2,813 research outputs found
Vortex-induced topological transition of the bilinear-biquadratic Heisenberg antiferromagnet on the triangular lattice
The ordering of the classical Heisenberg antiferromagnet on the triangular
lattice with the the bilinear-biquadratic interaction is studied by Monte Carlo
simulations. It is shown that the model exhibits a topological phase transition
at a finite-temperature driven by topologically stable vortices, while the spin
correlation length remains finite even at and below the transition point. The
relevant vortices could be of three different types, depending on the value of
the biquadratic coupling. Implications to recent experiments on the triangular
antiferromagnet NiGaS is discussed
Monte Carlo Simulation of the Heisenberg Antiferromagnet on a Triangular Lattice: Topological Excitations
We have simulated the classical Heisenberg antiferromagnet on a triangular
lattice using a local Monte Carlo algorithm. The behavior of the correlation
length , the susceptibility at the ordering wavevector , and
the spin stiffness clearly reflects the existence of two temperature
regimes -- a high temperature regime , in which the disordering
effect of vortices is dominant, and a low temperature regime ,
where correlations are controlled by small amplitude spin fluctuations. As has
previously been shown, in the last regime, the behavior of the above quantities
agrees well with the predictions of a renormalization group treatment of the
appropriate nonlinear sigma model. For , a satisfactory fit of the
data is achieved, if the temperature dependence of and is
assumed to be of the form predicted by the Kosterlitz--Thouless theory.
Surprisingly, the crossover between the two regimes appears to happen in a very
narrow temperature interval around .Comment: 13 pages, 8 Postscript figure
First-Order Transition to Incommensurate Phase with Broken Lattice Rotation Symmetry in Frustrated Heisenberg Model
We study a finite-temperature phase transition in the two-dimensional
classical Heisenberg model on a triangular lattice with a ferromagnetic
nearest-neighbor interaction and an antiferromagnetic
third-nearest-neighbor interaction using a Monte Carlo method. Apart from
a trivial degeneracy corresponding to O(3) spin rotations,the ground state for
has a threefold degeneracy corresponding to 120 degree lattice
rotations. We find that this model exhibits a first-order phase transition with
the breaking of the threefold symmetry when the interaction ratio is
.Comment: 4pages,5figure
Micromagnetic simulations of interacting dipoles on a fcc lattice: Application to nanoparticle assemblies
Micromagnetic simulations are used to examine the effects of cubic and axial
anisotropy, magnetostatic interactions and temperature on M-H loops for a
collection of magnetic dipoles on fcc and sc lattices. We employ a simple model
of interacting dipoles that represent single-domain particles in an attempt to
explain recent experimental data on ordered arrays of magnetoferritin
nanoparticles that demonstrate the crucial role of interactions between
particles in a fcc lattice. Significant agreement between the simulation and
experimental results is achieved, and the impact of intra-particle degrees of
freedom and surface effects on thermal fluctuations are investigated.Comment: 10 pages, 9 figure
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
Periodic features in the Dynamic Structure Factor of the Quasiperiodic Period-doubling Lattice
We present an exact real-space renormalization group (RSRG) method for
evaluating the dynamic structure factor of an infinite one-dimensional
quasiperiodic period-doubling (PD) lattice. We observe that for every normal
mode frequency of the chain, the dynamic structure factor always
exhibits periodicity with respect to the wave vector and the presence of
such periodicity even in absence of translational invariance in the system is
quite surprising. Our analysis shows that this periodicity in
actually indicates the presence of delocalized phonon modes in the PD chain.
The Brillouin Zones of the lattice are found to have a hierarchical structure
and the dispersion relation gives both the acoustic as well as optical
branches. The phonon dispersion curves have a nested structure and we have
shown that it is actually the superposition of the dispersion curves of an
infinite set of periodic lattices.Comment: 9 pages, 3 postscript figures, REVTeX, To appear in Phys. Rev. B (1
February 1998-I
Nonequilibrium relaxation study of the anisotropic antiferromagnetic Heisenberg model on the triangular lattice
Effect of exchange anisotropy on the relaxation time of spin and vector
chirality is studied for the antiferromagnetic classical Heisenberg model on
the triangular lattice by using the nonequilibrium relaxation Monte Carlo
method. We identify the Berezinskii-Kosterlitz-Thouless (BKT) transition and
the chiral transition in a wide range of the anisotropy, even for very small
anisotropy of 0.01%. As the anisotropy decreases, both the critical
temperatures steeply decrease, while the BKT critical region becomes
divergently wide. We elucidate a sharp "V shape" of the phase diagram around
the isotropic Heisenberg point, which suggests that the isotropic case is
exceptionally singular and the associated Z vortex transition will be isolated
from the BKT and chiral transitions. We discuss the relevance of our results to
peculiar behavior of the spin relaxation time observed experimentally in
triangular antiferromagnets.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
From one cell to the whole froth: a dynamical map
We investigate two and three-dimensional shell-structured-inflatable froths,
which can be constructed by a recursion procedure adding successive layers of
cells around a germ cell. We prove that any froth can be reduced into a system
of concentric shells. There is only a restricted set of local configurations
for which the recursive inflation transformation is not applicable. These
configurations are inclusions between successive layers and can be treated as
vertices and edges decorations of a shell-structure-inflatable skeleton. The
recursion procedure is described by a logistic map, which provides a natural
classification into Euclidean, hyperbolic and elliptic froths. Froths tiling
manifolds with different curvature can be classified simply by distinguishing
between those with a bounded or unbounded number of elements per shell, without
any a-priori knowledge on their curvature. A new result, associated with
maximal orientational entropy, is obtained on topological properties of natural
cellular systems. The topological characteristics of all experimentally known
tetrahedrally close-packed structures are retrieved.Comment: 20 Pages Tex, 11 Postscript figures, 1 Postscript tabl
The influence of critical behavior on the spin glass phase
We have argued in recent papers that Monte Carlo results for the equilibrium
properties of the Edwards-Anderson spin glass in three dimensions, which had
been interpreted earlier as providing evidence for replica symmetry breaking,
can be explained quite simply within the droplet model once finite size effects
and proximity to the critical point are taken into account. In this paper, we
show that similar considerations are sufficient to explain the Monte Carlo data
in four dimensions. In particular, we study the Parisi overlap and the link
overlap for the four-dimensional Ising spin glass in the Migdal-Kadanoff
approximation. Similar to what is seen in three dimensions, we find that
temperatures well below those studied in Monte Carlo simulations have to be
reached before the droplet model predictions become apparent. We also show that
the double-peak structure of the link overlap distribution function is related
to the difference between domain-wall excitations that cross the entire system
and droplet excitations that are confined to a smaller region.Comment: 8 pages, 8 figure
Genomic Organization, Splice Variants and Expression of CGMl, a CD66-related Member of the Carcinoembryonic Antigen Gene Family
The tumor marker carcinoembryonic antigen (CEA) belongs to a family of proteins which are composed of one immunogiobulin variable domain and a varying number of immunoglobulin constant-like domains. Most of the membrane-bound members, which are anchored either by a glycosylphosphatidylinositol moiety or a transmembrane domain, have been shown to convey cell adhesion in vitro. Here we describe two splice variants of CGMI. a transmembrane member of the CEA family without immunoglobulin constant.like domains. CGM1a and CGM1c contain cytopiasmic domains of 71 and 31 amino acids, respectively, The cytoplasmic region of CGM1a is encoded by four exons (Cyt1-Cyt4). Differential splicing of the Cyt1 exon (53 bp)..
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