Slow dynamics in a turbulent von Kármán swirling flow

An experimental study of a turbulent von Kármán flow in a cylinder is presented. The mean flow is stationary up to a Reynolds number Re=10(4) where a bifurcation takes place. The new regime breaks some symmetries of the problem and becomes time dependent because of equatorial vortices moving with a precession movement. In the exact counterrotating case, a bistable regime appears and spontaneous reversals of the azimuthal velocity are registered. A three-well potential model with additive noise reproduces this dynamic. A regime of periodic response is observed when a very weak forcing is applied.     

Measurement of tensor analyzing powers in deuteron photodisintegration

A new accurate measurement of the tensor analyzing powers T20, T21, and T22 in deuteron photodisintegration has been performed. Wide-aperture nonmagnetic detectors allowed broad kinematic coverage in a single set up: E(gamma)=25 to 600 MeV, and theta(p)(cm)=24 degrees to 48 degrees and 70 degrees to 102 degrees . The new data provide a significant improvement over the few existing measurements. The angular dependency of the tensor asymmetries in deuteron photodisintegration is extracted for the first time.     

Static and dynamic heterogeneities in a model for irreversible gelation

We study the structure and the dynamics in the formation of irreversible gels by means of molecular dynamics simulation of a model system where the gelation transition is due to the random percolation of permanent bonds between neighboring particles. We analyze the heterogeneities of the dynamics in terms of the fluctuations of the self-intermediate scattering functions: in the sol phase close to the percolation threshold, we find that this dynamic susceptibility increases with the time until it reaches a plateau. At the gelation threshold this plateau scales as a function of the wave vector k as k(eta-2), with eta being related to the decay of the percolation pair connectedness function. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent gamma as the mean cluster size. These findings suggest an alternative way of measuring critical exponents in a system undergoing chemical gelation.     

Production of ball-lightning-like luminous balls by electrical discharges in silicon

We performed electric arc discharges in pure Si to generate luminous balls with lifetime in the order of seconds and several properties usually reported for natural ball lightning. This simple experiment does not rely on energy sources and excitation mechanisms that are improbable in the natural phenomenon and clearly demonstrates the role of vaporization and oxidation of Si, as proposed by the Abrahamson-Dinniss theory for ball-lightning formation.     

Numerical experiments on quantum chaotic billiards

A recently proposed numerical technique for generation of high-quality unstructured meshes is combined with a finite-element method to solve the Helmholtz equation that describes the quantum mechanics of a particle confined in two-dimensional cavities. Different shapes are treated on equal footing, including Sinai, stadium, annular, threefold symmetric, mushroom, cardioid, triangle, and coupled billiards. The results are shown to be in excellent agreement with available measurements in flat microwave resonator counterparts with nonintegrable geometries.     

Heavy quarkonia from classical SU(3) Yang-Mills configurations

A generalized Cho-Faddeev-Niemi ansatz for SU(3) Yang-Mills is investigated. The corresponding classical field equations are solved for its simplest parametrization. From these solutions it is possible to define a confining non-relativistic central potential used to study heavy quarkonia. The associated spectra reproduces the experimental spectra with an error of less than 3% for charmonium and 1% for bottomonium. Moreover, the recently discovered new charmonium states can be accomodate in the spectra, keeping the same level of precision. The leptonic widths show good agreement with the recent measurements. The charmonium and bottomonium E1 electromagnetic transitions widths are computed and compared with the experimental values.     

Angle-resolved three-dimensional analysis of surface films by coherence scanning interferometry

The Fourier components of interference signals generated by scanning a high-numerical-aperture objective orthogonal to an object surface correspond to different angles of incidence on the surface. The phase and amplitude of these Fourier components relate to the structure of the object, including in particular the 3D topography and thickness profiles of thin-film layers.     

Theory of the time reversal cavity for electromagnetic fields

We derive a general expression of the electric dyadic Green function in a time-reversal cavity, based on vector diffraction theory in the frequency domain. Our theory gives a rigorous framework to time-reversal experiments using electromagnetic waves and suggests a methodology to design structures generating subwavelength focusing after time reversal.     

Effect of mating delay on the ovary of Melipona quadrifasciata anthidioides (Hymenoptera: Apidae) queens

The effect of mating delay on the ovary structure of virgin queens of Melipona quadrifasciata anthidioides (Hymenoptera: Apidae) was investigated. Virgin queens 15, 20, 25 and 30 days old were dissected to analyze their degree of ovary degeneration. To verify whether the degeneration would cause permanent sterility, virgin queens of the same ages (15, 20, 25 and 30 days) were mated and accompanied for at least 14 days to verify whether there was physogastry and then dissected. The ovaries were analyzed by histology, histochemical tests and TUNEL to verify programmed cell death. The results showed that mating delay interrupted oogenesis preventing vitellogenesis. Mating delay results in ovary degeneration which increased with queen age. However, even when there was ovariole degeneration, 25-day-old virgin queens after mating presented normal ovariole activation.