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1.
S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(3):285-289
We have studied the behavior of the energy gap of the 1D AF
spin-
XXZ model in a transverse magnetic field (h)
using the exact diagonalization technique. The ground state phase
diagram consists of two spin-flop and paramagnetic phases. Using a
modified finite-size scaling approach, we have computed the
critical exponent of the energy gap in the vicinity of the
critical transverse field hc(Δ). Our numerical results
confirm that the continuous phase transition from the spin-flop
phase to the paramagnetic one is in the universality class of the
Ising model in the transverse field (ITF). By applying conformal
estimates of a small perturbation (h≪1), we have also
justified our numerical results. 相似文献
2.
I. Titvinidze G.I. Japaridze 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(3):383-393
The quantum phase transition in the ground state of the extended spin S
= 1/2 XY model has been studied in detail. Using the exact solution of the model the low temperature thermodynamics, as well as the
ground state phase diagram of the model in the presence of applied uniform and/or staggered magnetic field are discussed.
Received 29 November 2002 / Received in final form 24 February 2003 Published online 11 April 2003
RID="a"
ID="a"e-mail: japa@iph.hepi.edu.ge 相似文献
3.
M. Arlego W. Brenig 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,53(2):193-198
Using analytical series expansion by continuous unitary
transformations we study the magnetic properties of a frustrated
tetrahedral spin-
chain. Starting from the limit of
isolated tetrahedra we analyze the evolution of the ground state
energy and the elementary triplet dispersion as a function of the
inter-tetrahedral coupling. The quantum phase diagram is evaluated
and is shown to incorporate a singlet product, a dimer, and a
Haldane phase. Comparison of our results with those from several
other techniques, such as density matrix renormalization group,
exact diagonalization, bond-operator theory and other numerical
series expansion are provided and convincing agreement is found. 相似文献
4.
G. Pawłowski 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,53(4):471-479
The extended Hubbard model in the atomic limit (AL-EHM) on a square lattice with periodic boundary conditions is studied with
use of the Monte Carlo (MC) method. Within the grand canonical ensemble the phase and order-order boundaries for charge orderings
are obtained. The phase diagrams include three types of charge ordered phases and the nonordered phase. The system exhibits
very rich structure and shows unusual multicritical behavior.
In the limiting case of tij=0, the EHM is equivalent to the pseudospin model with single-ion anisotropy
, exchange interaction W in an effective magnetic field
. This classical spin model is analyzed using the MC method for the canonical ensemble. The phase diagram is compared with
the known results for the Blume-Capel model. 相似文献
5.
P. F. Yu J. G. Cai J. M. Liu G. T. Shen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,44(1):151-158
We investigate the phase decoherence effects on
the entanglement of a two-qubit anisotropic Heisenberg
model
with a nonuniform magnetic field in the x–z-plane. As a
measure of the entanglement, the concurrence of the system is
calculated. It is shown that when the magnetic field is along the
z-axis, the nonuniform and uniform components of the field have
no influence on the entanglement for the cases of
and
, respectively. But when
the magnetic field is not along the z-axis, both the uniform and
the nonuniform components of the field will introduce the
decoherence effects. It is found that the effects of the
Heisenberg chain's anisotropy in the Z-direction on the
entanglement are dependent on the direction of the field.
Moreover, the larger the initial concurrence is, the higher value
it will exhibit during the time evolution of the system for a
proper set of the parameters ν, Δ, θ, γ
, B and b. 相似文献
6.
S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):371-376
The energy gap of the 1D AF-Heisenberg model in the presence of both uniform (H) and staggered (h) magnetic fields is investigated
using the exact
diagonalization technique. The opening of the gap in the presence of a
staggered field is found to scale with hν, where ν=ν(H) is the
critical exponent, and depends on the uniform field. With respect to the range
of the staggered magnetic field, two regimes are identified through which the
dependence of the real critical exponent ν(H) on H can be calculated
numerically. Our numerical results are in good agreement with the results
obtained by other theoretical approaches. 相似文献
7.
