Role Of Dipole-dipole Interactions Research Articles

The role of dipole-dipole interaction and the scattering strength in entanglement dynamics between atoms surrounding a microtoroidal cavity

In this work we have shown that the dipole-dipole interaction and the scattering strength have a role in entanglement dynamics between two and three two-level atoms with dipole-dipole interaction coupled to a whispering-gallery-mode (WGM) microtoroidal cavity for a given initial state. For this purpose, we present two different cases where the two atoms are coupled symmetrically or asymmetrically to the two WGMs through evanescent fields. Considering the symmetric regime and two types of initial states for the atoms, we have shown that for the initial entangled state, the sudden death and birth, as well as the freezing of the entanglement, can be obtained by adjusting both the scattering strength between modes and the dipole-dipole interaction, in contraposition to the product initial state. Moreover, we have noted that the atomic entanglement generation is more susceptible to the scattering strength variation between the modes than to the dipole-dipole interaction. In addition, for the asymmetric regime, the entanglement generation is strongly dependent on the atomic location and the scattering strength. Similar results are obtained for the case of three atoms coupled to a microtoroidal cavity, even in the presence of losses.

Collective resonance fluorescence in small and dense atom clouds: Comparison between theory and experiment

We study the emergence of a collective optical response of a cold and dense $^{87}$Rb atomic cloud to a near-resonant low-intensity light when the atom number is gradually increased. Experimental observations are compared with microscopic stochastic simulations of recurrent scattering processes between the atoms that incorporate the atomic multilevel structure and the optical measurement setup. We analyze the optical response of an inhomogeneously-broadened gas and find that the experimental observations of the resonance line shifts and the total collected scattered light intensity in cold atom clouds substantially deviate from those of thermal atomic ensembles, indicating strong light-induced resonant dipole-dipole interactions between the atoms. At high densities, the simulations also predict a significantly slower decay of light-induced excitations in cold than in thermal atom clouds. The role of dipole-dipole interactions is discussed in terms of resonant coupling examples and the collective radiative excitation eigenmodes of the system.

Universal Behavior of Dielectric Responses of Glass Formers: Role of Dipole-Dipole Interactions.

From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the α-loss peak at or near the glass transition temperature T_{g} is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strength Δε(T_{g}) of the system, the narrower is the α-loss peak. This remarkable property is explained by the contribution from the dipole-dipole interaction potential V_{dd}(r)=-Dr^{-6} to the attractive part of the intermolecular potential, making the resultant potential more harmonic, and the effect increases rapidly with the dipole moment μ and Δε(T_{g}) in view of the relation, D∝(μ^{4}/kT_{g})∝kT_{g}[Δε(T_{g})]^{2}. Since the novel correlation discovered encompasses practically all van der Waals molecular glass formers studied by dielectric spectroscopy, it impacts the large dielectric research community as well as those engaged in solving the glass transition problem.

Self-assembly of strongly dipolar molecules on metal surfaces.

The role of dipole-dipole interactions in the self-assembly of dipolar organic molecules on surfaces is investigated. As a model system, strongly dipolar model molecules, p-benzoquinonemonoimine zwitterions (ZI) of type C6H2(⋯ NHR)2(⋯ O)2 on crystalline coinage metal surfaces were investigated with scanning tunneling microscopy and first principles calculations. Depending on the substrate, the molecules assemble into small clusters, nano gratings, and stripes, as well as in two-dimensional islands. The alignment of the molecular dipoles in those assemblies only rarely assumes the lowest electrostatic energy configuration. Based on calculations of the electrostatic energy for various experimentally observed molecular arrangements and under consideration of computed dipole moments of adsorbed molecules, the electrostatic energy minimization is ruled out as the driving force in the self-assembly. The structures observed are mainly the result of a competition between chemical interactions and substrate effects. The substrate's role in the self-assembly is to (i) reduce and realign the molecular dipole through charge donation and back donation involving both the molecular HOMO and LUMO, (ii) dictate the epitaxial orientation of the adsorbates, specifically so on Cu(111), and (iii) inhibit attractive forces between neighboring chains in the system ZI/Cu(111), which results in regularly spaced molecular gratings.

Magnetic interactions in an ensemble of cubic nanoparticles: A Monte Carlo study

The ensemble of spatially disordered and randomly oriented spherical monodispersed single-domain magnetic nanoparticles with cubic anisotropy was studied by the Monte Carlo method. In the presence of dipole-dipole interactions, the effect of both particle volume and interparticle separation was investigated with respect to the characteristic parameters of hysteresis loops and zero field cooled and field cooled magnetization curves. The coercive field and remanent magnetic moment were shown to depend strongly on the dimensionless parameter $\ensuremath{\beta}={k}_{B}T/({K}_{1}V)$ ($T$ temperature, $V$ particle volume, ${K}_{1}$ cubic anisotropy constant). It was revealed that strong dipole-dipole interactions suppress both the coercive field and the remanent magnetic moment of densely packed nanoparticles. Yet, the effect quickly diminishes with the increasing interparticle distances and becomes rather insignificant for separations exceeding three particle diameters. The blocking temperature was found to be weakly affected by dipolar interactions, but mainly governed via $\ensuremath{\beta}$, i.e., by the nanoparticle volume and the strength of crystalline anisotropy. The role of dipole-dipole interactions on magnetic properties of nanoparticles was further elucidated by a comparison of the simulation results for a single cluster with an infinite periodic arrangement of such clusters.

