Showing papers on "Dipole published in 2012"

Optimization of the additive CHARMM all-atom protein force field targeting improved sampling of the backbone φ, ψ and side-chain χ(1) and χ(2) dihedral angles.

Abstract: While the quality of the current CHARMM22/CMAP additive force field for proteins has been demonstrated in a large number of applications, limitations in the model with respect to the equilibrium between the sampling of helical and extended conformations in folding simulations have been noted. To overcome this, as well as make other improvements in the model, we present a combination of refinements that should result in enhanced accuracy in simulations of proteins. The common (non Gly, Pro) backbone CMAP potential has been refined against experimental solution NMR data for weakly structured peptides, resulting in a rebalancing of the energies of the α-helix and extended regions of the Ramachandran map, correcting the α-helical bias of CHARMM22/CMAP. The Gly and Pro CMAPs have been refitted to more accurate quantum-mechanical energy surfaces. Side-chain torsion parameters have been optimized by fitting to backbone-dependent quantum-mechanical energy surfaces, followed by additional empirical optimization targeting NMR scalar couplings for unfolded proteins. A comprehensive validation of the revised force field was then performed against data not used to guide parametrization: (i) comparison of simulations of eight proteins in their crystal environments with crystal structures; (ii) comparison with backbone scalar couplings for weakly structured peptides; (iii) comparison with NMR residual dipolar couplings and scalar couplings for both backbone and side-chains in folded proteins; (iv) equilibrium folding of mini-proteins. The results indicate that the revised CHARMM 36 parameters represent an improved model for the modeling and simulation studies of proteins, including studies of protein folding, assembly and functionally relevant conformational changes.

Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region

Abstract: Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.

Principles of nano-optics

Abstract: 1. Introduction 2. Theoretical foundations 3. Propagation and focusing of optical fields 4. Spatial resolution and position accuracy 5. Nanoscale optical microscopy 6. Near-field optical probes 7. Probe-sample distance control 8. Light emission and optical interaction in nanoscale environments 9. Quantum emitters 10. Dipole emission near planar interfaces 11. Photonic crystals and resonators 12. Surface plasmons 13. Forces in confined fields 14. Fluctuation-induced phenomena 15. Theoretical methods in nano-optics Appendices Index.

Rational synthesis of noncentrosymmetric metal-organic frameworks for second-order nonlinear optics.

