Dipole Behavior Research Articles

Detecting Electric Dipoles Interaction at the Interface of Ferroelectric and Electrolyte Using Graphene Field Effect Transistors.

Graphene was inserted into the interface between electric dipole layers from DEME-TFSI ionic liquid (top-gate) and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT, back-gate) to probe the interface dipole-dipole interaction in response to DC and pulsed gate voltages. A highly complicated behavior of the interface dipole-dipole interaction has been revealed as a combination of electrostatic and electrochemical effects. The interfacial polar molecules in the DEME-TFSI electrical double layer are pinned with assistance from the PLZT back-gate in response to a DC top-gate pump, leading to strong nonlinear electrochemical behavior. In contrast, depinning of these molecules can be facilitated by a faster pulsed top-gate pump, which results in a characteristic linear electrostatic behavior. This result not only sheds light on the dynamic dipole-dipole interactions on the interface between functional materials but also prototypes a unique pump and probe approach using graphene field effect transistors to detect the interface dipole-dipole interaction.

Multiscale Coarse-Graining of Polarizable Models through Force-Matched Dipole Fluctuations.

An extension of the multiscale coarse-graining method (MS-CG) to polarizable coarse-grain (CG) models is presented. In the extension, force matching is used to derive charged dimers that mimic the dipole behavior, including electronic polarizability, of fine resolution systems. The extended MS-CG method separates short-range and electrostatic forces and treats the polarization interactions by representing dipole fluctuations with a harmonic bond reminiscent of the Drude oscillator. The new method is first tested on several flexible alcohols, where the transferable head groups in the atomistic field lead to transferable electrostatics in the CG field. The method is then applied to a polarizable methanol model, where the CG dimer is able to match the atomistic dipole distribution. Force fields are benchmarked with radial distribution functions and dielectric constants. The static dielectric constants agree well with atomistic models, but the faster dynamics in the CG ensembles leads to significant deviations in the frequency-dependent permittivity. Model transferability is checked by measuring response to static external fields through a dipole coupling order parameter. Effective polarizabilities are inferred from the dimer harmonic bonds, and an ad hoc correction is used to improve one CG force field's dipole magnitude response to electric fields. Broader applicability is explored through a small set of amines, and several multibead models are tested where CG dipole orientations are restricted through intramolecular forces, capturing constructive and destructive dipole interactions within single molecules.

A gaussian model for simulated geomagnetic field reversals

Field reversals are the most spectacular events in the geomagnetic history but remain little understood. Here we explore the dipole behaviour in particularly long numerical dynamo simulations to reveal statistically significant conditions required for reversals and excursions to happen. We find that changes in the axial dipole moment behaviour are crucial while the equatorial dipole moment plays a negligible role. For small Rayleigh numbers, the axial dipole always remains strong and stable and obeys a clearly Gaussian probability distribution. Only when the Rayleigh number is increased sufficiently the axial dipole can reverse and its distribution becomes decisively non-Gaussian. Increased likelihoods around zero indicate a pronounced lingering in a new low dipole moment state. Reversals and excursions can only happen when axial dipole fluctuations are large enough to drive the system from the high dipole moment state assumed during stable polarity epochs into the low dipole moment state. Since it is just a matter of chance which polarity is amplified during dipole recovery, reversals and grand excursions, i.e. excursions during which the dipole assumes reverse polarity, are equally likely. While the overall reversal behaviour seems Earth-like, a closer comparison to palaeomagnetic findings suggests that the simulated events last too long and that grand excursions are too rare. For a particularly large Ekman number we find a second but less Earth-like type of reversals where the total field decays and recovers after a certain time.

Pinch-off of axisymmetric vortex pairs in the limit of vanishing vortex line curvature

Pinch-off of axisymmetric vortex pairs generated by flow between concentric cylinders with radial separation ΔR was studied numerically and compared with planar vortex dipole behavior. The axisymmetric case approaches planar vortex dipole behavior in the limit of vanishing ΔR. The flow was simulated at a jet Reynolds number of 1000 (based on ΔR and the jet velocity), jet pulse length-to-gap ratio (LΔR) in the range 10–20, and gap-to-outer radius ratio (ΔRRo) in the range 0.01-0.1. Contrary to investigations of strictly planar flows, vortex pinch-off was observed for all gap sizes investigated. This difference was attributed to the less constrained geometry considered, suggesting that even very small amounts of vortex line curvature and/or vortex stretching may disrupt the absence of pinch-off observed in strictly planar vortex dipoles.

