Lamellar-like structures in ferrofluids placed in strong magnetic fields

We consider a ferrofluid system consisting of magnetic particles interacting with a magnetic dipole–dipole interaction. We study the strong magnetic field regime where all magnetic dipoles are completely polarized in the direction of the magnetic field. We introduce a lattice gas model that serves to describe space ordering phenomena in such systems. It is found that, within mean field theory, this model predicts a second order phase transition to a phase with inhomogeneous lamellar-like ordering below a certain critical temperature.     

Vibration induced electric dipole in a weakly bound molecular complex

A strong electric susceptibility is measured as a function of temperature in a molecular dimer which is weakly bound by a pair of hydrogen bonds. This system is a remarkable example where the dominant term in the susceptibility is due to a vibration induced electric dipole. As a consequence, the averaged square of the dipole moment varies linearly with the temperature and the susceptibility does not follow the usual 1/T Curie law. this spectacular effect demonstrates the importance of dynamics to interpret the properties of weakly bound complexes.     

Simulation of two dimensional systems with two dimensional electrostatics

Some of the problems associated with Monte Carlo and molecular dynamics simulations of three dimensional ionic and dipolar systems are discussed, with emphasis on the use of periodic boundary conditions (PBC). It is shown that analogous problems may arise in two dimensional systems provided that the interactions are two dimensional electrostatic interactions, that is, interactions derived from the two dimensional Laplace equation. The PBC hamiltonian is evaluated by considering the appropriate two dimensional lattice sums, and a computable form for the effective pair interactions in PBC developed. The idea of an external dielectric constant is introduced and its effects included in the PBC hamiltonian. Formulae for evaluating the dielectric constant from a simulation with any external dielectric constant are given. Perturbation formulae showing the effects on the structure and mean square dipole moment of a dipolar system which are caused by a change in external dielectric constant are derived. A formula for the shift in mean square dipole moment of an ionic system is also developed. The problem of interpreting results from such simulations is discussed briefly.     

Dynamics of Condensation in the Totally Asymmetric Inclusion Process

We study the dynamics of condensation of the inclusion process on a one-dimensional periodic lattice in the thermodynamic limit, generalising recent results on finite lattices for symmetric dynamics. Our main focus is on totally asymmetric dynamics which have not been studied before, and which we also compare to exact solutions for symmetric systems. We identify all relevant dynamical regimes and corresponding time scales as a function of the system size, including a coarsening regime where clusters move on the lattice and exchange particles, leading to a growing average cluster size. The second moment of the occupation numbers is a suitable observable to characterise the transition, and exhibits a power law scaling in this regime before saturating to stationarity following an exponential decay depending on the system size. Our results are based on heuristic derivations and exact computations for symmetric systems, and are supported by detailed simulation data.     

Canonical Analysis of Condensation in Factorised Steady States

We study the phenomenon of real space condensation in the steady state of a class of mass transport models where the steady state factorises. The grand canonical ensemble may be used to derive the criterion for the occurrence of a condensation transition but does not shed light on the nature of the condensate. Here, within the canonical ensemble, we analyse the condensation transition and the structure of the condensate, determining the precise shape and the size of the condensate in the condensed phase. We find two distinct condensate regimes: one where the condensate is gaussian distributed and the particle number fluctuations scale normally as L ^1/2 where L is the system size, and a second regime where the particle number fluctuations become anomalously large and the condensate peak is non-gaussian. Our results are asymptotically exact and can also be interpreted within the framework of sums of random variables. We further analyse two additional cases: one where the condensation transition is somewhat different from the usual second order phase transition and one where there is no true condensation transition but instead a pseudocondensate appears at superextensive densities. PACS numbers: 05.40.-a, 02.50.Ey, 64.60.-i.     

Subwavelength displacement of the far-field image of a radiating dipole

The field lines of the Poynting vector for light emitted by a dipole with a rotating dipole moment show a vortex pattern near the location of the dipole. In the far field, each field line approaches a straight line, but this line does not appear to come exactly from the location of the dipole. As a result, the image of the dipole in its plane of rotation seems displaced. Secondly, the image in the far field is displaced as compared with the image of a source for which the field lines run radially outward. It turns out that both image displacements are the same. The displacements are of subwavelength scale, and they depend on the angles of observation. The maximum displacement occurs for observation in the plane of rotation and equals lambda/pi, where lambda is the wavelength of the light.     

Radiation of the Point Source from an Anisotropic Plasma Cylinder

We obtain and analyze an analytical solution to the problem of electromagnetic-wave radiation of the point electric dipole from an anisotropic plasma cylinder to free space. Two cases of the dipole orientation are considered, where the electric dipole is directed along and across a horizontal magnetic field whose direction does not coincide with the axes of a cylindrical coordinate system. We analyze how the conditions and characteristics of the resonance influence of the anisotropic plasma cylinder depend on the strength of the magnetic field and its direction with respect to the dipole moment of the source. Comparative analysis of the resonance influence of the plasma cylinder with horizontal and axial external magnetic fields is performed.     

