Radiation zeros in high-energy e^+e^- annihilation into hadrons

The process contains radiation zeros, i.e. configurations of the four–momenta for which the scattering amplitude vanishes. We calculate the positions of these zeros for –quark and –quark production and assess the feasibility of identifying the zeros in experiments at high energies. The radiation zeros are shown to occur also for massive quarks, and we discuss how the final state may offer a particularly clean environment in which to observe them. Received: 16 December 1997 / Published online: 10 March 1998     

Stable vortex-antivortex molecules in mesoscopic superconducting triangles

A thermodynamically stable vortex-antivortex pattern has been revealed in equilateral mesoscopic type I superconducting triangles, contrary to type II superconductors where similar patterns are unstable. The stable vortex-antivortex "molecule" appears due to the interplay between two factors: a repulsive vortex-antivortex interaction in type I superconductors and the vortex confinement in the mesoscopic triangle.     

Dynamic vortex-antivortex interaction in a single cross-tie wall

A fascinating property of micromagnetism comes from the possibility to control the domain and vortex configuration through the sample shape and size. For instance, in a rectangular platelet a configuration containing a stable combination of vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. Here we report on its magnetization dynamics including the vortex-antivortex interactions. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. We show that the vortex dynamics can be used to identify the core configuration, which is not directly accessible to x-ray microscopy because of its limited spatial resolution.     

Thermal activation of vortex-antivortex pairs in quasi-two-dimensional Bose-Einstein condensates

Here we show, by performing ab initio classical field simulations that two distinct superfluid phases, separated by thermal vortex-antivortex pair creation, exist in experimentally producible quasi-2D Bose gas. These results resolve the debate on the nature of the low temperature phase(s) of a trapped interacting 2D Bose gas.     

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

Monochromatic neutrino signals from dark matter annihilation

Dark matter (DM) annihilations in the Sun to neutrino–antineutrino pairs are known to have potentially observable signatures in neutrino telescopes such as IceCube and KM3. We propose a model independent analysis in which the monochomatic neutrino signal from dark matter (DM) annihilations in the Sun is related to the direct detection spin-independent and spin-dependent cross sections rather than assuming cross sections from a particular model. We propagate the neutrinos from the center of the Sun to the Earth taking into account matter effects on neutrino oscillations. For DM capture in the Sun via a large spin-dependent DM capture cross section the discovery prospects of the IceCube experiment are found to be promising.     

Competing symmetries and broken bonds in superconducting vortex-antivortex molecular crystals

Hall probe microscopy has been used to image vortex-antivortex molecules induced in superconducting Pb films by the stray fields from square arrays of magnetic dots. We have directly observed spontaneous vortex-antivortex pairs and studied how they interact with added free (anti)fluxons in an applied magnetic field. We observe a variety of phenomena arising from competing symmetries which either drive added antivortices to join antivortex shells around dots or stabilize the translationally symmetric antivortex lattice between the dots. Added vortices annihilate antivortex shells, leading first to a stable "nulling state" with no free fluxons and then, at high densities, to vortex shells around the dots stabilized by the asymmetric antipinning potential. Our experimental findings are in good agreement with Ginzburg-Landau calculations.     

Radiation patterns of circular apertures with Gaussian illumination

We analyze the radiation pattern from circular apertures with Gaussian electric field distribution. Patterns are calculated using the Fraunhofer diffraction integral and we obtain a convenient expression for the FWHM beamwidth as a function of the edge taper. The aperture efficiency can be calculated for blocked and unblocked apertures in analytic form. The beam efficiency must be computed numerically, but can be obtained in a straightforward way from the beamwidth.     

Symmetric and asymmetric vortex-antivortex molecules in a fourfold superconducting geometry

In submicron superconducting squares in a homogeneous magnetic field, Ginzburg-Landau theory may admit solutions of the vortex-antivortex type, conforming to the symmetry of the sample [L. F. Chibotaru, Nature (London) 408, 833 (2000)10.1038/35048521]. Here we show that these fascinating, but never experimentally observed states, can be enforced by artificial fourfold pinning, with their diagnostic features enhanced by orders of magnitude. The second-order nucleation of vortex-antivortex molecules can be driven by either temperature or an applied magnetic field, with stable asymmetric vortex-antivortex equilibria found on its path.