Macroscopic Dipole Moment Research Articles

Determination of the macroscopic dipole moment of associated solvents

The solubility (crystallization) of n-alkanes and mixtures (paraffins) in unassociated (chloroalkanes, toluene, hexane) and associated (ketones, alcohols, carboxylic acids) solvents and in binary mixtures of these solvents was investigated. The values of the macroscopic dipole moment (arbitrary polarity) were determined for the first time for acetone and its homologs, up to methyl hexyl ketone, for C 4 -C 7 aliphatic alcohols, and for carboxylic acids - from butyric to caprylic. Using the dependences of the arbitrary polarity on the molecular weight and molar refraction of the solvents, the polarity values were calculated for water, methanol, formic acid, and other associated substances. The mechanism of the increase in the solubility of water, glucose, and glycine with a decrease in the difference between the polarity of the solvent and the dissolved substance was demonstrated.

Theory of relaxation dynamics in glass-forming hydrogen-bonded liquids

We address the relaxation dynamics in hydrogen-bonded supercooled liquids near (but above) the glass transition, measured via broadband dielectric spectroscopy (BDS). We propose a theory based on decomposing the relaxation of the macroscopic dipole moment into contributions from hydrogen-bonded clusters of s molecules, with s(min) < or = s < or = s(max) . The existence of s(max) is translated into a sum rule on the concentrations of clusters of size s . We construct the statistical mechanics of the supercooled liquid subject to this sum rule as a constraint, to estimate the temperature-dependent density of clusters of size s . With a theoretical estimate of the relaxation time of each cluster, we provide predictions for the real and imaginary parts of the frequency-dependent dielectric response. The predicted spectra and their temperature dependence are in accord with measurements, explaining a host of phenomenological fits like the Vogel-Fulcher fit and the stretched exponential fit. Using glycerol as a particular example, we demonstrate quantitative correspondence between theory and experiments. The theory also demonstrates that the alpha peak and the "excess wing" stem from the same physics in this material. The theory also shows that in other hydrogen-bonded glass formers the excess wing can develop into a beta peak, depending on the molecular material parameters (predominantly the surface energy of the clusters). We thus argue that alpha and beta peaks can stem from the same physics. We address the BDS in constrained geometries (pores) and explain why recent experiments on glycerol did not show a deviation from bulk spectra. Finally, we discuss the dc part of the BDS spectrum and argue why it scales with the frequency of the alpha peak, providing an explanation for the remarkable data collapse observed in experiments.

The elastic field of a surface step: The Marchenko–Parshin formula in the linear case

Strain has significance for both the growth characteristics and material properties of thin epitaxial films. In this work, the method of lattice statics is applied to an epitaxial system with cubic symmetry, using harmonic potentials. The energy density and force balance equations are written using a finite difference formalism that clearly shows their consistency with continuum elasticity. For simplicity, the atomic interactions are assumed to be maximally localized. For a layered material system with a material/vacuum interface and with surface steps, force balance equations are derived, and intrinsic surface stress at the material/vacuum interface is included by treating the atoms at the surface as having different elastic properties. By defining the strain relative to an appropriately chosen nonequilibrium lattice, as in the method of eigenstrains, analytic formulas in terms of microscopic parameters are found for the local force field near a step and for the macroscopic monopole and dipole moment forces due to a step. These results provide an atomistic validation of the Marchenko–Parshin formula for the dipole moment in terms of the elastic surface stress.

Pure component properties from group contribution: Hydrogen-bond basicity, hydrogen-bond acidity, Hildebrand solubility parameter, macroscopic surface tension, dipole moment, refractive index and dielectric constant

The estimation of two primary properties by group contribution and five secondary properties by correlation is proposed in this work. The primary properties are Abraham’s hydrogen-bond basicity and acidity. The secondary properties are the Hildebrand solubility parameter, the macroscopic surface tension, the dipole moment, the refractive index and the dielectric constant. Temperature-dependent properties are estimated at 298 K. A database of experimental data for up to 870 compounds that can be represented by 41 UNIFAC groups is developed in order to perform regressions. The only information required to estimate the properties is the number and type of UNIFAC groups in the compound of interest. Several statistics, including confidence intervals on group parameters, are considered during the development of the techniques. Because no connectivity information is required, the methods are expected to perform well for compounds where there are no significant proximity effects.

