949 research outputs found
Magnetic polarons in Ca_(1-x)Y_xMnO_3
Experimental evidence show that in the magnetoresistive manganite Ca_(1-x)
Y_xMnO_3, ferromagnetic (FM) polarons arises in an antiferromagnetic (AF)
background, as a result of the doping with Yttrium. This hypothesis is
supported in this work by classical Monte Carlo (MC) calculations performed on
a model where FM Double Exchange (DE) and AF Superexhange (SE) compite.Comment: 3 pages, 3 figs, submitted to LAW3M conferenc
Double exchange model for RuSr_2(Eu,Gd)Cu_2O_8
We propose a double exchange model to describe the RuO_2 planes of
RuSr_2(Eu,Gd)Cu_2O_8. The Ru^+5 ions are described by localized spins, and
additional electrons provided by the superconducting CuO_2 planes are coupled
ferromagnetically to them by Hund rules coupling. We calculate the spin
structure factor, magnetic susceptibility and magnetization as a function of
magnetic field and temperature, using a Monte Carlo algorithm in which the
Ru^+5 spins are treated as classical. Several experiments which seemed in
contradiction with one another are explained by the theory.Comment: 3 pages, 3 figs., submitted to LAW3M conferenc
Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder
Using Monte Carlo simulations and the two eg-orbital model for manganites,
the stability of the CE and A-type antiferromagnetic insulating states is
analyzed when quenched disorder in the superexchange JAF between the t2g
localized spins and in the on-site energies is introduced. At vanishing or
small values of the electron-(Jahn-Teller)phonon coupling, the previously
hinted "fragility" of these insulating states is studied in detail, focusing on
their charge transport properties. This fragility is here found to induce a
rapid transition from the insulator to a (poor) metallic state upon the
introduction of disorder. A possible qualitative explanation is presented based
on the close proximity in energy of ferromagnetic metallic phases, and also on
percolative ideas valid at large disorder strength. The scenario is compared
with previously discussed insulator-to-metal transitions in other contexts. It
is argued that the effect unveiled here has unique properties that may define a
new class of giant effects in complex oxides. This particularly severe effect
of disorder must be present in other materials as well, in cases involving
phases that arise as a compromise between very different tendencies, as it
occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio
Time-dependent local Green's operator and its applications to manganites
An algorithm is presented to calculate the electronic local time-dependent
Green's operator for manganites-related hamiltonians. This algorithm is proved
to scale with the number of states in the Hilbert-space to the 1.55 power,
is able of parallel implementation, and outperforms computationally the Exact
Diagonalization (ED) method for clusters larger than 64 sites (using
parallelization). This method together with the Monte Carlo (MC) technique is
used to derive new results for the manganites phase diagram for the spatial
dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain
as a function of an insulating parameter, the sequence of ground states given
by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and
AF-type G, which are in remarkable agreement with experimental results.Comment: 9 pages, 11 figure
Food, microbes, sex and old age: on the plasticity of gastrointestinal innervation
The gastrointestinal tract is innervated by its own enteric nervous system and by extrinsic neurons that connect it with the central nervous system. Innervation allows the gastrointestinal tract to sense and respond to diverse stimuli, adjusting motility and secretion, but also affecting our physiology, behaviour and immunity. The mechanisms underlying the formation of gastrointestinal neurons are beginning to be elucidated; those that keep them plastic over an organism's lifetime remain to be explored. Here, we review the effects of microbiota, nutrients, sex and ageing on the morphology and function of gastrointestinal innervation in mammals, and discuss how this plasticity shapes gut-brain crosstalk and whole-body physiology. We also highlight insights gained by nascent studies of the enteric innervation of Drosophila melanogaster
Phase diagram for Ca_{1-x}Y_xMnO_3 type crystals
We present a simple model to study the electron doped manganese perovskites.
The model considers the competition between double exchange mechanism for
itinerant electrons and antiferromagnetic superexchange interaction for
localized electrons. It represents each Mn^{4+} ion by a spin 1/2, on which an
electron can be added to produce Mn^{3+}; we include a hopping energy t, a
strong intratomic interaction exchange J (in the limit J/t>>1), and an
interatomic antiferromagnetic interaction K between the local spins. Using the
Renormalized Perturbation Expansion and a Mean Field Approximation on the
hopping terms and on the superexchange interaction we calculate the free
energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic,
and novel spin glass phases can be determined as functions of the parameters
characterizing the system. The model results can be expressed in terms of t and
K for each value of the doping x in phase diagrams. The magnetization m and
canting angle can also be calculated as fuctions of temperature for fixed
values of doping and model parameters.Comment: 4 figure
- …