Abstract: The term nonlinear optics (NLO) was coined to describe the nonlinear relationship between dielectric polarization P and electric field E in optical media. NLO is a cornerstone of the emerging field of photonics, in which photons instead of electrons are used for signal transmission and processing. The vision of photonic signal transmission, processing, and storage has attracted a great deal of attention from both the engineering and the scientific communities because of its great impact in many of the existing and future information technologies. The first step toward realization of these revolutionary technologies is to develop tools to manipulate photons. For example, it is desirable to develop materials with the ability to alter the frequency of light, to amplify light signal, and to modulate light intensity or phase factors. NLO phenomena can be the key to achieving these important functions. One of the most common NLO behaviors is second-harmonic generation (SHG), in which a NLO material mediates the “adding-up” of two photons to form a new one with twice the frequency. The SHGphenomenonwas first demonstrated by Franken et al. in 1961. In their pioneering work, a laser beam with a wavelength of 694.2 nm was irradiated through a quartz crystal and an output ultraviolet radiation with a wavelength of 347.1 nm (double frequency) was detected. After this discovery, numerous nonlinear optical phenomena have been studied and a number of NLO-active materials have been developed. Second-harmonic generation can be quantitatively described by the second-order nonlinear optical susceptibility χ, a third-rank tensor with 27 components. The tensor elements are related to each other tomeet the requirements of both inherent and structural symmetries, which greatly reduces the number of independent components of the susceptibility tensor. Only crystals in noncentrosymmetric crystal classes can have nonvanishing χ. Moreover, for material crystallizing in the noncentrosymmetric 422, 622, and 432 crystal classes, the second-order NLO response might also vanish due to structural symmetry as well as Kleinman’s symmetry. Many inorganic compounds crystallize in noncentrosymmetric space groups and have been found to be SHG active. Some important examples are potassium dihydrogen phosphate (KDP = KH2PO4), lithium niobate (LiNbO3), and barium sodium niobate (Ba2NaNb5O15). 7 New inorganic compounds have been explored for NLO applications including but not limited to metal borates 12 and metal oxides. Recent structural studies on the inorganic systems have led to a better understanding of crystal growth/packing, paving the way for potentially manipulating their crystallization tendency to form noncentrosymmetric structures. Since the 1970s molecular NLO materials, including organic, organometallic, and inorganic complexes, have been of increasing interest to synthetic chemists. 19 The existing library of organic compounds was first screened, and the urea crystal has become a SHG standard because of its high SHG efficiency and usual availability. In a microscopic view, the second-order NLO susceptibility χ is related to the first hyperpolarizability β of a molecule. According to the classical two-level model, β is enhanced by a large transition moment and a large dipole moment difference between the ground and the charge transfer excited state. A donor acceptor type of molecule often possesses both a large transition moment and a large excited state dipole moment. As a result, most of the organic SHG chromophors belong to this category. However, most of the molecules with large β values also possess a large dipole moment, which induces formation of centrosymmetric assemblies of molecules due to dipole dipole interactions. One of the methods to avoid the centrosymmetric alignment of molecular dipoles is to trap them inside the channels of asymmetric porous host structures. 28 Other methods include formation of poled polymers in which the required asymmetry is imposed by the external electric field 35 and the Langmuir Blodgett (LB)

Charge Model 5: An Extension of Hirshfeld Population Analysis for the Accurate Description of Molecular Interactions in Gaseous and Condensed Phases.

Abstract: We propose a novel approach to deriving partial atomic charges from population analysis. The new model, called Charge Model 5 (CM5), yields class IV partial atomic charges by mapping from those obtained by Hirshfeld population analysis of density functional electronic charge distributions. The CM5 model utilizes a single set of parameters derived by fitting to reference values of the gas-phase dipole moments of 614 molecular structures. An additional test set (not included in the CM5 parametrization) contained 107 singly charged ions with nonzero dipole moments, calculated from the accurate electronic charge density, with respect to the center of nuclear charges. The CM5 model is applicable to any charged or uncharged molecule composed of any element of the periodic table in the gas phase or in solution. The CM5 model predicts dipole moments for the tested molecules that are more accurate on average than those from the original Hirshfeld method or from many other popular schemes including atomic polar ten...

Collective Dipole Oscillations of a Spin-Orbit Coupled Bose-Einstein Condensate

Abstract: In this Letter, we present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. The dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region as a function of spin-orbit coupling parameters as well as the oscillation amplitude. The anharmonic properties beyond the effective-mass approximation are revealed, such as the amplitude-dependent frequency and finite oscillation frequency at a place with a divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of the spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility---a static physical quantity---can be measured via the dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates the magnetic nonzero-momentum condensate from the nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a benchmark for recently developed theoretical approaches.

Condensed Matter Theory of Dipolar Quantum Gases

Abstract: Recent experimental breakthroughs in trapping, cooling and controlling ultracold gases of polar molecules, magnetic and Rydberg atoms have paved the way toward the investigation of highly tunable quantum systems, where anisotropic, long-range dipolar interactions play a prominent role at the many-body level. In this article we review recent theoretical studies concerning the physics of such systems. Starting from a general discussion on interaction design techniques and microscopic Hamiltonians, we provide a summary of recent work focused on many-body properties of dipolar systems, including: weakly interacting Bose gases, weakly interacting Fermi gases, multilayer systems, strongly interacting dipolar gases and dipolar gases in 1D and quasi-1D geometries. Within each of these topics, purely dipolar effects and connections with experimental realizations are emphasized.