Small oscillations of a 3D electric dipole in the presence of a uniform magnetic field

The classical behavior of a 3D electric dipole in the presence of a uniform magnetic field is studied in the small oscillations approximation. Using the Lagrangian formulation, the equations of motion are obtained, as well as their solutions and constants of motion. Normal modes of oscillation and their corresponding normal coordinates are obtained. Furthermore, the existence of a type of bound states without turning points, so-called trapped states conjectured by Troncoso and Curilef [Eur. J. Phys 27 (2006) 1315-1322], is investigated.

Solution-processed organic trilayer solar cells incorporating conjugated polyelectrolytes

We report solution-processed organic trilayer solar cells consisting of a bottom poly(3-hexylthiophene) (P3HT) layer, a conjugated polyelectrolyte (CPE) interlayer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) top layer, wherein the CPE exists as an interlayer within the donor–acceptor junction. The influence of interlayer thickness on device properties was investigated, as well as the behavior of molecular dipoles in the trilayer solar cells when influenced by external electrical stimuli. We found that incorporation of an interlayer which is too thick results in decreased performance due to reduced short-circuit current (JSC), open-circuit voltage (VOC), and fill factor (FF). However the VOC is found to increase significantly when a thin CPE layer is used in conjunction with an external electric field. These results provide an experimental approach to probe the influence of interfacial dipoles on the solar cell parameters and behavior of charge separating organic donor/acceptor junctions, yielding fundamental information about the influence of electrical dipoles on the donor/acceptor interface in organic solar cells.

Assessment of split-charge equilibration model for development of polarizable force fields

Performance of split-charge equilibration model (SQE) for force field development was tested by computing static and response characteristics of water clusters. Results of the study show that SQE yields reliable results for the molecular electrostatic potential, dipole, and polarizability of clusters of increasing size. Intermolecular charge transfer needs to be considered for hydrogen bonded systems in order to accurately reproduce the dipole versus system size behavior. It was found that the model quantitatively mimics the IR intensities of intermolecular vibrational modes, but fails in the computation of the IR and Raman intensities of intramolecular vibrations. The response of charge distribution to perturbation by point charge is confidently modeled by SQE in the conditions corresponding to the linear-response regime.

The Coupling Behavior of Multiple Dipoles and Localized Surface Plasmons in Ag Nanoparticles Array

In this work, the coupling behavior of multiple dipoles and localized surface plasmons (LSPs) in Ag nanoparticle arrays is explored based on experimental results and 3D finite difference time domain (FDTD) simulations. The Ag nanoparticles (NPs) located inside the hexagonal photonic crystal (PhC) array holes are embedded in a green light-emitting diode (LED), which enhances emission efficiency significantly. In the simulation of the 3D FDTD, five spaced x-polarized dipoles are approximated as five quantum wells. The internal quantum efficiency (IQE) and light extraction efficiency (LEE) of the LSP-coupled LED are deduced respectively from the original IQE of the bare LED and the FDTD simulation results. Besides, the dynamic LSP-dipole coupling behavior is also explored considering the interaction of the five dipoles and their feedback effect to LSP, which lead to the magnification of the LSP-dipole coupling enhancement effect and the reduction of energy dissipation in Ag NPs.

Effects of apparent mass on the radiated sound power from fluid-loaded structures

Prediction of radiated noise from underwater vehicles due to propeller excitation is of significant interest to maritime defence industries. In mathematical and numerical modelling, the pressure hull of an underwater vehicle is usually modelled with additional mass to achieve neutral buoyancy in water. However, the hull can also be regarded as a buoyant structure surrounded by free-flood spaces whose boundaries are acoustically transparent when flooded. In this work, apparent relative mass describes the ratio of the apparent mass of a longitudinal cylinder section to the mass of the water displaced by that section, where apparent indicates the fact that the effective structural mass is a function of frequency. The overall apparent mass and the apparent sectional mass distribution have an important effect on sound radiation. A rigid cylindrical hull with hemispherical ends is initially examined. Departures from dipole behaviour for a rigid body are shown to depend on apparent relative mass, cylinder length and direction of excitation. The analysis is then extended to higher frequencies and to a representative submarine pressure hull with stiffeners, bulkheads and added mass. The contributions of the rigid body and flexible modes to the radiated sound power for axial and transverse excitation cases are presented.

Large and Tunable Polar-Toroidal Coupling in Ferroelectric Composite Nanowires toward Superior Electromechanical Responses.