S. Alipour V. Karimipour L. Memarzadeh 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(2):159-169
We use the matrix product formalism to find exact ground states of
two new spin-1 quantum chains with nearest neighbor interactions.
One of the models, model I, describes a one-parameter family of
quantum chains for which the ground state can be found exactly. In
certain limit of the parameter, the Hamiltonian turns into the
interesting case
. The
other model which we label as model II, corresponds to a family of
solvable three-state vertex models on square lattices. The ground
state of this model is highly degenerate and the matrix product
states is a generating state of such degenerate states. The simple
structure of the matrix product state allows us to determine the
properties of degenerate states which are otherwise difficult to
determine. For both models we find exact expressions for
correlation functions. 相似文献
8.
The magnetic properties of an anti-ferromagnetic and ferrimagnetic mixed spin-1/2 and spin-5/2 Ising model with a crystal field in a longitudinal magnetic field on the honeycomb (z=3) and square lattice (z=4) are studied by using the effective-field theory with correlations. The ground state phase diagram of the model is obtained in the longitudinal magnetic field (h) and a single-ion potential or crystal-field interaction (Δ) plane. We also investigate the thermal variations of the sublattice and total magnetizations, and present the phase diagrams in the (Δ/|J|, ) plane. The phase diagrams have one, two or even three compensation temperatures depending on the values of the crystal-field interaction. Moreover, the susceptibility, internal energy and specific heat of the system are numerically examined, and some interesting phenomena in these quantities are found due to the applied longitudinal magnetic field. 相似文献
9.
M. Badehdah S. Bekhechi A. Benyoussef M. Touzani 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):431-440
A two dimensional antiferromagnetic spin-1 Ising model with negative next- nearest neighbour interaction (J
2
<0) and under an external magnetic field is investigated by two methods: The mean-field theory and Finite-Size-Scaling based
on transfer matrix (TMFSS) calculations. The ground state diagrams exhibit several new phases including frustrated ones. At
finite temperature we obtain by these two methods quite rich phase diagrams, with several multicritical points. While Mean
field approximation yields phase diagrams which are sometimes even qualitatively incorrect, accurate results are obtained
from transfer matrix finite size scaling calculations. For a certain range of interaction parameters, the model is shown to
violate the ordinary universality hypothesis.
Received: 3 November 1997 / Revised: 31 March 1998 / Accepted: 7 April 1998 相似文献
10.
11.
12.
The universal phase diagram of a 2D surface superconductor with generic Rashba interaction in a parallel magnetic field is found. In addition to the uniform BCS state, we find two inhomogeneous superconductive states, the stripe phase with Δ (r) ∝ cos(Qr) at high magnetic fields, and a new “helical” phase with Δ(r) ∝ exp(iQr) which intervenes between the BCS state and stripe phase at an intermediate magnetic field and temperature. We prove that the ground state for helical phase carries no current. 相似文献
13.
F. Capraro C. Gros 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):35-40
Using the density matrix renormalization group technique, we evaluate the low-energy spectrum (ground state and first excited
states) of the anisotropic antiferromagnetic spin-one-half chain under magnetic fields. We study both homogeneous longitudinal
and transversal fields as well as the influence of a transversal staggered field on the opening of a spin-gap. We find that
only a staggered transversal field opens a substantial gap.
Received 16 April 2002 / Received in final form 4 July 2002 Published online 17 September 2002 相似文献
14.
