Modern Physics Letters B

Some topological aspects of the magnetic reconnection phenomenon are summarized and recent numerical results, derived within a two-fluid model, of two-dimensional collisionless magnetic reconnection in presence of a strong guide field are reported. Both the Alfvèn and the whistler frequency range are investigated by including electron parallel compressibility effects that are related respectively to thermal effects and to density fluctuations. The Hamiltonian character of the system is emphasized as it drives the small scale dynamics through the presence of topological invariants. These determine the formation and the shape of small scale current and vorticity layers inside the magnetic island. Secondary fluid instabilities, mainly of the Kelvin–Helmholtz type, can destabilize these layers when a hydrodynamic type regime is achieved. The inclusion of parallel electron compressibility has stabilizing effects. In view of the limitations of the two-fluid modelling, possible developments are briefly discussed such as the inclusion of Larmor-radius corrections, in lieu of a fully kinetic approach.

The phase transition induced by pressure in the PbS and HgS nanocrystals was studied by in situ high pressure X-ray powder diffraction measurements. PbS underwent a phase transition from B1-B16 at 7 GPa whereas no transition was observed in HgS up to 15 GPa. The transition remained unaltered on the release of pressure. The volume decreased with an increase in pressure. There is a shift in transition pressure towards the higher side when compared with the bulk materials.

The Next Nearest Neighbor (NNN) effect in Ni²⁺-[2×2] using the isotropic Heisenberg model has been investigated in this article. Although the NNN effect is commonly very weak in this kind of grid, it was calculated that its response is quite big to the outsider perturbations in certain regions. Two different antiferromagnetic arrangements of spins interacting via the Heisenberg exchange type in a square were discussed in terms of their stability regions. Using the isotropic Heisenberg exchange Hamiltonian, the zero field energy spectrum has been calculated for this particular Ni²⁺-[2×2] grid geometry. The average magnetic moments with and without the next nearest neighbors interactions were also calculated. In order to verify the calculations, the results were compared with experimental data which had been published in literature.

The damage spreading of the Ising model on a set of trivalent structures is studied with competing Glauber and Kawasaki dynamics. We find that the damage transition temperature T_d increases as the number of rectangles increases at the pure Glauber dynamics case and T_d decreases sharply as the probability of occurrence of Kawasaki dynamics increases. A heuristic argument is provided to explain the numerical results.

Measurements have been performed on the resistivity of samples MgB₂, AlB₂ and AgB₂. The samples show presence of impurities. By analyzing the data in terms of impurity scattering, electron-phonon scattering, and weak localization, it has been found that the AlB₂ (AgB₂) sample involves maximum (minimum) effect of the impurity, electron-phonon interaction and weak localization.

A novel solution to the singly quantized vortex is presented for big Bose–Einstein condensates in axis-symmetric harmonic traps, which is valid in the regime of strong coupling parameters. This solution manifests the vortex as a topological soliton and describes correctly its global structure for the whole region of large condensates including the vortex-core, except for the thin boundary region. An improved expression for thermodynamical critical rotation frequency for vortex formation is derived based on our solution, conforming to the known frequency formula given by Lundh et al. in the Thomas–Fermi limit.