The collective dipole behaviors in (BaTiO3)m/(SrTiO3)n composite nanowires are investigated based on the first-principles-derived simulations. It demonstrates that such nanowire systems exhibit intriguing dipole orders, due to the combining effect of the anisotropic electrostatic interaction of the nanowire, the SrTiO3-layer-modified electrostatic interaction and the multiphase ground state of BaTiO3 layer. Particularly, a strong polar-toroidal coupling that is tunable by the SrTiO3-layer thickness, temperature, external strains and electric fields is found to exist in the nanowires, with the appearance of fruitful dipole states (including those being purely polar, purely toroidal, both polar and toroidal, or distorted toroidal) and phase boundaries. As a consequence, an efficient cross control of the toroidal (polar) order by static (curled) electric field, and superior piezoelectric and piezotoroidal responses, can be achieved in the nanowires. The result provides new insights into the collective dipole behaviors in nanowire systems.

Lorentz coherence and the proton form factor

The dipole cutoff behavior for the proton form factor has been and still is one of the major issues in high-energy physics. It is shown that this dipole behavior comes from the coherence between the Lorentz contraction of the proton size and the decreasing wavelength of the incoming photon signal. The contraction rates are the same for both cases. This form of coherence is studied also in the momentum–energy space. The coherence effect in this space can be explained in terms of two overlapping wave functions.

Spatial spectra of the geomagnetic field in the observations and geodynamo models

The correlation coefficients of the geomagnetic field on different spatial scales are estimated for the observations since 1590 and for a three-dimensional spherical-shell geodynamo model. It is shown that in the latter case, despite the presence of the geomagnetic reversals, the correlation of the magnetic field on different scales is low. The time behavior of the magnetic dipole is correlated to the evolution of the integral magnetic energy in the liquid core, whereas the dependence of the behavior of the integral kinetic energy on the magnetic field evolution is weak. Two-dimensional spectra of the magnetic field, velocity field, and cross-helicity in the liquid core of the Earth are presented.

A contribution from dielectric analysis to the study of the formation of multi-wall carbon nanotubes percolated networks in epoxy resin under an electric field

The formation of percolation networks in epoxy matrix nanocomposites reinforced with multi-wall carbon nanotubes (MWNT) during the curing process, at different MWNT contents, was studied by using a parallel plate cell subjected to a 300 V/cm AC electric field at 1 kHz. The percolation was verified by the electrical current output measured during and after the resin curing. The behavior of electric dipoles was characterized by impedance spectroscopy and followed the Debye first order dispersion model, by which an average relaxation time of 6.0 × 10−4 s and a cut-off frequency of 1.7 kHz were experimentally found. By applying the theory of percolation, a critical probability, pc, equal to 0.038 vol% and an exponent of conductivity of 2.0 were found. Both aligned and random samples showed dipole relaxation times typical of interfacial and/or charge-hopping polarization, while the permittivity exhibited an exponential decrease with frequency. This behavior can be related to the increased ability to trap electrical charges due to the formation of the carbon nanotubes network. Optical and electron microscopies confirm the theoretical prediction that the application of an electric field during cure helps the process of MWNT debundling in epoxy resin.

Surface plasmon coupled light-emitting diode: Experimental and numerical studies

First, the experimental implementations and theoretical/numerical investigations of surface plasmon (SP) coupled InGaN/GaN quantum-well light-emitting diodes (LEDs) are reviewed. If the p-GaN layer in an LED can be thin, surface metal nanoparticle (NP) is an inexpensive structure for inducing effective SP coupling. When the p-GaN layer is thick, a few metal structures, including metal protrusion, buried metal NP, and embedded metal NP, can be used for effective SP coupling. In the numerical study, an algorithm, including the feedback effect of the induced SP resonance on the radiating behavior of the source dipole, has been proposed for studying the SP coupling effects with an embedded metal NP, a surface metal NP, and a metal protrusion. Then, the theoretical formulations and numerical algorithms for evaluating the radiated power enhancement in the coupling process between two radiating dipoles and the localized surface plasmon (LSP) induced on a nearby Ag NP are built. Three mechanisms are considered in the coupling process for radiated power enhancement, including the interference of the two phase-retarded radiation contributions from the two dipoles, the interaction between the two dipoles, and the LSP resonant coupling.

Cell dipole behaviour revealed by ECM sub-cellular geometry.

Cells sense and respond to their mechanical environment by exerting forces on their surroundings. The way forces are modulated by extra-cellular matrix (ECM) properties plays a key role in tissue homoeostasis. Using highly resolved micropatterns that constrain cells into the same square envelope but vary the adhesive geometry, here we investigate how the adhesive micro-environment affects the architecture of actin cytoskeleton and the orientation of traction forces. Our data demonstrate that local adhesive changes can trigger orientational ordering of stress fibres throughout the cell, suggesting that cells are capable of integrating information on ECM geometry at the whole-cell level. Finally, we show that cells tend to generate highly polarized force pattern, that is, unidirectional pinching, in response to adequate adhesive conditions. Hence, the geometry of adhesive environment can induce cellular orientation, a process which may have significant implications for the formation and mechanical properties of tissues.

