191 research outputs found
An introduced effective-field theory study of spin-1 transverse Ising model with crystal field anisotropy in a longitudinal magnetic field
A spin-1 transverse Ising model with longitudinal crystal field in a
longitudinal magnetic field is examined by introducing an effective field
approximation (IEFT) which includes the correlations between different spins
that emerge when expanding the identities. The effects of the crystal field as
well as the transverse and longitudinal magnetic fields on the thermal and
magnetic properties of the spin system are discussed in detail. The order
parameters, Helmholtz free energy and entropy curves are calculated numerically
as functions of the temperature and Hamiltonian parameters. A number of
interesting phenomena such as reentrant phenomena originating from the
temperature, crystal field, transverse and longitudinal magnetic fields have
been found.Comment: 11 pages, 15 figure
Heisenberg model in a random field: phase diagram and tricritical behavior
By using the differential operator technique and the effective field theory
scheme we study the tricritical behavior of Heisenberg classical model of
spin-1/2 in a random field. The phase diagram in the T-h plane on a square and
simple cubic lattice for a cluster with two spins is obtained when the random
field is bimodal distributed.Comment: 10 pages, 1 figur
Hysteretic response characteristics and dynamic phase transition via site dilution in the kinetic Ising model
The decay of the hysteresis loop area of the system, which is obeying a site
diluted kinetic Ising model, is considered by the disorder parameter using the
effective field theory analysis. The exhibition focuses on the understanding of
external field frequency, amplitude and the site concentration dependency of
the hysteresis loop area for several powerful treatments. Important
characteristics of the hysteretic response, such as frequency dispersion,
effect of domain nucleation phenomenon on the dynamic process etc. has been
introduced together with well known other characteristics. An attempt has been
made to explain the relations between the competing time scales (intrinsic
microscopic relaxation time of the system and the time period of the external
oscillatory field) and the shape of the response. As a result of the detailed
investigations, existence of essentially three, particularly four types of
dispersion curves have been propounded.Comment: 16 pages, 8 figure
Investigation of oscillation frequency and disorder induced dynamic phase transitions in a quenched-bond diluted Ising ferromagnet
Frequency evolutions of hysteresis loop area and hysteresis tools such as
remanence and coercivity of a kinetic Ising model in the presence of quenched
bond dilution are investigated in detail. The kinetic equation describing the
time dependence of the magnetization is derived by means of effective-field
theory with single-site correlations. It is found that the frequency
dispersions of hysteresis loop area, remanence and coercivity strongly depend
on the quenched bond randomness, as well as applied field amplitude and
oscillation frequency. In addition, the shape of the hysteresis curves for a
wide variety of Hamiltonian parameters is studied and some interesting
behaviors are found. Finally, a comparison of our observations with those of
recently published studies is represented and it is shown that there exists a
qualitatively good agreement.Comment: 11 pages, 8 figure
Stationary State Solutions of a Bond Diluted Kinetic Ising Model: An Effective-Field Theory Analysis
We have examined the stationary state solutions of a bond diluted kinetic
Ising model under a time dependent oscillating magnetic field within the
effective-field theory (EFT) for a honeycomb lattice . Time evolution of
the system has been modeled with a formalism of master equation. The effects of
the bond dilution, as well as the frequency and amplitude of
the external field on the dynamic phase diagrams have been discussed in detail.
We have found that the system exhibits the first order phase transition with a
dynamic tricritical point (DTCP) at low temperature and high amplitude regions,
in contrast to the previously published results for the pure case \cite{Ling}.
Bond dilution process on the kinetic Ising model gives rise to a number of
interesting and unusual phenomena such as reentrant phenomena and has a
tendency to destruct the first-order transitions and the DTCP. Moreover, we
have investigated the variation of the bond percolation threshold as functions
of the amplitude and frequency of the oscillating field.Comment: 8 pages, 4 figure
The extended Hubbard model in the ionic limit
In this paper, we study the Hubbard model with intersite Coulomb interaction
in the ionic limit (i.e. no kinetic energy). It is shown that this model is
isomorphic to the spin-1 Ising model in presence of a crystal field and an
external magnetic field. We show that for such models it is possible to find,
for any dimension, a finite complete set of eigenoperators and eigenvalues of
the Hamiltonian. Then, the hierarchy of the equations of motion closes and
analytical expressions for the relevant Green's functions and correlation
functions can be obtained. These expressions are formal because these functions
depend on a finite set of unknown parameters, and only a set of exact relations
among the correlation functions can be derived. In the one-dimensional case we
show that by means of algebraic constraints it is possible to obtain extra
equations which close the set and allow us to obtain a complete exact solution
of the model. The behavior of the relevant physical properties for the 1D
system is reported.Comment: 19 pages, 9 figures, 16 panel
Curie Temperatures for Three-Dimensional Binary Ising Ferromagnets
Using the Swendsen and Wang algorithm, high accuracy Monte Carlo simulations
were performed to study the concentration dependence of the Curie temperature
in binary, ferromagnetic Ising systems on the simple-cubic lattice. Our results
are in good agreement with known mean-field like approaches. Based on former
theoretical formulas we propose a new way of estimating the Curie temperature
of these systems.Comment: nr. of pages:13, LATEX. Version 2.09, Scientific Report :02/1994
(Univ. of Bergen, Norway), 7 figures upon reques
Critical behavior of 2 and 3 dimensional ferro- and antiferromagnetic spin ice systems in the framework of the Effective Field Renormalization Group technique
In this work we generalize and subsequently apply the Effective Field
Renormalization Group technique to the problem of ferro- and
antiferromagnetically coupled Ising spins with local anisotropy axes in
geometrically frustrated geometries (kagome and pyrochlore lattices). In this
framework, we calculate the various ground states of these systems and the
corresponding critical points. Excellent agreement is found with exact and
Monte Carlo results. The effects of frustration are discussed. As pointed out
by other authors, it turns out that the spin ice model can be exactly mapped to
the standard Ising model but with effective interactions of the opposite sign
to those in the original Hamiltonian. Therefore, the ferromagnetic spin ice is
frustrated, and does not order. Antiferromagnetic spin ice (in both 2 and 3
dimensions), is found to undergo a transition to a long range ordered state.
The thermal and magnetic critical exponents for this transition are calculated.
It is found that the thermal exponent is that of the Ising universality class,
whereas the magnetic critical exponent is different, as expected from the fact
that the Zeeman term has a different symmetry in these systems. In addition,
the recently introduced Generalized Constant Coupling method is also applied to
the calculation of the critical points and ground state configurations. Again,
a very good agreement is found with both exact, Monte Carlo, and
renormalization group calculations for the critical points. Incidentally, we
show that the generalized constant coupling approach can be regarded as the
lowest order limit of the EFRG technique, in which correlations outside a
frustrated unit are neglected, and scaling is substituted by strict equality of
the thermodynamic quantities.Comment: 28 pages, 9 figures, RevTeX 4 Some minor changes in the conclussions.
One reference adde
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