulliken analysis shows clear evidence of the charge distribution in different molecules of the HBLC system.

Exact Solutions to Schrödinger Equation for a Charged Particle on a Torus in Uniform Electric and Magnetic Fields

Abstract: We present exact solutions of the Schrodinger equation for a charged particle constrained to move along a toroidal surface in the presence of uniform electric and magnetic fields

Effect of the OH Groups on Spectroscopic Parameters of the Er 3+ -Doped Glasses

Abstract: In this work, the PAN, PANB, PANK, and SNAB glass matrices with nominal compositions 60P205.10Al2O3.30Na2O, 60P205.10Al2O3.15Na2O.15BaO, 40P2O5.20Al2O3.35Na2O.5K2O, and 40SiO2.30Na2CO3.1Al2O3.29B2O3 (mol%), respectively, doped with Er3+ ions, were studied. To determine the Er3+ spectroscopic properties in these matrices, the optical absorption, luminescence, and experimental lifetime were measured, and based on the obtained data, the Judd-Ofelt theory was applied. The experimental lifetime and quantum efficiency of the 4I13/2 → 4I15/2 transition were analyzed as a function of the Er3+ concentration. Among the studied samples, the phosphate glass PANK presents the higher lifetime (~ 8.8 ms) and quantum efficiency (~ 88%), due to the small hydroxyl content.

Inclusion of Mass Density Fluctuations in the Landau-de Gennes Theory of Nematic-Isotropic Phase Transition

Abstract: Experimental evidences have shown that mass density gradients couple energetically to the liquid crystal director. In this paper, we incorporate this mechanism into the Landau-de Gennes theory of nematic-isotropic transition. In conformity with empirical evidence, the extended theory enables us to explain the enhancement to the amplitude of the singular part of the specific heat above the nematic-isotropic phase transition temperature TNI. It also permits us to obtain a correction term for the Kerr constant B in isotropic phase proportional to 1/(T − T∗)2, where T∗ is the virtual transition temperature. The curve B versus T thus corrected provides a very good fit to experimental data.

Recent Advances of Epitaxial BiVO 4 Thin Film: Preparation and Physical and Photoelectrochemical Properties

Abstract: Semiconductor photocatalysis has two important applications: environmental remediation and H2 production. Bismuth vanadate has attracted great interests as an ideal candidate for use as high-performance photocatalysts for visible light-driven water splitting and photoanode in photoelectrochemical cells. Single crystal film is invaluable to deepen the fundamental understanding of materials. The object of this article was to provide a brief review on the advances in the preparation of epitaxial BiVO4 thin film, the physical and photoelectrochemical properties. The epitaxial BiVO4 thin film could be fabricated on (001) yttria-stabilized cubic zirconia or SrTiO3(001) by molecular beam epitaxy, chemical vapor deposition, and, pulsed laser deposition. Three crystal structures, namely, monoclinic scheelite, monoclinic clinobisvanite, and orthorhombic, were mentioned in the literatures. The optical band gap was reported to be 2.5~2.7 eV for a direct transition. The highest photocurrent of 4.83 mA cm−2at 1.23 V vs. the RHE was obtained at the sample with Lu2O3 as a hole blocking layer.

New Effects in the Vicinity of a Perfectly Conducting Plate in a Non-minimal Lorentz Violation Scenario

Abstract: In this paper we consider some physical phenomena in the vicinity of a conducting plate in a non-minimal Lorentz-violating scenario in 3 + 1 dimensions. We consider a model where the dynamics of the abelian gauge field is given by the standard Maxwell Lagrangian added by a Lorentz symmetry breaking term with the presence of higher order derivatives in the gauge field. The Lorentz symmetry breaking is due to the presence of a single background vector dλ and we perform the calculations perturbatively up to first order in dλ. We obtain the propagator for the gauge field in the presence of a mirror and the interaction force between the mirror and a point-like electric charge, as well as the interaction force between the mirror and an electric dipole. We show that, on the contrary to what happens in a minimal Lorentz-violating scenario with one single background field (L.H.C. Borges, F.A. Barone, Eur. Phys. J. C 693, 77, 2017), the image method is not valid in the model that we consider in this paper. As a consequence, the symmetry of spatial reflection on the mirror is broken. We also investigate the emergence of a torque in a system composed by a mirror and a point-like electric charge, placed at a fixed distance apart each other. The results obtained in this work have no counterpart in the Maxwell electrodynamics and were not verified in any Lorentz-violating scenario previously.