G. I. Japaridze S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,68(1):59-66
The ground-state magnetic phase diagram of a spin S=1/2 two-leg ladder with alternating rung exchange J⊥(n)=J⊥[1 + (-1)n δ] is studied using the analytical and numerical approaches. In the limit where the rung exchange is dominant, we have mapped
the model onto the effective quantum sine-Gordon model with topological term and identified two quantum phase transitions
at magnetization equal to the half of saturation value from a gapped to the gapless regime. These quantum transitions belong
to the universality class of the commensurate-incommensurate phase transition. We have also shown that the magnetization curve
of the system exhibits a plateau at magnetization equal to the half of the saturation value. We also present a detailed numerical
analysis of the low energy excitation spectrum and the ground state magnetic phase diagram of the ladder with rung-exchange
alternation using Lanczos method of numerical diagonalizations for ladders with number of sites up to N = 28. We have calculated
numerically the magnetic field dependence of the low-energy excitation spectrum, magnetization and the on-rung spin-spin correlation
function. We have also calculated the width of the magnetization plateau and show that it scales as δν, where critical exponent varies from ν = 0.87±0.01 in the case of a ladder with isotropic antiferromagnetic legs to ν = 1.82±0.01
in the case of ladder with ferromagnetic legs. Obtained numerical results are in an complete agreement with estimations made
within the continuum-limit approach. 相似文献
15.
A.S.T. Pires 《Solid State Communications》2004,129(5):297-298
We calculate the temperature dependent energy gap in the magnetic excitation spectrum of the one-dimensional spin 1/2 antiferromagnetic Heisenberg model in the presence of a staggered magnetic field, using the phase Hamiltonian, a self consistent harmonic approximation, and the thermal-Green function technique. 相似文献
16.
T. Sommer M. Vojta K.W. Becker 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(3):329-339
The two-layer square lattice quantum antiferromagnet with spins 12 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding
a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar
behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate
ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be
efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas
they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations
throughout the whole phase diagram.
Received 23 April 2001 相似文献
17.
We propose an extension of the spectroscopic Monte Carlo method to realistic effective interactions. The scheme is applied to the recently introduced GXPF1 interaction for fp nuclei for the ground state of 60Fe, 56Ni, 64Ni and 60Zn. The method hinges on the use of Hartree-Fock-Bogoliubov wave functions (properly projected before variation) and on a reformulation of the effective interaction so that it is a sum of negative squares of Hermitian one-quasi-particle operators, so the application of the Hubbard-Stratonovich transformation to the elementary propagator exp[-
] gives a functional integral over a Hermitian propagator. Limitations and difficulties encountered in the calculation are discussed. 相似文献
18.
We discuss the ground state phase transition between an antiferromagnet and a valence-bond solid in a two-dimensional spin-1/2 XY model with a four-spin interaction. This transition has been proposed as a candidate for a deconfined quantum-critical point. We analyze quantum Monte Carlo data in order to accurately characterize the transition. The central question that remains to be answered is whether the transition really is continuous, or whether it is actually weakly first-order. We present the current status of both ground state and finite-temperature calculations. Based on the results, we discuss possible scenarios for the transition, none of which is consistent with deconfined quantum-criticality. However, we argue that a deconfined quantum-critical point may be located nearby in an extended parameter space.We also discuss the staggered Ising phase obtaining in the limit of strong four-spin coupling. 相似文献
19.
The electronic structure and magnetic states for hexagonal-MnFeAs have been studied by a first-principles density functional theory (DFT) calculation. The ground state is ferromagnetic and the calculated magnetic moments for Fe and Mn are 1.1 and 3.1μB, respectively, leading to a total magnetization of 4.1μB per formula unit due to the small negative moments of As atoms. The exchange interaction between Fe and Mn layers () is positive and tends to form the ferromagnetic ordering. On the other hand, the exchange interaction at the Fe-As1 layer () is negative while that at the Mn-As2 layer () is positive. The field induced first order magnetic transition at TC is related to the competed exchange interaction in the compound. 相似文献
20.
We present Electron-Spin-Resonance experiments in the frequency range 35–420 GHz in magnetic fields up to 14 T and with temperatures down to 1.5 K. The quasi-onedimensionalS=1 Spin Systems NENP and NINO exhibit similar resonance-modes, which can be associated with transitions between excited states atq= and excitations from the groundstate. Groundstateexcitation can be explained with regard to a transverse staggered field due to inequivalent Ni2+-sites along the chains. Several features are discussed like polarisation-and temperature dependence of line-intensity and the angular and temperature-dependence of resonance-field. 相似文献