Modern Physics Letters B

Crossover from individual Rabi dynamics to collective Josephson dynamics in two-coupled Bose–Einstein condensates is studied as a phase transition process. We obtain the critical value of the parameter Λ (ratio of the mean field energy to the tunneling matrix element) for the π-phase oscillations from the non-linear perturbation expansion of small oscillations around the saddle point of the first-order Euclidean (imaginary time) differential equations for polar and azimuthal angles θ and ϕ without approximation.

In this paper we report influence of titanium substitution on the structural properties and d.c. conduction behavior of Li-Zn ferrites. The samples having the compositional formula Li_0.45+0.5tZn_0.1Ti_tFe_2.45-1.5tO₄ with t varying from 0 to 1.2 in steps of 0.2 were prepared by the conventional dry ceramic method. XRD analysis showed all the samples to be single phase spinel and the lattice parameter was found to increase with increasing t. D.C. resistivity increases with Ti concentration. The possible mechanisms involved are discussed.

Glasses of the system Bi₂O₃·As₂O₃ doped with MnO up to 20 mol% were studied by means of EPR. The distribution of Mn²⁺ ions on different structural units of the matrix was studied as function of the MnO content of the glass. The revealed valence states of manganese ions and the interactions involving them are discussed.

Crystals with different faceting behavior were observed in the strontium titanate borosilicate glass ceramic system with a small amount of bismuth oxide addition and proper heat treatment schedule. These types of faceting morphology were generally observed in phase-separated glasses followed by heterogeneous nucleation. X-ray diffraction and electron microscopic techniques were used to characterize these samples. X-ray diffraction patterns for bulk and pulverized samples were observed to be readily different from each other. Peaks in the X-ray diffraction pattern for bulk samples can be indexed for icosahedral symmetry.

La_0.5Ca_0.5MnO₃ (LCMO) thin films grown by pulsed laser deposition (PLD) and annealed at different temperatures were investigated by high angle X-ray diffraction, atomic force microscope (AFM), scanning electron microscope (SEM), and energy dispersive spectroscopy (SEM-EDS). The lattice parameters, surface morphology as well as the metal compositions of the films were obtained. It was found that the surface morphology of the films strongly depends on the annealing temperatures. The difference of the thermal expansion coefficients between the film and the substrate plays an important role in determining the morphology of the film surface. It induces an in-plane compressive stress in the LCMO films. The strains in the film can be relaxed by nanoscale grains and cracks.

Systematic measurements of electrical magnetoconductivity near the coherence transition of granular DyBa₂Cu₃O_7-δ ultra-thin films are reported. Two type of experiments were performed: diverse current densities up to 80 A/cm² were applied in the absence of magnetic field H and magnetic fields ranging from 0 to 2.5 kOe were applied perpendicular to the structural Cu–O planes to study the phase-glass transition. Close to the zero resistance state, the measurements reveal the occurrence of a coherence transition, where the phases of the order parameter in individual grains become long-range ordered. The critical temperature, T_c0(H), for this transition is close to the point where the resistivity vanishes. A strong enlargement of the fluctuation interval preceding the coherence transition is caused by the applied magnetic field. The critical conductivity behavior for the coherence transition is interpreted within a 3D-XY model where disorder and frustration are relevant. It was evidenced that close to the coherence transition, the correlation length scales as a power law of temperature with a temperature-dependent critical exponent ν. In the presence of external magnetic fields, the corresponding value of ν is consistent with the 2D-gauge-glass model. In the absence of applied magnetic fields, the value of ν is not the same as for the 2D-phase-glass model. A scaling into the framework of the vortex-glass theory shows that the chiral- and gauge-glass behaviors are in different dynamic universality classes.