Motion in an ultralong-range potential in cold-Rydberg-atom collisions

In a recent letter Li et al. [Phys. Rev. Lett. 94, 173001 (2005)] have studied experimentally the role of dipole-dipole interactions in the ionization of Rydberg atoms and the formation of a cold plasma. The authors associated their observations with atomic motion due to such interaction. In this work, we adapt the dynamical model proposed by De Oliveira et al. [Phys. Rev. Lett. 90, 143002 (2003)] based on a two atom interaction, which reproduce qualitatively well some experimental results observed by Li and co-workers. This comparison presents clear evidences of motion in such ultralong range potential between cold-Rydberg atoms, which may be important for some applications, as quantum computation.

Spin-system radio-frequency superradiation: A phenomenological study and comparison with numeric simulations

We discuss the coherent behavior of a polarized, nuclear or electron, spin system for which the magnetic dipole radiation emitted in the radio-frequency region, has approximately quadratic dependence on the number of spins. An effective method of describing these phenomena is provided by computer simulation of a microscopic model of the spin system. Important aspects of this numeric simulation are described, together with a comparison with the theoretical predictions. The behavior of the transverse component of the magnetic moment, ${M}^{+}(t)$, in super-radiant conditions is studied. In addition, the role of dipole-dipole interactions in super-radiation phenomena is investigated in detail. It is shown that some important features of super-radiation cannot be described with the Bloch equations.

On Extreme Density of Data Storage in Patterned Magnetic Media

ABSTRACTA patterned magnetic medium is the next step to significantly increasing data storage density. To attain extreme density, it is necessary to deal with the smallest granules (each storing a bit of information) admissible from the physical point of view. It is well known that the volume of such granules, and their aspect ratio, have to be large enough to prevent thermally activated magnetization relaxation. However, these requirements are not exhaustive. A patterned recording medium is a set of closely-spaced, aligned magnetic granules, with the random (“up” and “down”) orientations of granules' magnetic moments. Though the exchange interactions between the granules may be neglected, there are long-range intergranular dipole-dipole interactions (DDI) which lead to significant lowering of activation barrier preventing thermal relaxation. Those interactions limit the acceptable density of ferromagnetic granules, and hence, reduce the extreme density of data storage. Our numerical calculations estimate the negative role of DDI for the particular case of a two-dimensional set of ellipsoidal magnetic granules and allow us to determine the extreme density of data storage.

Role of Dipole-Dipole Interaction in Phase Transition of Organic Antiferromagnet Motmp

The role of the dipole-dipole (DD) interaction in organic molecular antiferromagnet MOTMP is elucidated. The anisotropy energy due to the DD interaction ca.n explain well the experimental values of the spin-flop field and the saturation field along each crystal axis if we use the nearest neighbor (n.n.) interchain exchange coupling Jc = -0.0087 K and if we introduce small anisotropy for the intrachain exchange coupling. The uniform magnetic susceptibility in the paramagnetic state has been calculated by taking into account the DD interaction on the basis of the dynamical correlated-effective-field approximation. The observed susceptibility are well reproduced by assuming the n.n. intrachain exchange coupling Ja =0.45 K and the next n.n. interchain exchange coupling Jb =0 K. The Néel temperature T N is determined to be 0.15 K, which agrees with the observation.

Dipole-dipole interaction of Josephson diamagnetic moments

The role of dipole-dipole interaction between Josephson diamagnetic moments is considered within a model system of two clusters (each cluster contains three weakly connected superconducting grains). The sign of the resulting critical current is shown to depend on the orientation between clusters, allowing for both 0 and π type junction behavior. The possibility of the experimental verification of the model predictions is discussed.

Role of dipole-dipole interaction in the decay kinetics of spin-polarized atomic hydrogen and the problem of Bose-condensation region attainment

Abstract The significant role of the dipole-dipole interaction in the decay kinetics of spin-polarized atomic hydrogen is revealed. It is shown that the possibility to overcome the exchange recombination on the surface and to attain ultra-low temperatures, corresponding to Bose-condensation, only remains when the atoms accumulate in the state with parallel electron and nuclear spins.

The influence of aromatic fragments on the intermolecular interaction of phosphoryl compounds with the medium

1. The interaction of a number of phosphoryl compounds containing phenyl and phenoxyl radicals at the phosphorus with the medium was investigated by the method of IR spectroscopy. 2. The presence of aromatic fragments, both in phosphoryl compounds and in solvent molecules, weakens the influence of the polarity of the medium on the vibration frequency of the P=O bonds. This is explained by a decrease in the role of dipole-dipole interactions on account of specific interactions with the aromatic fragments.

Method of determining the relative stability of different conformational states of biopolymer molecules

A method for estimating the relative stability of conformational states of biopolymer molecules from the degree of sharpness of the transition between them has been suggested. The method is applied to the estimation of stability of the DNA helical structure under physiological conditions as well as to the estimation of stability of an α-helical structure of polypeptide chains in different solvents. The stability of an α-helical structure of polypeptide chains is relatively low not only in aqueous media but also in water-organic mixtures and even in inert organic solvents in which the energy of intramolecular hydrogen bonds is essentially higher than in aqueous medium. This is an evidence that the formation of the helical structure in organic solvents must be accompanied by a noticeably greater decrease in the conformational free energy of monomer units than in water. This conclusion is confirmed by independent experimental data on free energies of initiation of helical regions of polypeptide chains. For the explanation of this effect an assumption is made on the role of dipole-dipole interactions of peptide groups of the backbone.