Spectroscopic manifestations of orientational phase transition in adsorbate monolayer

In the framework of the virtual crystal approximation, the temperature dependences of frequencies and integral intensities of infrared absorption spectral lines are established for an adsorbate monolayer near the point of the order-to-disorder orientational phase transition. The results obtained are consistent with the experimentally observed temperature dependences of the vibrational spectra for the 2 × 1 monolayer CO/NaCl (100). For this system, the Davydov splitting does not vanish on the phase transition due to a nonzero value of the average dipole moment (unlike molecular crystals and orientationally planar monolayers in which the average dipole moment becomes zero at the phase transition temperature).     

Equilibrium states and dynamics of the dipole moment of square arrays of dipoles

The dipole lattices in the form of 3 × 3–8 × 8 dipole square arrays have been considered. It has been shown that, after turning off the external field orienting dipoles along the edges of the array, two types of equilibrium configurations of dipole moments can exist depending on the size of the system, namely, the state symmetric with respect to the diagonal of the array and the state with the total dipole moment directed along the edges of the array. It has been found that there are differences in these types of configurations with respect to the alternating-field-induced oscillation modes of the total dipole moment of the system. The dependence of the oscillation modes on the direction of linear polarization of the alternating field has been investigated.     

Phase ordering induced by defects in chaotic bistable media

The phase ordering dynamics of coupled chaotic bistable maps on lattices with defects is investigated. The statistical properties of the system are characterized by means of the average normalized size of spatial domains of equivalent spin variables that define the phases. It is found that spatial defects can induce the formation of domains in bistable spatiotemporal systems. The minimum distance between defects acts as parameter for a transition from a homogeneous state to a heterogeneous regime where two phases coexist The critical exponent of this transition also exhibits a transition when the coupling is increased, indicating the presence of a new class of domain where both phases coexist forming a chessboard pattern.     

Mesoscopic fluctuations of the dipole moment and the electric polarizability

We consider mesoscopic fluctuations of the dipole moment and the electric polarizability of a disordered thin film in the diffusive regime. It is shown, that the requirement of a fixed particle number is crucial for obtaining finite fluctuations at low temperatures. For systems with sufficient disorder and a long screening length a spontaneous dipole moment should be observable with an atomic force microscope.Dedicated to Prof. H. Wagner on the occasion of his 60th birthday     

A new association state of solutes in nanoconfined aqueous solutions

Recently, we have found a reversible transition between the dispersion and aggregation states of solute molecules in aqueous solutions confined in nanoscale geometry, where solutes exhibit distinct behavior in a new association state from that in the dispersion and aggregation states observed usually in macroscopic systems. However, it remains unknown whether this new association state of solute molecules found in nanoconfined systems would vanish with the system size increasing and approaching the macroscopic scale. Here, we achieve the phase diagram of solute association states by making the analyses of Gibbs free energy of solutes in nanoconfined aqueous solutions in detail. In the phase diagram, we observe a closed regime with a finite system size of nanoconfined aqueous solutions and a solute concentration range, only in which there exists the new association state of solutes with the reversible transition between the aggregation and dispersion states, and there indeed exists an upper limit of the system size for the new association state, around several tens nanometers. These findings regarding the intimate connection between the system size and the solute association behavior provides the comprehensive understanding of the association dynamics of solutes in nanoconfined environment.     

The Orientation of the Phosphorescence Dipole Moment of Erythrosine B Within Its Molecular Frame

For an unequivocal interpretation of time-resolved phosphorescence depolarization measurements on dye molecules in biophysical systems, the orientation of the transition dipole moments within the frame of the dye has to be known. The goal of this work is to find the orientation of the phosphorescence dipole moment of erythrosine B within its molecular frame. To this end, we first determine four difference angles, exciting a molecule in two of its absorption bands and measuring the fluorescence and phosphorescence anisotropy for each excitation wavelength. The next step is to position the transition dipole moments within the molecular frame, combining the results of angle-resolved fluorescence depolarization measurements with the measured difference angles. It is shown that the phosphorescence dipole moment is tilted out of the plane of the molecule. Additionally, we find that the phosphorescence dipole moment is located above the fluorescence dipole moment and not above the absorption dipole moment. This induces an extra difference angle between the absorption and the phosphorescence dipole moments which in the past has been interpreted erroneously as a larger tilt out of the plane of the molecule.     