Microscopic origin of polarity in quasiamorphousBaTiO3

The recent observation of pyroelectricity in quasiamorphous thin films of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ introduced a previously unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied by long-range crystal-like order. This poses a question regarding the mechanism of polarity in noncrystalline ionic systems and the nature of their local dipoles. By combining x-ray diffraction and x-ray-absorption fine-structure spectroscopy techniques we have identified the local dipoles as stable but distorted $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ were found to be nearly twice as large as those in bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$. We propose that the mechanism of macroscopic polarity in quasiamorphous $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is in a weak orientational ordering of the $\mathrm{Ti}{\mathrm{O}}_{6}$ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example $\mathrm{Nb}{\mathrm{O}}_{6}$, $\mathrm{Ti}{\mathrm{O}}_{6}$, or $\mathrm{V}{\mathrm{O}}_{6}$, may also form noncrystalline polar phases.

Electron redistribution in low-dimensional oxide structures

We discuss several aspects of electron redistributions in low-dimensional oxides: surfaces, ultra-thin films or clusters. We rely upon a combination of first principles density functional calculations, Bader analysis and an analytical model, the bond electron transfer model, that we derive and which is valid whatever the covalent strength of the anion–cation bonding. We show that, despite the fact that nearly bulk-like charges are found on low-coordinated sites, there usually exists a strong increase of the covalent character of the oxygen–cation bonds, which is reflected in experimentally measurable quantities. On polar orientations, we discuss the efficiency of various processes yielding a cancellation of the macroscopic dipole moment.

Electric polarization of heteropolar nanotubes as a geometric phase.

The threefold symmetry of planar boron nitride (BN), the III-V analog to graphene, prohibits an electric polarization in its ground state, but this symmetry is broken when the sheet is wrapped to form a BN nanotube. We show that this leads to an electric polarization along the nanotube axis which is controlled by the quantum mechanical boundary conditions on its electronic states around the tube circumference. Thus the macroscopic dipole moment has an intrinsically nonlocal quantum mechanical origin from the wrapped dimension. We formulate this novel phenomenon using the Berry's phase approach and discuss its experimental consequences.

Photophysical and Structural Properties of Saddle-Shaped Free Base Porphyrins: Evidence for an “Orthogonal” Dipole Moment

Ground- and excited-state absorption and fluorescence properties of three free base porphyrins with graded degrees of a macrocycle distortion have been studied. The different degrees of nonplanarity were introduced by successive addition of ethyl groups at the ‚-pyrrole positions of free base 5,10,15,20-tetraphenylporphyrin (H2TPP): from four ethyl groups in cis-tetraethyl-TPP (H2cTETPP) to six ethyl groups in hexaethyl-TPP (H2HETPP) and eight ethyl groups in octaethyl-TPP (H2OETPP). The static and dynamic optical properties of the compounds change systematically with an increase of the porphyrin macrocycle nonplanarity. These perturbations include significant broadening of the absorption and fluorescence bands, an increased spacing between the long-wavelength absorption and short-wavelength emission maxima, and reduced excited-state lifetimes. In nonpolar solvents, these perturbations directly reflect the steric/electronic consequences of the distortion of the porphyrin ﷿-system. In polar media, all the photophysical consequences of nonplanar distortion are markedly enhanced as a function of solvent polarity. These effects derive from electronic interactions between the polar solvent molecules and the polar S1(﷿,﷿*) excited state of the nonplanar free base porphyrin. The origin of the polar nature of these nonplanar chromophores is indicated by semiempirical calculations, which show that free-base porphyrins with saddle-type macrocycle distortions have a permanent dipole moment (1-2 D) with a significant projection orthogonal to the nitrogen mean plane. Among the contributions to this macroscopic dipole moment are structural/electronic asymmetries derived from the pyrrole rings, N-H bonds and nitrogen lone pairs. The specific factors and the macroscopic dipole moment provide foci for solvent interactions that are amplified in nonplanar porphyrins relative to their planar counterparts. The studies demonstrate the strong interrelated effects of the conformation(s) involving the porphyrin macrocycle and its peripheral substituents, electronic structure, and solvent interactions (including macrocycle-nonplanarity-induced dipole moments) in dictating the photophysical properties of distorted porphyrins. The findings have implications for the function of tetrapyrrole cofactors in the biological proteins and for the use of nonplanar porphyrins in molecular optoelectronics.