Hydrodynamics of self-propulsion near a boundary: predictions and accuracy of far-field approximations

Abstract: The swimming trajectories of self-propelled organisms or synthetic devices in a viscous fluid can be altered by hydrodynamic interactions with nearby boundaries. We explore a multipole description of swimming bodies and provide a general framework for studying the fluid-mediated modifications to swimming trajectories. A general axisymmetric swimmer is described as a linear combination of fundamental solutions to the Stokes equations: a Stokeslet dipole, a source dipole, a Stokeslet quadrupole, and a rotlet dipole. The effects of nearby walls or stress-free surfaces on swimming trajectories are described through the contribution of each singularity, and we address the question of how accurately this multipole approach captures the wall effects observed in full numerical solutions of the Stokes equations. The reduced model is used to provide simple but accurate predictions of the wall-induced attraction and pitching dynamics for model Janus particles, ciliated organisms, and bacteria-like polar swimmers. Transitions in attraction and pitching behaviour as functions of body geometry and propulsive mechanism are described. The reduced model may help to explain a number of recent experimental results.

Solar Cell Efficiency, Self-Assembly, and Dipole–Dipole Interactions of Isomorphic Narrow-Band-Gap Molecules

Abstract: We examine the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution-processable molecules. These charge-transfer chromophores are described by a D1-A-D-A-D1 structure comprising electron-rich 2-hexylbithiophene and 3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene moieties as the donor units D1 and D, respectively. The building blocks 2,1,3-benzothiadiazole (BT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) were used as the electron-deficient acceptor units A. Using a combination of UV–visible spectroscopy, field-effect transistors, solar cell devices, grazing incident wide-angle X-ray scattering, and transmission electron microscopy, three PT-containing compounds (1–3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (4), are compared and contrasted in solution, in t...

Quantum dynamics of impurities in a one-dimensional Bose gas

Abstract: Using a species-selective dipole potential, we create initially localized impurities and investigate their interactions with a majority species of bosonic atoms in a one-dimensional configuration during expansion. We find an interaction-dependent amplitude reduction of the oscillation of the impurities' size with no measurable frequency shift, and study it as a function of the interaction strength. We discuss possible theoretical interpretations of the data. We compare, in particular, with a polaronic mass shift model derived following Feynman variational approach.

What is the “best” atomic charge model to describe through-space charge-transfer excitations?

Abstract: We investigate the efficiency of several partial atomic charge models (Mulliken, Hirshfeld, Bader, Natural, Merz–Kollman and ChelpG) for investigating the through-space charge-transfer in push–pull organic compounds with Time-Dependent Density Functional Theory approaches. The results of these models are compared to benchmark values obtained by determining the difference of total densities between the ground and excited states. Both model push–pull oligomers and two classes of “real-life” organic dyes (indoline and diketopyrrolopyrrole) used as sensitisers in solar cell applications have been considered. Though the difference of dipole moments between the ground and excited states is reproduced by most approaches, no atomic charge model is fully satisfactory for reproducing the distance and amount of charge transferred that are provided by the density picture. Overall, the partitioning schemes fitting the electrostatic potential (e.g. Merz–Kollman) stand as the most consistent compromises in the framework of simulating through-space charge-transfer, whereas the other models tend to yield qualitatively inconsistent values.

Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer Boron–Nitride Sheets

Abstract: Born–Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron–nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net pos...

An invisible metal-semiconductor photodetector

Abstract: Researchers demonstrate a silicon nanowire photodetector whose gold electrical contacts render the device ‘invisible’. The wire and gold coating have opposing dipole moments that almost cancel each other out in the far-field. Although the net dipole moment is zero, a significant photocarrier population is generated in the wire.

Dark plasmonic breathing modes in silver nanodisks.

