Reconstructing phase dynamics of oscillator networks

We generalize our recent approach to the reconstruction of phase dynamics of coupled oscillators from data [B. Kralemann et al., Phys. Rev. E 77, 066205 (2008)] to cover the case of small networks of coupled periodic units. Starting from a multivariate time series, we first reconstruct genuine phases and then obtain the coupling functions in terms of these phases. Partial norms of these coupling functions quantify directed coupling between oscillators. We illustrate the method by different network motifs for three coupled oscillators and for random networks of five and nine units. We also discuss nonlinear effects in coupling.     

Dipolar Order in Molecular Fluids: I. Toward an Understanding

The origin behind the dipolar order in molecular fluids is investigated by using a simple dipolar fluid and Monte Carlo simulation technique. A penalty function is employed to separately manipulate the positional and orientational structure of the fluid. By considering the distance-dependent Kirkwood function G _k, which in turn is related to the dielectric permittivity of the fluid, it is observed that both positional and orientational ordering are involved to establish dipolar order.     

Dipolar Order in Molecular Fluids: II. Molecular Influence

The influence on the short-range packing in dipolar fluids by molecular shape and by additional higher order electrostatic moments has been investigated by molecular dynamic simulations. The dipole polarization was found to decrease as the particles were elongated parallel to the dipole and to increase for elongation perpendicular to the dipole, eventually forming a nematic order. The addition of a quadrupole lead to a reduction of the polarization, and the influence of an axial octupole was weaker and more complex. Both a decrease and an increase of the polarization is possible depending on the relative dipole–dipole and octupole–octupole interaction strengths and the relative direction of the symmetry axes of the moments. These observations were attributed to the different parity of a dipole and a quadrupole and the same parity of a dipole and an axial octupole under reflection. In addition, further insights into the formation of dipole polarization were obtained. Short polar and long equatorial radii and strong dipole–dipole interaction are particle properties that promote a fluid with a high dipole polarization.     

Evolution of the interfacial structure of LaAlO3 on SrTiO3

The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. Based on these structures, the threshold for formation of a two-dimensional electron system at a film thickness of 4 monolayers is quantitatively explained. The findings are also qualitatively reproduced by density-functional-theory calculations.     

The effect of oxidation on the surface-near lattice relaxation in FeNi nanoparticles

The near-surface oxidation-induced lattice relaxation and compositional changes of FeNi alloy nano-particles are investigated. Using a newly developed transfer system, the particle structure was characterised by means of aberration-corrected HR-TEM prior to exposing the particles to ambient air. This allows for a comparison of oxidised and un-oxidised particles, respectively. Independent of the oxidation, the surface-near and/or interface-near metal lattice was found to be expanded by up to 3%. EELS profiles clearly reveal an enrichment of Fe at the particle surfaces. MD simulations in combination with HR-TEM contrast simulations were conducted to investigate the effect of the Fe enrichment on the structural relaxation. The results show that a surface-near over-stoichiometric enrichment of Fe indeed causes a dilation that counteracts a compression of the lattice at the particle surface as obtained for homogeneously alloyed particles. Besides, the large lattice mismatch between the metallic cores and the NiFe₂O₄ shells causes the formation of step dislocations in the close vicinities of the interface. In essence, the surface-near lattice relaxation in oxide free particles is found to be due to a segregation of Fe to the surface, whereas in the case of shell–core particles, no systematic influence of the oxide on the lattice relaxation was found.     

Index of refraction for cold lithium- and diatomic sodium waves traveling through cold noble gases

In the present paper we propose to measure the index of refraction for diatomic sodium molecules traveling through a cold helium gas. Theoretical calculations of the index of refraction for this system are presented as a function of the molecule velocity and atom gas temperature. Whereas previous theoretical efforts to compute the refractive index have been concerned with atomic systems and atomic matter waves, we extend the investigation to diatomic molecules in the present work. To enable such calculations the potential energy surface for the atom-molecule interaction is calculated ab initio, along with the long range dispersion coefficients for the atom-molecule system. The full close-coupled equations, describing the atom-molecule collisions, are solved numerically to work out the influence of the collisions on the matter waves. We investigate the sensitivity of the results upon changes and inaccuracies in the potential energy surface. Several molecular rotational levels are included in the present study, and the index of refraction is found to depend on the rotational state. In addition, the index of refraction for atomic lithium matter waves traveling through the cold noble gases helium and argon are computed, motivated by a recent experiment with atomic lithium matter waves. Different resonances (glory- and scattering resonances) are identified from the results. Such resonances offer an important opportunity for the comparison of experiment and theory.     

Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects

We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical.     

Influence of organic capping layers on the performance of transparent organic light-emitting diodes

We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05.     

Semi-Parametric Probability-Weighted Moments Estimation Revisited

In this paper, for heavy-tailed models and through the use of probability weighted moments based on the largest observations, we deal essentially with the semi-parametric estimation of the Value-at-Risk at a level p, the size of the loss occurred with a small probability p, as well as the dual problem of estimation of the probability of exceedance of a high level x. These estimation procedures depend crucially on the estimation of the extreme value index, the primary parameter in Statistics of Extremes, also done on the basis of the same weighted moments. Under regular variation conditions on the right-tail of the underlying distribution function F, we prove the consistency and asymptotic normality of the estimators under consideration in this paper, through the usual link of their asymptotic behaviour to the one of the extreme value index estimator they are based on. The performance of these estimators, for finite samples, is illustrated through Monte-Carlo simulations. An adaptive choice of thresholds is put forward. Applications to a real data set in the field of insurance as well as to simulated data are also provided.