A new mechanism of phase locking in a single-mode laser

We study the motion of the laser phase in presence of the noise-correlation identified in a recent work [Phys. Lett. A 260 (1999) 126]. The laser phase is found to be locked around a fixed value and this phase locking is induced by the correlation between Γ t , the noise of the macroscopic atomic dipole moment, and Γ z t , the noise of atomic inversion. Furthermore, when the laser operates from an intensity level I 0<1/ A ( A is the self-saturation coefficient) to a level I 0>1/ A, the locked phase value φ 0 exhibits a discontinuity, jumping up a finite value π, at I 0=1/ A.

Concepts of NMR in quantum optics

Coherent, continuous-wave and pulsed-laser response of optical two-level systems are reviewed in terms of optical Bloch-Maxwell equations. The concept of the macroscopic magnetic moment reaction “radiation damping” field in NMR is extended to account for the threshold condition for maser-laser action in a cavity and for propagating optical, coherent, transient free-induction signals emitted by macroscopic electric dipole moments. Analogues of NMR in coherent optical transients are listed. A brief history is presented of early developments in magnetic resonace, leading to application of the Bloch equations to the semiclassical description of electric dipole two-level systems. © 1997 John Wiley & Sons, Inc.

The concept of weak polarity: an application to the SrTiO 3(001) surface

We propose the term “weak polarity” to characterize the electrostatic properties of some ternary (or more complex) insulating compound slabs, which are not fully ionic and in which covalency (i.e. anion-cation electron sharing) is responsible for the presence of a non-vanishing macroscopic dipole moment perpendicular to the surface. We exemplify this situation by the SrTiO 3(001) surface, which we study by means of a semi-empirical Hartree-Fock approach. The bond-breaking processes at the surface are shown to yield the charge redistribution required for the cancellation of the macroscopic electrostatic field. There is no need for strong modifications of the electronic structure such as the surface metallization predicted on the polar stoichiometric MgO(111) surface.

Diffusion-induced domain inversion in ferroelectrics

Abstract The basic property of ferroelectric crystals is switching of the direction of the macroscopic electric dipole moment under applying of an external electric field. Recently the polarization reversal effect without the external electric field has been discovered. It occurs under some treatments, namely, during proton-exchange. Ti,Rh-indiffusion in LiNbO3, out(in)-diffusion in LiNbO4, and LiTaO3, under Rb ionexchange and out(in)-diffusion at elevated temperature in KTiOPO4. The effect takes place at any polar surface and it is observed both far and near the Curie temperature. As a result a bidomain structure is formed. The diffusion-induced polarization reversal in ferroelectrics represents a new physical phenomenon and it is widely used in modern electronics and nonlinear optics. The studies of the effect are based on several traditional and new methods providing necessary data about the spontaneous polarization distribution and electrical parameters of these structures. In this paper, we review recent progress in the understanding of this phenomenon.

On the Phase Transition of SmCA*Liquid Crystals

A simple microscopic model of the antiferroelectric SmC A * liquid crystals is proposed so as to analyse a few possible phase transitions between smectic subphases with various macroscopic dipole moments. It is theoretically implied that SmC A * liquid crystals show a universal phase sequence SmC A * (Antiferroelectric), SmCγ * (Ferrielectric), and SmC * (Ferroelectric) with increasing temperature in general. This is considered as a consequence of a competition between an attractive dipolar interaction and an excluded volume effect which results in a packing entropy related to the steric hindrance between the biaxial molecules. Also the effect of an external electric field on the phase transition is investigated.

Ferroelectric ordering in KNO 3

The results of mid infrared absorption experiments on the phase transitions in KNO 3 are described and compared with the behaviour of NaNO 3. The first order phase II-Phase I transition is observed in KNO 3 upon heating to 401 K through the appearance of an additional peak next to the ν 2 peak. Upon cooling phase I, the ferroelectric I–III transition is observed by discontinuities in the frequency and linewidth of the ν 2 mode. However, the actual change in these properties at the phase transition is very small (particularly for the frequency shift which is only about 0.5 cm −1), and hence it is suggested that phases I and III have very similar structures as far as the local symmetry of the nitrate group is concerned. These results agree with earlier suggestions that phase I consists of ferroelectric microdomains which order to produce a macroscopic dipole moment at the I–III transition. The sudden increase in the linewidth of the ν 2 mode at T c is interpreted as being due to solitonic excitations which result in domain growth and reorganisation.

