Range Dipolar Interactions Research Articles

Structure evolution in a paramagnetic latex suspension

We present the aggregation of a model system of superparamagnetic latex particles. We measure the kinetics of the nonequilibrium aggregation process. At low volume fraction, when the magnetic field is applied, particles experience an attractive dipolar interaction and form linear chains by aggregating tip-to-tip parallel to the applied field. We find that the aggregation is transport limited and the mean cluster size increases with the square root of time. The long range dipolar interactions modify the flux in such a way that we must derive a characteristic time scale accounting for the ballistic trajectories experienced by aggregating dipoles. At surface fractions above a few percent, lateral interactions alter the aggregation mechanism; a sideways coalescence of the chains similar to a “zippering” motion is observed. This phenomenon results in a thickening of the chains to create fibers of particles and is eventually responsible for the crosslinking of chains. We present a new observation of a fully interconnected fibrous structure in concentrated suspensions.

Very low energy electron-H(2p) interaction studied in H-rare-gas collisions by translation electron spectroscopy

Detached electron energy spectral have been observed in the forward direction for collisions of H- ions with helium, neon and argon gas targets at 4 keV. Specifically, the channel corresponding to H(2p) excitation has been examined by recording the electron energy distributions in coincidence with Lyman- alpha photons. Near the 0' observation angle, electrons with very low energy in the projectile frame (1<Ep<100 meV) are observed that are kinematically shifted in the laboratory and which yield important information on the H(n=2)-e- interaction near threshold. The spectra reveal, in addition to the well known 1Po shape resonance of H-, a cusp typical of convoy electrons which is attributed to the long range dipole interaction between H(n=2) and an electron. In addition, weak structure near 4 meV above the H(2p) threshold could constitute further evidence in support of the existence of dipole oscillations.

Conformations and dynamics of surfactants in micelles and liquid crystals by nuclear magnetic relaxation

The order parameters as well as the rates of overall and internal motions of aggregated surfactants can be obtained from deuteron and carbon-13 nuclear relaxation experiments. The main contribution to the relaxation is generally the quadrupolar coupling (2H) or the short range dipolar interaction with protons (13C). In some cases it is convenient to derive the same information from the13C relaxation induced by long range dipolar interactions with a paramagnetic probe exchanging rapidly among the polar heads of surfactant molecules. This paper outlines the methods of interpretation of relaxation data by means of a rotational jump model of internal motions, taking into account most of the accessible conformers. The conformational and dynamical parameters are obtained from the magnetic field dependence of the longitudinal relaxation rates (micelles) or from the simultaneous fit of these rates and of the dipolar or quadrupolar splittings (liquid crystals). Some examples of application of these methods are given from recent works on single and double detailed surfactants.

Light scattering at the solid-liquid interface of cyclohexane

Anomalous Rayleigh scattering in the interface layer of cyclohexane crystals has been studied during crystal growth. For the first time we have observed anomalous Rayleigh scattering in the interface layer during melting. Cyclohexane has been chosen because the molecule has no permanent dipole moment. Thus all models based on the existence of long range dipolar interaction to describe these light scattering phenomena can be eliminated. A new crystal growth geometry has been developed to eliminate any possibility of contamination. A liquid sphere inside of a multiple zone refined single crystal is melted. The same light scattering phenomena are observed during inward growth, which we know from experiments during zone refining. Light scattering is also observed during melting of the solid. The solid is not superheated and thus no instabilities can occur at the interface. Thus it is concluded that light is not scattered by surface corrugations, small scale convections or by segregated impurities. Quasielastic light scattering techniques have been combined with elastic light scattering measurements in our studies of the freezing process. Model calculations for the angular dependence of the intensities of the scattered light have been compared with the experimental data. Calculations have been performed on the basis of 3 models: Mie theory for spherical particles, continuous fluctuations with an Ornstein-Zernike type pair correlation function and continuous fluctuations with a Debye-Bueche type pair correlation function. Only continuous fluctuations are compatible with the experiment. Thus models based on the assumption of gas bubbles or crystallites in front of the crystal-liquid interface can also be eliminated. The experimental results with cyclohexane are compatible with earlier experiments with H 2O and salol, indicating that the interface layer is a general phenomenon of growing dielectric crystals.