Abstract: We map the complete plasmonic spectrum of silver nanodisks by electron energy loss spectroscopy and show that the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Therefore, this mode does not couple to light and has escaped from observation in optical experiments. This radial breathing mode has the character of an extended two-dimensional surface plasmon with a wavenumber determined by the circular disk confinement. Its strong near fields can impact the hybridization in coupled plasmonic nanoparticles as well as couplings with nearby quantum emitters.

Low‐degree structure in Mercury's planetary magnetic field

Abstract: The structure of Mercury's internal magnetic field has been determined from analysis of orbital Magnetometer measurements by the MESSENGER spacecraft. We identified the magnetic equator on 531 low-altitude and 120 high-altitude equator crossings from the zero in the radial cylindrical magnetic field component, Beta (sub rho). The low-altitude crossings are offset 479 +/- 6 km northward, indicating an offset of the planetary dipole. The tilt of the magnetic pole relative to the planetary spin axis is less than 0.8 deg.. The high-altitude crossings yield a northward offset of the magnetic equator of 486 +/- 74 km. A field with only nonzero dipole and octupole coefficients also matches the low-altitude observations but cannot yield off-equatorial Beta (sub rho) = 0 at radial distances greater than 3520 km. We compared offset dipole and other descriptions of the field with vector field observations below 600 km for 13 longitudinally distributed, magnetically quiet orbits. An offset dipole with southward directed moment of 190 nT-R-cube (sub M) yields root-mean-square (RMS) residuals below 14 nT, whereas a field with only dipole and octupole terms tuned to match the polar field and the low-altitude magnetic equator crossings yields RMS residuals up to 68 nT. Attributing the residuals from the offset-dipole field to axial degree 3 and 4 contributions we estimate that the Gauss coefficient magnitudes for the additional terms are less than 4% and 7%, respectively, relative to the dipole. The axial alignment and prominent quadrupole are consistent with a non-convecting layer above a deep dynamo in Mercury's fluid outer core.

Magnetic braking formulation for sun-like stars: dependence on dipole field strength and rotation rate

Abstract: We use two-dimensional axisymmetric magnetohydrodynamic simulations to compute steady-state solutions for solar-like stellar winds from rotating stars with dipolar magnetic fields. Our parameter study includes 50 simulations covering a wide range of relative magnetic field strengths and rotation rates, extending from the slow- and approaching the fast-magnetic-rotator regimes. Using the simulations to compute the angular momentum loss, we derive a semi-analytic formulation for the external torque on the star that fits all of the simulations to a precision of a few percent. This formula provides a simple method for computing the magnetic braking of Sun-like stars due to magnetized stellar winds, which properly includes the dependence on the strength of the magnetic field, mass loss rate, stellar radius, surface gravity, and spin rate, and which is valid for both slow and fast rotators.

Boosting the directivity of optical antennas with magnetic and electric dipolar resonant particles

Abstract: Dielectric particles supporting both magnetic and electric Mie resonances are shown to be able to either reflect or collect the light emitted by a single photon source. An analytical model accurately predicts the scattering behavior of a single dielectric particle electromagnetically coupled to the electric dipole transition moment of a quantum emitter. We derive near field extensions of the Kerker conditions in order to determine the conditions that strongly reduce scattering in either the forward or backward directions. This concept is then employed to design a lossless dielectric collector element whose directivity is boosted by the coherent scattering of both electric and magnetic dipoles.