The Domain Mode in Ferroelectric Liquid Crystals a Novel Electrooptical Process

ABSTRACTNovel electrooptical effects in ferroelectric liquid crystals with high spontaneous polarization (Ps ) are observed. They are connected with the formation of a modulated distribution of the macroscopic dipole moments of smectic layers if going along the layer normal. The period (D) of this modulation (ferroelectric domains) near the electrode surfaces (DS) and in the bulk (DB) of a FLC-specimen can differ appreciably, depending on an applied external voltage. The main role of the value of the effective spontaneous polarization is established. A variuos number of domain structures and corresponding diffraction patterns are observed under variation of frequency and amplitude of the external electric field. A simple explanation of the new electrooptical responses is proposed on the basis of interference and incommensurability of bulk and surface domains.

Intrinsic dipole moments in reflection-asymmetric nuclei

Low-energy collective E1 moments in nuclei from the Ra-Th and Ba-Sm region are analysed within the shell-correction method based on the reflectio-asymmetric Woods-Saxon model. Extensive systematics of experimental intrinsic dipole moments are presented. Shell effects on the isovector E1 moment explain local quenchings in E1 strength observed experimentally. It is demonstrated that the inclusion of the neutron-skin contribution to the macroscopic dipole moment is crucial for the understanding of experimental trends.

Ferroelectric liquid crystal with extreme polarization: X‐Ray structure analysis of the crystalline phase

Ciral liquid crystalline compounds which adopt the S phase are of interest as they can possess a spontaneous, permanent and macroscopic dipole moment (ferroelectric polarization) even in the absence of external fields. The compound reported here (see figure) has the highest known spontaneous polarization (520 nC cm−2) and therefore could improve the switching speeds of electro‐optic devices substantially. equation image

Measurements of local fields of molecular dipoles in Langmuir-Blodgett films

The technique of a molecular Stark probe is applied to a study of local fields induced by a monolayer of polar molecules in a Langmuir-Blodgett multilayer. Polar monolayers with various directions of the macroscopic dipole moment were prepared from amphiphilic azo dyes, and a bilayer of an anthraquinone dye was chosen as a Stark probe. It is shown that the sign and the magnitude of the local field may be controlled by varying the orientation of the polar monolayer and the thickness of a multilayer respectively.

Nuclear-quadrupole induction of atomic polarization.

Nuclear-quadrupole-induced electric signals are measured as a new resonance response mechanism that is the reciprocal of the Stark effect in magnetic resonance. In single crystals that have noncentrosymmetry with respect to the sites of precessing nuclear-quadrupole moments, the electronic polarizability of atoms and chemical bonds on opposite sides of the nuclear-quadrupole moment is not the same. The oscillating electric field produced by the quadrupole moment induces a net electric dipole moment in its neighboring electronic environment. The coherent summation of these dipole moments over the Boltzmann distribution of the nuclear ensemble produces an oscillating macroscopic electric dipole moment. The sample is placed between the plates of a capacitor that is tuned with an inductance to the nuclear precession frequency. Coherent nuclear precession is initiated following a rf magnetic field pulse that tips the nuclear spins into the precession mode. The voltage signal from the capacitor gives rise to an oscillating current in the series circuit and magnetic flux in the inductor. Stray magnetic induction pickup signals are balanced out. The flux is coupled to a dc superconducting quantum interference device (SQUID), which produces a voltage output at the nuclear-quadrupole resonance frequency of \ensuremath{\sim}30 MHz for $^{35}\mathrm{Cl}$ nuclei in ${\mathrm{NaClO}}_{3}$ at 4.2 K. The $^{35}\mathrm{Cl}$ nucleus induces electric dipole moments in nearby oxygen atoms bonded to the Cl atom. Measured free-precession electric signals are compared to the predictions of a point-oxygen-atom polarizability model applied to the Cl---O bond. The technique is sensitive to chirality and to bond angles.

Rapid radiative decay and enhanced optical nonlinearity of excitons in a quantum well.

An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.