Mode coupling theory for the critical dynamics of dipolar ferromagnets

The critical dynamics of ferromagnets has been studied by various experimental methods sampling different regions in wave vector space. Nevertheless there seemed to be a list of contradictions between these experiments. Within a mode coupling theory including both short range exchange and long range dipolar interaction we are able to resolve the seemingly contradictory situation.

Surface phonons and surface reconstructions of ZnS structure crystals

Abstract We present surface phonon calculations for the (111), (110) and (100) surfaces of ZnS structure crystals with special emphasis lying on Si and GaAs. In our potential model we treat short range interactions within a force constant model. Additionally, we take into account long range forces arising from dipole moments, which are induced dynamically on an atom by a displacement of the atom itself or its nearest neighbors. Surface phonons and resonances are calculated by a Green function method yielding the exact solution of the equation of motion for the semi-infinite crystal. We give a comparison of our results to helium time-of-flight and EELS measurements for Si and GaAs. Allowing for changes of the surface layer force constants and dipole parameters we investigate the possibility of a surface phonon softening. Setting up models for the surface parameters we find characteristic features connected with the short range forces and the long range dipole interactions. Relating these soft modes to phase transitions causing surface reconstructions we find evidence for many experimentally observed surface structures. We discuss in detail surface reconstructions of silicon, namely the (2 × 1) and (7 × 7) structures of the (111) surface.

Surface phonons and surface reconstructions of ZnS structure crystals

We present surface phonon calculations for the (111), (110) and (100) surfaces of ZnS structure crystals with special emphasis lying on Si and GaAs. In our potential model we treat short range interactions within a force constant model. Additionally, we take into account long range forces arising from dipole moments, which are induced dynamically on an atom by a displacement of the atom itself or its nearest neighbors. Surface phonons and resonances are calculated by a Green function method yielding the exact solution of the equation of motion for the semi-infinite crystal. We give a comparison of our results to helium time-of-flight and EELS measurements for Si and GaAs. Allowing for changes of the surface layer force constants and dipole parameters we investigate the possibility of a surface phonon softening. Setting up models for the surface parameters we find characteristic features connected with the short range forces and the long range dipole interactions. Relating these soft modes to phase transitions causing surface reconstructions we find evidence for many experimentally observed surface structures. We discuss in detail surface reconstructions of silicon, namely the (2 × 1) and (7 × 7) structures of the (111) surface.

Shape functions of dipolar ferromagnets at the Curie point

We present a complete mode coupling theory for the critical dynamics of ferromagnets above the Curie point with both short range exchange and long range dipolar interaction. This theory allows us to determine the full Kubo relaxation functions at the critical point. In particular, we are able to explain recent spin echo measurements.

Magnetic coupling contributions to the thermal boundary resistance between3He and metals

In a recent experiment, Perry, DeConde, Sauls, and Stein found a thermal boundary resistance between3He and platinum that was inversely proportional to temperature and decreased with application of a magnetic field. These authors suggested an explanation of the experimental observations in terms of a hyperfine contact interaction between the helium nuclear spins in the first surface monolayer and the conduction electrons of the metal. In the present work the magnetic coupling between the electron-nuclear spin systems is investigated for the contact interaction using a realistic representation of the metallic spin susceptibility at the helium site. The longer range dipole interaction is also considered and results for copper, silver, gold, and platinum indicate that both magnetic coupling mechanisms are orders of magnitude too weak to account for experimentally observed heat transfer rates. The much larger heat transfer rate obtained by Perryet al. is traced to their overestimation of the conduction electron local density of states at the helium site.

Signal propagation in a bubble lattice with long range dipole interaction

The propagation of an information bit along a dense packed bubble lattice is simulated by computer. Each column of bubbles is assumed to be a non-linear oscillator interacting with other ones by short and long range dipole forces. The results show how a bit, represented by a transition layer separating two regions of differing response, moves through the lattice with velocity depending on the amplitude of the external driving force and on the ratio between short and long range interactions.