Vibration-rotation intensities for “hot” bands

Vibration-rotation transition moments for “hot” bands are presented assuming the Dunham potential and a power series expansion for the dipole moment function. These results are applied to an analysis of CO intensities where a unique dipole moment function is determined from the experimental slope of the Herman-Wallis factors and the integrated intensities of the fundamental and overtone bands. This is then used together with the theoretical expressions to calculate the “hot” band dipole moment matrix elements which are found to be in good agreement with the experimental results.     

Tunable negative refraction without absorption via electromagnetically induced chirality

We show that negative refraction with minimal absorption can be obtained by means of quantum interference effects similar to electromagnetically induced transparency (EIT). Coupling a magnetic dipole transition coherently with an electric dipole transition leads to electromagnetically induced chirality, which can provide negative refraction without requiring negative permeability and also suppress absorption. This technique allows negative refraction in the optical regime at densities where the magnetic susceptibility is still small and with refraction/absorption ratios that are orders of magnitude larger than those achievable previously. Furthermore, the refractive index can be fine-tuned, which is essential for practical realization of subdiffraction-limit imaging. As with EIT, electromagnetically induced chirality should be applicable to a wide range of systems.     

Doppler cooling to the quantum limit

Doppler cooling on a narrow transition is limited by the noise of single scattering events. It shows novel features, which are in sharp contrast with cooling on a broad transition, such as a non-gaussian momentum distribution, and divergence of its mean square value close to the resonance. We have observed those features using 1D cooling on an intercombination transition in strontium, and compared the measurements with theoretical predictions and Monte Carlo simulations. We also find that for very a narrow transition, cooling can be improved using a dipole trap, where the clock shift is canceled.     

The effect of bending vibrations on the dipole moment of a linear polyatomic molecule: A theoretical treatment of the vibrational changes and transition moments

The vibrational effect on the dipole moment of a linear molecule is theoretically considered from the aspects of the dipole moment changes with the excitation of bending vibrations and the transition moments for the overtone, combination, and difference bands associated with bending modes. Such dipole moment changes and transition moments consist of two components, one depending on the first dipole moment derivatives with respect to bond lengths and the other depending on the second dipole moment derivatives with respect to bond angles. We show that the first component normally contributes little, and propose an approximation in which only the second component is retained. This approximation is practically important because the second component can be calculated without the anharmonic force constants. We derive formulas for the dipole moment changes and transition moments to facilitate a simultaneous analysis of different isotopic species. We introduce the concept of the equivalent mode, by which we may readily understand the correlation between the dipole moment change for a bending mode and the transition moment for a vibrational band.     

Sum rule for bremsstrahlung cross section for 6Li in the resonating-group method

In the method of resonating-group structure, the wave function of ⁶Li is assumed to have a single channel of alpha and deuteron substructures in the ground state. It is shown that the intercluster exchange of nucleons is an important effect which causes significant change in the root mean square radius and the dipole transition cross section. Due to the lack of symmetry in space coordinates of ⁶Li, the dipole operator is not identical to the mean square radius operator for this sum rule calculation and is expected to display like behavior in similar systems. It is also shown that the deuteron substructure in this nucleus is substantially larger than the alpha substructure.     

Effect of molecular structure nonrigidity on anisotropy of dipole absorption cross section

The influence of the nonrigidity of free complex asymmetric top molecules on the anisotropy of the absorption cross section over various rotational configurations has been studied under conditions where the total angular momentum is conserved. It is shown that the nonrigidity-induced cross section anisotropy is maximal for the optical transition with the dipole moment collinear to the axis of the molecule’s mean moment of inertia and rapidly decreases with the rise of the intramolecular flexibility. For all types of tops, disturbances from internal motions are stronger in the region where the principal axes of inertia are collinear with the total rotational angular momentum.     

Electrooptical Absorption Measurements (EOAM) Testify Existence of two Conformers of Prodan and Laurdan with Different Dipole Moments in Equilibrium Ground and Franck-Condon Excited State

The results from the electrooptical absorption measurements (EOAM) on the equilibrium ground and excited Franck-Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane are presented. As follows from experiments Prodan and Laurdan in the equilibrium ground and excited Franck-Condon state have two conformers with considerably different dipole moments. The electrical dipole moments and the transition dipole moment, obtained from the short-wavelength region of the absorption spectrum are parallel. The electrical dipole moments measured at the long-wavelength spectral region are parallel to each other but not parallel to the transition dipole moment m _a. The angle θ between the transition dipole moment m _a and the dipole moment in the equilibrium ground state μ _g of the long-wavelength conformer is about 30⁰ for both probes. Obtained results evidence that donor-acceptor pairs of the short-wavelength and long-wavelength conformers are not located on the same axis. Two low-energy conformers of Prodan have been found by density functional theory (DFT) calculations, differing in the orientation of the carbonyl group towards the naphthalene system.