Ants mediate the structure of phytotelm communities in an ant-garden bromeliad

The main theories explaining the biological diversity of rain forests often confer a limited understanding of the contribution of interspecific interactions to the observed patterns. We show how two-species mutualisms can affect much larger segments of the invertebrate community in tropical rain forests. Aechmea mertensii (Bromeliaceae) is both a phytotelm (plant-held water) and an ant-garden epiphyte. We studied the influence of its associated ant species (Pachycondyla goeldii and Camponotus femoratus) on the physical characteristics of the plants, and, subsequently, on the diversity of the invertebrate communities that inhabit their tanks. As dispersal agents for the bromeliads, P. goeldii and C. femoratus influence the shape and size of the bromeliad by determining the location of the seedling, from exposed to partially shaded areas. By coexisting on a local scale, the two ant species generate a gradient of habitat conditions in terms of available resources (space and food) for aquatic invertebrates, the diversity of the invertebrate communities increasing with greater volumes of water and fine detritus. Two-species mutualisms are widespread in nature, but their influence on the diversity of entire communities remains largely unexplored. Because macroinvertebrates constitute an important part of animal production in all ecosystem types, further investigations should address the functional implications of such indirect effects

Theory of interlayer exchange interactions in magnetic multilayers

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.Comment: 18 pages, 4 PS figures, LaTeX with IOP package; v2: ref. added. Further (p)reprints available from http://www.mpi-halle.de/~theory

Cryopreservation and long-term in vitro maintenance of second-stage larvae of Toxocara canis.

Second stage larvae of Toxocara canis were isolated from developed eggs, frozen in Eagle's Minimal Essential Medium with 5% dimethyl sulfoxide or 10% glycerol as cryoprotectants according to two cooling schedules and maintained in liquid nitrogen for 1 week. After thawing, the previously frozen larvae (FL) and unfrozen controls (CL) were maintained in a chemically defined medium in vitro for 35 weeks. While CL had motility rates around 95% to 97% throughout the experiment, previously frozen larvae (FL) exhibited rates of 48%-58% at the beginning and of 19%-39% at the end of the 35 week in vitro maintenance period. The surviving FL and CL larvae proved to be infective for mice. Excretory/secretory (ES) antigens isolated from several batches of culture medium in which FL and CL had been maintained reacted in the ELISA with human sera containing antibodies against Toxocara. Antigens from FL and CL separated by SDS-PAGE and silver-stained showed some differences in polypeptide patterns. Western-blot analysis revealed that these differences were not related to antigenic polypeptides but were most likely caused by substances without antigenic properties originating from dead and/or degenerating larvae. It can be concluded that ES antigens produced by previously frozen larvae are essentially the same as those derived from unfrozen controls. The value of cryopreservation of T. canis larvae for routine production of ES antigens will be further evaluated

Transformations of coordinates and Hamiltonian formalism in deformed Special Relativity

We investigate the transformation laws of coordinates in generalizations of special relativity with two observer-independent scales. The request of covariance leads to simple formulas if one assumes noncanonical Poisson brackets, corresponding to noncommuting spacetime coordinates.Comment: 11 pages, plain LaTe

Echoes in classical dynamical systems

Echoes arise when external manipulations to a system induce a reversal of its time evolution that leads to a more or less perfect recovery of the initial state. We discuss the accuracy with which a cloud of trajectories returns to the initial state in classical dynamical systems that are exposed to additive noise and small differences in the equations of motion for forward and backward evolution. The cases of integrable and chaotic motion and small or large noise are studied in some detail and many different dynamical laws are identified. Experimental tests in 2-d flows that show chaotic advection are proposed.Comment: to be published in J. Phys.

Charge Distributions in Metallic Alloys: a Charge Excess Functional theory approach

Charge Distributions in Metallic Alloys: a Charge Excess Functional theory approachComment: 13 pages, 5 figure

Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.Comment: 4 pages, 5 figures, to be published in Phys. Rev. Let

On the definition of velocity in doubly special relativity theories

We discuss the definition of particle velocity in doubly relativity theories. The general formula relating velocity and four-momentum of particle is given.Comment: 7 page

Twisted exchange interaction between localized spins embedded in a one- or two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in one- and two-dimensions in presence of a Rashba spin-orbit (SO) coupling. We show that rotation of the spin of conduction electrons due to SO coupling causes a twisted RKKY interaction between localized spins which consists of three different terms: Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. We also show that the effective spin Hamiltonian reduces to the usual RKKY interaction Hamiltonian in the twisted spin space where the spin quantization axis of one localized spin is rotated.Comment: 4pages, no figur

Magnetic Anisotropy of Isolated Cobalt Nanoplatelets

Motivated in part by experiments performed by M.H. Pan et al. (nanoletters, v.5, p 83, 2005), we have undertaken a theoretical study of the the magnetic properties of two-monolayer thick Co nanoplatelets with an equilateral triangular shape. The analysis is carried out using a microscopic Slater-Koster tight-binding model with atomic exchange and spin-orbit interactions designed to realistically capture the salient magnetic features of large nanoclusters containing up to 350 atoms. Two different truncations of the FCC lattice are studied, in which the nanoplatelet surface is aligned parallel to the FCC (111) and (001)crystal planes respectively. We find that the higher coordination number in the (111) truncated crystal is more likely to reproduce the perpendicular easy direction found in experiment. Qualitatively, the most important parameter governing the anisotropy of the model is found to be the value of the intra-atomic exchange integral J. If we set the value of J near the bulk value in order to reproduce the experimentally observed magnitude of the magnetic moments, we find both quasi-easy-planes and perpendicular easy directions. At larger values of J we find that the easy-axis of magnetization is perpendicular to the surface, and the value of the magnetic anisotropy energy per atom is larger. The possible role of hybridization with substrate surface states in the experimental systems is discussed.Comment: 15 pages, 13 figure