Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies

We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films.Comment: 10 pages, 9 figures, 2 table

Generalized Neighbor-Interaction Models Induced by Nonlinear Lattices

It is shown that the tight-binding approximation of the nonlinear Schr\"odinger equation with a periodic linear potential and periodic in space nonlinearity coefficient gives rise to a number of nonlinear lattices with complex, both linear and nonlinear, neighbor interactions. The obtained lattices present non-standard possibilities, among which we mention a quasi-linear regime, where the pulse dynamics obeys essentially the linear Schr{\"o}dinger equation. We analyze the properties of such models both in connection with their modulational stability, as well as in regard to the existence and stability of their localized solitary wave solutions

Resistive and magnetoresistive properties of CrO2 pressed powders with different types of inter-granular dielectric layers

Resistive, magnetoresistive and magnetic properties of four kinds of pressed CrO2 powders, synthesized by hydrothermal method of chromic anhydride have been investigated. The particles in powders constituted of rounded particles (diameter 120 nm) or needle-shaped crystals with an average diameter of 22.9 nm and average length of 302 nm. All of the particles had a surface dielectric shell of varying thickness and different types (such as oxyhydroxide -CrOOH or chromium oxide Cr2O3). For all the samples at low temperatures we found non-metallic temperature dependence of resistivity and giant negative magnetoresistance (MR). The maximum value of MR at low temperatures (T \approx 5 K) is \approx 37% in relatively small fields (0.5 T). At higher temperatures there was a rapid decrease of MR (up to \approx 1% / T at T \approx 200 K). The main objective of this work was studying the influence of properties and thickness of the intergranular dielectric layers, as well as CrO2 particle shape, on the magnitude of the tunneling resistance and MR of the pressed powder. The new results obtained in this study include: (1) detection at low temperatures in powders with needle-like particles a new type of MR hysteresis, and nonmonotonic MR behaviour with increasing magnetic field (absolute value of the MR at first grows rather rapidly with the field, and then begins diminishing markedly, forming a maximum), and (2) detection of non-monotonic temperature dependence, where - a field in which the resistance in a magnetic field has a maximum, as well as finding discrepancies in values of and coercivity fields, (3) detection of the anisotropy of MR, depending on the relative orientation of the transport current and the magnetic field, (4) a new method of synthesis, to regulate the thickness of dielectric coating.Comment: 19 pages, 8 figure

Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice

The similarity between matter waves in periodic potential and solid-state physics processes has triggered the interest in quantum simulation using Bose-Fermi ultracold gases in optical lattices. The present work evidences the similarity between electrons moving under the application of oscillating electromagnetic fields and matter waves experiencing an optical lattice modulated by a frequency difference, equivalent to a spatially shaken periodic potential. We demonstrate that the tunneling properties of a Bose-Einstein condensate in shaken periodic potentials can be precisely controlled. We take additional crucial steps towards future applications of this method by proving that the strong shaking of the optical lattice preserves the coherence of the matter wavefunction and that the shaking parameters can be changed adiabatically, even in the presence of interactions. We induce reversibly the quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press

Numerical instability of the Akhmediev breather and a finite-gap model of it

In this paper we study the numerical instabilities of the NLS Akhmediev breather, the simplest space periodic, one-mode perturbation of the unstable background, limiting our considerations to the simplest case of one unstable mode. In agreement with recent theoretical findings of the authors, in the situation in which the round-off errors are negligible with respect to the perturbations due to the discrete scheme used in the numerical experiments, the split-step Fourier method (SSFM), the numerical output is well-described by a suitable genus 2 finite-gap solution of NLS. This solution can be written in terms of different elementary functions in different time regions and, ultimately, it shows an exact recurrence of rogue waves described, at each appearance, by the Akhmediev breather. We discover a remarkable empirical formula connecting the recurrence time with the number of time steps used in the SSFM and, via our recent theoretical findings, we establish that the SSFM opens up a vertical unstable gap whose length can be computed with high accuracy, and is proportional to the inverse of the square of the number of time steps used in the SSFM. This neat picture essentially changes when the round-off error is sufficiently large. Indeed experiments in standard double precision show serious instabilities in both the periods and phases of the recurrence. In contrast with it, as predicted by the theory, replacing the exact Akhmediev Cauchy datum by its first harmonic approximation, we only slightly modify the numerical output. Let us also remark, that the first rogue wave appearance is completely stable in all experiments and is in perfect agreement with the Akhmediev formula and with the theoretical prediction in terms of the Cauchy data.Comment: 27 pages, 8 figures, Formula (30) at page 11 was corrected, arXiv admin note: text overlap with arXiv:1707.0565

Features of the magnetic properties of quasi-one-dimensional beta-TeVO4 magnets at low temperatures

The effect of a magnetic field on the magnetic susceptibility chi(T) and the temperature of the three magnetic phase transitions in single-crystal beta-TeVO4 is studied at low temperatures. The H-T phase diagram is obtained for H parallel to b and H perpendicular to b. A tricritical point with coordinates H* = 3.2 +/- 0.1 T and T* = 3.0 +/- 0.1 K is found for H parallel to b. (C) 2015 AIP Publishing LLC