18 research outputs found
Atomic structure of grain boundaries in iron modeled using the atomic density function
A model based on the continuous atomic density function (ADF) approach is
applied to predict the atomic structure of grain boundaries (GBs) in iron.
Symmetrical [100] and [110] tilt GBs in bcc iron are modeled with the ADF
method and relaxed afterwards in molecular dynamics (MD) simulations. The shape
of the GB energy curve obtained in the ADF model reproduces well the
peculiarities of the angles of 70.53 deg. [ 3(112)] and 129.52 deg.
[ 11(332)] for [110] tilt GBs. The results of MD relaxation with an
embedded-atom method potential for iron confirm that the atomic GB
configurations obtained in ADF modeling are very close to equilibrium ones. The
developed model provides well-localized atomic positions for GBs of various
geometries.Comment: 8 pages, 8 figures, revised versio
Spin vortices and vacancies: interactions and pinning on a square lattice
The study gives a decisive answer to the recently risen question about the
type and origin of interaction between spin vortices and spin vacancies in 2D
spin models. The approach is based on the low-temperature approximation of the
2D XY model known as the Villain model and does not involve any additional
approximations, thus preserving the lattice structure. The exact form of the
Hamiltonian describing a system of topological charges and a vacant site
supports the attractive type of interaction between the vacancy and the
charges. The quantitative difference between the characteristics of the vortex
behavior in the 2D XY and Villain models due to the different energy of the
vortex "cores" in the two models is pointed out. This leads to a conclusion
that the interaction between a vortex and a spin vacancy and between a vortex
and the antivortex differs quantitatively for small separations in the two
mentioned models.Comment: 13 pages, 5 figure
Perturbation expansion for the diluted two-dimensional XY model
We study the quasi-long-range ordered phase of a 2D XY model with quenched
site-dilution using the spin-wave approximation and expansion in the parameter
which characterizes the deviation from completely homogeneous dilution. The
results, obtained by keeping the terms up to the third order in the expansion,
show good accordance with Monte Carlo data in a wide range of dilution
concentrations far enough from the percolation threshold. We discuss different
types of expansion.Comment: 8 pages, 1 eps figure, style file include
The 2D XY model on a finite lattice with structural disorder: quasi-long-range ordering under realistic conditions
We present an analytic approach to study concurrent influence of quenched
non-magnetic site-dilution and finiteness of the lattice on the 2D XY model.
Two significant deeply connected features of this spin model are: a special
type of ordering (quasi-long-range order) below a certain temperature and a
size-dependent mean value of magnetisation in the low-temperature phase that
goes to zero (according to the Mermin-Wagner-Hohenberg theorem) in the
thermodynamic limit. We focus our attention on the asymptotic behaviour of the
spin-spin correlation function and the probability distribution of
magnetisation. The analytic approach is based on the spin-wave approximation
valid for the low-temperature regime and an expansion in the parameters which
characterise the deviation from completely homogeneous configuration of
impurities. We further support the analytic considerations by Monte Carlo
simulations performed for different concentrations of impurities and compare
analytic and MC results. We present as the main quantitative result of the work
the exponent of the spin-spin correlation function power law decay. It is non
universal depending not only on temperature as in the pure model but also on
concentration of magnetic sites. This exponent characterises also the vanishing
of magnetisation with increasing lattice size.Comment: 13 pages, 7 eps figures, style files include
Interplay of topological and structural defects in the 2D XY model
The present work is devoted to the investigation of the interaction between
vortices (topological defects) and site-impurities (structural defects) in the
2D XY model and its influence on the well-known properties of the pure system.
The main goal is a theoretical description of the
Berezinskii-Kosterlitz-Thouless (BKT) temperature reduction by quenched
non-magnetic impurities, based on the vacancy-vortex interactions and the
vortex-pair dissociation mechanism of the transition. The non-magnetic impurity
interaction with a system of vortices can be found either from the
phenomenological theory of topological defects or from the Villain model. We
take both paths and compare the results obtained. Our prediction for the BKT
temperature reduction is confirmed by the available Monte Carlo data.Comment: 13 pages, 1 eps figure, style files include
Quasi-long-range ordering in a finite-size 2D Heisenberg model
We analyse the low-temperature behaviour of the Heisenberg model on a
two-dimensional lattice of finite size. Presence of a residual magnetisation in
a finite-size system enables us to use the spin wave approximation, which is
known to give reliable results for the XY model at low temperatures T. For the
system considered, we find that the spin-spin correlation function decays as
1/r^eta(T) for large separations r bringing about presence of a
quasi-long-range ordering. We give analytic estimates for the exponent eta(T)
in different regimes and support our findings by Monte Carlo simulations of the
model on lattices of different sizes at different temperatures.Comment: 9 pages, 3 postscript figs, style files include
Consistent model of magnetism in ferropnictides
The discovery of superconductivity in LaFeAsO introduced the ferropnictides
as a major new class of superconducting compounds with critical temperatures
second only to cuprates. The presence of magnetic iron makes ferropnictides
radically different from cuprates. Antiferromagnetism of the parent compounds
strongly suggests that superconductivity and magnetism are closely related.
However, the character of magnetic interactions and spin fluctuations in
ferropnictides, in spite of vigorous efforts, has until now resisted
understanding within any conventional model of magnetism. Here we show that the
most puzzling features can be naturally reconciled within a rather simple
effective spin model with biquadratic interactions, which is consistent with
electronic structure calculations. By going beyond the Heisenberg model, this
description explains numerous experimentally observed properties, including the
peculiarities of the spin wave spectrum, thin domain walls, crossover from
first to second order phase transition under doping in some compounds, and
offers new insight in the occurrence of the nematic phase above the
antiferromagnetic phase transition.Comment: 5 pages, 3 figures, revtex
First-Order Phase Transition with Breaking of Lattice Rotation Symmetry in Continuous-Spin Model on Triangular Lattice
Using a Monte Carlo method, we study the finite-temperature phase transition
in the two-dimensional classical Heisenberg model on a triangular lattice with
or without easy-plane anisotropy. The model takes account of competing
interactions: a ferromagnetic nearest-neighbor interaction and an
antiferromagnetic third nearest-neighbor interaction . As a result, the
ground state is a spiral spin configuration for . In this
structure, global spin rotation cannot compensate for the effect of 120-degree
lattice rotation, in contrast to the conventional 120-degree structure of the
nearest-neighbor interaction model. We find that this model exhibits a
first-order phase transition with breaking of the lattice rotation symmetry at
a finite temperature. The transition is characterized as a vortex
dissociation in the isotropic case, whereas it can be viewed as a vortex
dissociation in the anisotropic case. Remarkably, the latter is continuously
connected to the former as the magnitude of anisotropy decreases, in contrast
to the recent work by Misawa and Motome [J. Phys. Soc. Jpn. \textbf{79} (2010)
073001.] in which both the transitions were found to be continuous.Comment: 11pages, 16figures, accepted to JPS