Dipole-dipole coupling in adsorbate vibrational mode spectra

Separating chemical effects from those due to long range dipole interactions is a recurring problem in the analysis of adsorbate vibrational mode spectra (ref. 1,2,3). For adsorbate systems containing only one site and one type of adsorbate (homogeneous systems), agreement between theory and experiment has been obtained (ref.3). For systems containing inequivalent occupied sites or more than one adsorbate species (inhomogeneous systems), the analysis of effects due to dipole coupling can become complicated. We will show that a simple effective medium model, similar to those used for thin film coatings (ref.4,5), can be used to analyze such systems. The validity of this model is tested by comparing its results to those obtained by numerically solving the exact equations of motion for arrays of coupled point dipole oscillators. Dramatic line narrowing due to dipole coupling, as well as the more well known frequency (ref.3) and intensity (ref.2) shifts are exhibited in both models. Using a sample prescription to determine the effective thickness and density for the effective medium model, we obtain results which are both qualitatively and quantitatively equivalent to those of the much less tractable point dipole model. (This equivalence can be viewed as a consequence of the long range collective nature of the long wavelength modes in a dipole coupled system). We conclude that, despite its extreme simplicity, an effective medium model can be used to accurately treat the effects of dipole coupling in inhomogeneous as well as homogeneous adsorbate systems.

Dielectric Properties of Heavy Ice Ih (D2O Ice)

Dielectric anisotropy of heavy ice Ih is examined in a wide temperature range by measuring the complex dielectric constants parallel and normal to the c axis, and it is compared with that of ordinary ice Ih (H 2 O ice). The increased anisotropy in the static dielectric constants including only the shift of the extrapolated Curie-Weiss temperature to 55 K was observed. Under these experimental results, a phenomenological treatment as ice is a dielectric is successfully tried. The difference in the dielectric properties between heavy and ordinary ice is mainly attributed to the difference of the strength in the long range dipolar interaction in the c direction. The predominancy between the proton semiconductive aspect and the dielectric aspect on ice is discussed.

Stark broadening of Hβ: wings calculation

Starting from the quantum one-perturber theory, the 'exact resonance' approximation is used. This accounts exactly for the long range dipole interaction. The validity of this approximation is analysed. In contrast to usual dipole semiclassical approaches, it is found that dipolar quantum calculations give an asymmetry to the wings that is in good agreement with measurements of Preston (1977). This is because the full quantum approach provides a proper dynamical treatment of the electron-atom interaction.

Ultrasonic attenuation and kinetic coefficients in pure and random uniaxial ferromagnets with dipolar interactions

The ultrasonic attenuation and kinetic coefficients in pure and random uniaxial ferromagnets with both short range exchange and long range dipolar interactions are calculated near the critical point using renormalization group theory. The dispersion and temperature dependences found differ logarithmically from classical theory and suggest new experiments. In the pure system a recent phenomenological calculation of the ultrasonic attenuation in the low frequency limit is confirmed.

Phase transitions in random uniaxial systems with dipolar interactions

The critical behaviour of random uniaxial ferromagnetic (ferroelectric) systems with both short range and long range dipolar interactions is investigated, using the field theoretic renormalization method of Brezin et al. for the free energy above and below the transition pointTc. The randomness is due to externally introduced fluctuations in the short range interactions (quenched case) or (and) magneto-elastic coupling to the lattice (annealed case). Strong deviations in the critical behaviour with respect to the pure systems are found. In the quenched case e.g. the specific heatC and the coefficientf2 (ofM3 in the equation of state, whereM is the magnetization) change fromC ∝|ln|t∥1/3,f2 ∝|ln|t∥−1 in the pure system toC\( = A_ \pm + C_ \pm \exp \left[ { - 4\sqrt {\frac{3}{{106}}|ln|t||} } \right],f_2 \propto |ln|t||^{ - \tfrac{1}{2}} (where t = (T - T_c )/T_c\) is the reduced temperature andA±,C± are constants) in the random situation. This change should e.g. be experimentally observable by deuterization of the ferroelectric tri-glycine sulfate where the logarithmic behaviour off2 has already been detected in the pure case. For nonvanishing magnetoelastic coupling a complex critical behaviour is obtained and discussed. We find the interesting result that if both quenched randomness and a weak magnetoelastic coupling are present the quenched random critical behaviour dominates in the close vicinity ofTc. Finally the influence of the magnetoelastic coupling on the longitudinal phonons in investigated and it is found that the relative changes in the corresponding elastic constant and structure factor are proportional to the specific heat and the wavevector dependent energy-energy correlation function respectively, suggesting new experiments.