Magnetic dipole and electric quadrupole transitions in the trivalent lanthanide series: Calculated emission rates and oscillator strengths

Abstract: Given growing interest in optical-frequency magnetic dipole transitions, we use intermediate coupling calculations to identify strong magnetic dipole emission lines that are well suited for experimental study. The energy levels for all trivalent lanthanide ions in the $4{f}^{\text{n}}$ configuration are calculated using a detailed free ion Hamiltonian, including electrostatic and spin-orbit terms as well as two-body, three-body, spin-spin, spin-other-orbit, and electrostatically correlated spin-orbit interactions. These free ion energy levels and eigenstates are then used to calculate the oscillator strengths for all ground-state magnetic dipole absorption lines and the spontaneous emission rates for all magnetic dipole emission lines including transitions between excited states. A large number of strong magnetic dipole transitions are predicted throughout the visible and near-infrared spectrum, including many at longer wavelengths that would be ideal for experimental investigation of magnetic light-matter interactions with optical metamaterials and plasmonic antennas.

Van der Waals interactions in solids using the exchange-hole dipole moment model.

Abstract: The exchange-hole dipole moment model of dispersion interactions of Becke and Johnson [J. Chem. Phys. 127 154108 (2007)] is implemented for calculations in solids using the pseudopotentials/plane-waves approach. The resulting functional retains the simplicity and efficiency of semilocal functionals while accurately treating dispersion interactions via a semiempirical asymptotic expansion. The dispersion coefficients are calculated completely ab initio using local quantities alone (density, gradient, Laplacian, and kinetic energy density). The two empirical parameters in the damping function are calculated by fit to a 65-molecule training set recalculated under periodic boundary conditions. Calculations in simple solids offer good results with minimal computational cost compared to electronic relaxation.

Dielectric antennas - a suitable platform for controlling magnetic dipolar emission

Abstract: Plasmonic nanoparticles are commonly used to tune and direct the radiation from electric dipolar emitters. Less progress has been made towards understanding complementary systems of magnetic nature. However, it has been recently shown that high-index dielectric spheres can act as effective magnetic antennas. Here we explore the concept of coupling dielectric magnetic antennas with either an electric or magnetic dipolar emitter in a similar fashion to the purely electric systems reported previously. We investigate the enhancement of radiation from systems comprising admixtures of these electric and magnetic elements and perform a full study of its dependence on the distance and polarization of the emitter with respect to the antenna. A comparison to the plasmon antennas reveals remarkable symmetries between electric and magnetic systems, which might lead to novel paradigms in the design of nanophotonic devices that involve magnetic activity.

Optimal electromagnetic-wave absorption by enhanced dipole polarization in Ni/C nanocapsules

Abstract: Electromagnetic-wave (EMW) absorption by Ni/C nanocapsules with similar permeability but different permittivity mainly due to differences in the graphite-shell thickness has been investigated. The optimal working frequency could appear at S-band and C-band and considerable strong EMW absorption was achieved. For the optimal Ni/C nanocapsules, a reflection loss exceeding −20 dB was reached from 2.6 to 8.2 GHz with a maximum value of −40 dB at 3 GHz. The improved absorption can be attributed to an optimal electromagnetic match and an enhanced dipole polarization upon increasing of shell thickness.

Profile of the static permittivity tensor of water at interfaces: consequences for capacitance, hydration interaction and ion adsorption.

Abstract: We derive the theoretical framework to calculate the dielectric response tensor and determine its components for water adjacent to hydrophilic and hydrophobic surfaces using molecular dynamics simulations. For the nonpolarizable water model used, linear response theory is found to be applicable up to an external perpendicular field strength of ∼2 V/nm, which is well beyond the experimental dielectric breakdown threshold. The dipole contribution dominates the dielectric response parallel to the interface, whereas for the perpendicular component it is essential to keep the quadrupole and octupole terms. Including the space-dependent dielectric function in a mean-field description of the ion distribution at a single charged interface, we reproduce experimental values of the interfacial capacitance. At the same time, the dielectric function decreases the electrostatic part of the disjoining pressure between two charged surfaces, unlike previously thought. The difference in interfacial polarizability between h...