Precipitate size dependence of Ni/Ni3Al interface energy

Abstract The interface energy of Ni/Ni3Al dictates the morphology of Ni3Al precipitate and thereby the creep resistance of the Ni-base superalloy. Based on classical molecular dynamics simulations, we report that the interface formation energy depends on the size of Ni3Al precipitate. This dependence is non-monotonic below 20 nm, and converges to a constant value beyond 20 nm. Such dependence is a result of competition between the attractive interaction of dislocations from neighboring interfaces and the strain energy in the Ni3Al precipitate. Further, from elasticity theory, taking into account of the effect of misfit strain, a similar transformation behavior of dislocation dipole's elastic energy versus interface distance was also obtained, which shows the transition point of about 6.2 nm.

Photoionization of Ca 4s in a spherical attractive well potential: dipole, quadrupole and relativistic effects

A study of the outer 4s subshell of the Ca atom in a spherical attractive potential well (SAW) of variable depth is performed to determine the effect of an external potential on dipole (E1) and quadrupole (E2) photoionization processes. As the depth of the potential well increases, dramatic changes are observed in the 4s cross-section, as well as in the photoelectron angular distribution. The existence of Cooper minima in the 4s dipole channels give rise to very significant effects of quadrupole interactions, even at extremely low energies, in the vicinity of the E1 Cooper minimum. It is shown that the entrapment of Ca in a spherical attractive well (Ca@SAW) further enhances the importance of quadrupole interactions determining the photoionization parameters. The complicated behavior of both dipole and quadrupole Cooper minima, as functions of well depth, is delineated, along with the importance of relativistic effects.

Classical states of an electric dipole in an external magnetic field: Complete solution for the center of mass and trapped states

We study the classical behavior of an electric dipole in the presence of a uniform magnetic field. Using the Lagrangian formulation, we obtain the equations of motion, whose solutions are represented in terms of Jacobi functions. We also identify two constants of motion, namely, the energy E and a pseudomomentumC→. We obtain a relation between the constants that allows us to suggest the existence of a type of bound states without turning points, which are called trapped states. These results are consistent with and complementary to previous results.

Electroporation as a New Cancer Treatment Technique: A Review on the Mechanisms of Action

Electroporation is a highly effective method to increase permeability of cell membrane by using series of short intense electric pulses. Using this technique, we can introduce small and large molecules into cells. The electroporation of biological membranes has various applications in molecular biology and medicine. Despite the numerous applications of electroporation, the detailed effects of electric pulse on biological membranes as well as exact mechanisms of pore formation in living cells are not well understood. Several in-vitro and in-vivo experimental studies have been conducted to determine the mechanisms of action of electroporation in various types of membranes. Because of the small spatial and fast temporal scales of this process, direct observation of electroporation is difficult, theoretical models and molecular dynamic simulations have been developed to facilitate the interpretation of experimental data and the understanding of the mechanisms of action of electroporation. The role of phospholipids in respond to external electric fields, behavior of water dipoles in the complex electric field landscape of the membrane interface and reorganization of water dipoles in pore formation process have been proposed in these studies. Pore characteristics such as life time, ion selectivity, size, kinetics of formation as well as number of pores are significant factors in the proposed mechanism of action. The present study reviews the different mechanisms of action of electroporation proposed by different experimental and modeling studies.

A traveling-wave stripline dipole antenna on a substrate lens at terahertz frequency

In this paper, a study of a stripline dipole antenna on a substrate lens used as a photoconductive detector in a terahertz system is presented. The traveling-wave behavior of the stripline dipole and the influence of the substrate lens are investigated over a broad frequency range up to 5.0 THz. The numerical results show that the lens shape represented by the ratio of the extension length to the lens radius plays an important role in maximizing the antenna gain and radiation spectral bandwidth. The gain response exhibited an increased level of sensitivity to the lens shape as the lens size increased, and this is particularly important in optimizing large substrate lenses. Improvements in the gain level over the entire frequency range of interest were observed as the lens diameter increased. This study provides helpful guidelines in choosing and optimizing a substrate lens designed for a terahertz photoconductive antenna, which is particularly useful for specific applications requiring a miniaturized photoconductive antenna design.