Phospholipid monolayers at the n-heptane/water interface. Part 2.—Dilute monolayers of saturated 1,2-diacyl-lecithins and -cephalins

Surface pressure against area isotherms for dilute monolayers of a homologous series of 1,2-diacyl-lecithins spread at the oil/water interface are presented as a function of temperature and salt concentration. Comparative results for di-C14-cephalin monolayers are used to distinguish head-group effects. Over the whole range of areas studied the isotherms are independent of chain length and salt concentration but vary with head-group, with lecithins giving the higher pressures. From conventional Amagat-type plots the monolayers are non-ideal. Apparent co-areas for both lecithins and cephalins are large with the lecithins showing a marked positive temperature coefficient. Negative entropies of compression calculated from the lecithin data are larger than those for single chain ionised molecules spread at the oil/water interface. These phospholipids show little evidence of cluster formation at areas higher than the phase transitions reported in part 1, but undergo complex configurational changes on compression which depend on medium range dipolar interactions and steric interaction of the chains.

A least-squares method for the identification of resonant groups from NMR powder spectra. Identification of oxonium and hydroxyl ions in hydrated uranates

A general method to calculate NMR powder spectra with a predetermined precision is described. The calculated spectrum for one resonant group depends on short and long range dipolar interactions, α and β and on the proportion of the resonant group in the chemical formula. We propose a least-squares method to determine the best values of the parameters. To choose initial values, we use some easily measurable characteristic of the experimental spectrum and relations between them and the various parameters. Thus, the mathematical method suggested can be used to identify resonant groups and to state precisely the chemical formula of a polycrystalline solid. The theoretical calculation is applied to the hydrated uranates. The hydration water is shown to exist in the form of oxonium and hydroxyl ions according to Brasseur's hypothesis. The proposed method is used to refute some formulas as well as to specify the oxonium and hydroxyl parameters. The proton-proton distance in the equilateral triangle of the oxonium ions is found to be 1.72 Å. The limits of the method are discussed. Dehydration experiments show that the first dehydration step corresponds to the departure of the oxonium ions.

Solvent Effects upon Fluorescence Spectra and the Dipolemoments of Excited Molecules

Abstract A general formula for the difference of solvent shifts of fluorescence and absorption spectra in the approximation of long range dipolar interaction was derived using Ooshika’s theory of light absorption in solution. Measurements of fluorescence and absorption spectra of some naphthalene derivatives in various organic solvents were undertaken, and the data were analysed by the theoretical formula. The formula reproduces the experimental data satisfactorily, and from this fact it was concluded that the most predominant factor which determines the difference of solvent shifts of fluorescence and absorption spectra of these molecules is the interaction energy between solute and solvent molecules due to orientation polarization. The incremental values of dipolemoments in the excited state were estimated, and those for α-, β-naphthols and β-naphthyl methyl ether were interpreted as due to the increase of electron migration from the substituent in the excited state.

DEVIATION OF VAN DER WAALS INTERACTION FROM ADDITIVE RELATIONS

With a lattice of isotropic point-oscillators as an idealized model, the error of the usual additive approximation for the van der Waals interaction in solids is estimated. For a few common crystals, the estimated errors for van der Waals energy and its first and second derivatives have values up to 13%, 34% and 70% respectively. The model moreover points to the possible importance of the long range dipolar interaction as a mechanism determining the structure of the exciton-band in. insulators.