Spontaneous emission spectra and quantum light-matter interactions from a strongly coupled quantum dot metal-nanoparticle system

Abstract: We investigate the quantum optical properties of a quantum-dot dipole emitter coupled to a finite-size metal nanoparticle using a photon Green-function technique that rigorously quantizes the electromagnetic fields. We first obtain pronounced Purcell factors and photonic Lamb shifts for both a 7- and 20-nm-radius metal nanoparticle, without adopting a dipole approximation. We then consider a quantum-dot photon emitter positioned sufficiently near the metal nanoparticle so that the strong-coupling regime is possible. Accounting for nondipole interactions, quenching, and photon transport from the dot to the detector, we demonstrate that the strong-coupling regime should be observable in the far-field spontaneous emission spectrum, even at room temperature. The vacuum-induced emission spectra show that the usual vacuum Rabi doublet becomes a rich spectral triplet or quartet with two of the four peaks anticrossing, which survives in spite of significant nonradiative decays. We discuss the emitted light spectrum and the effects of quenching for two different dipole polarizations.

Role of dipole-dipole interactions for hyperthermia heating of magnetic nanoparticle ensembles

Abstract: For clinical hyperthermia treatment the heating efficiency of magnetic nanoparticle ensembles is a crucial element. Using efficient algorithms, this heating is studied numerically with a focus on the effects of dipole-dipole interparticle interactions. For the time evolution of realistically modeled systems an approach based on the Landau-Lifschitz-Gilbert equation of motion with Langevin dynamics is taken. Our results suggest a widely negative influence of dipole-dipole interactions on the heating power of nanoparticles . However, considering ensembles within a fixed, given sample volume an optimal particle density exists. The presented results may have important implications for the medical use of magnetic hyperthermia treatment. I. INTRODUCTION

Dipole showers and automated NLO matching in Herwig

Abstract: We report on the implementation of a coherent dipole shower algorithm along with an automated implementation for dipole subtraction and for performing powheg- and mc@nlo-type matching to next-to-leading order (NLO) calculations. Both programs are implemented as add-on modules to the event generator Herwig++. A preliminary tune of parameters to data acquired at LEP, HERA and Drell-Yan pair production at the Tevatron has been performed, and we find an overall very good description which is slightly improved by the NLO matching.

Comparison of vertical and adiabatic harmonic approaches for the calculation of the vibrational structure of electronic spectra.

Abstract: The calculation of the vibrational structure associated to electronic spectra in large molecules requires a Taylor expansion of the initial and final state potential energy surface (PES) around some reference nuclear structure. Vertical (V) and adiabatic (A) approaches expand the final state PES around the initial-state (V) or final-state (A) equilibrium structure. Simplest models only take into account displacements of initial- and final-state minima, intermediate ones also allow for difference in frequencies and more accurate models introduce the Dushinsky effect through the computation of the Hessians of both the initial and final state. In this contribution we summarize and compare the mathematical expressions of the complete hierarchy of V and A harmonic models and we implement them in a numerical code, presenting a detailed comparison of their performance on a number of prototypical systems. We also address non-Condon effects through linear expansions of the transition dipole as a function of nuclear coordinates (Herzberg–Teller effect) and compare the results of expansions around initial and final state equilibrium geometries. By a throughout analysis of our results we highlight a number of general trends in the relative performance of the models that can provide hints for their proper choice. Moreover we show that A and V models including final state PES Hessian outperform the simpler ones and that discrepancies in their predictions are diagnostic for failure of harmonic approximation and/or of Born–Oppenheimer approximation (existence of remarkable geometry-dependent mixing of electronic states).

Non-Trivial Non-Radiating Excitation as a Mechanism of Resonant Transparency in Toroidal Metamaterials

Abstract: We demonstrate theoretically and confirm experimentally a new mechanism of resonant electromagnetic transparency, which yields extremely narrow isolated symmetric Lorentzian lines of full transmission in metamaterials. It exploits the long sought non-trivial non-radiating charge-current excitation based on toroidal dipole moment, predicted to generate waves of gauge-irreducible vector potential in the complete absence of scattered electromagnetic fields.