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1.
S. Duhot R. Mélin 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(2):257-266
The three-dimensional bimodal random-field Ising model
is investigated using the N-fold version of the Wang-Landau
algorithm. The essential energy subspaces are determined by the
recently developed critical minimum energy subspace technique, and
two implementations of this scheme are utilized. The random fields
are obtained from a bimodal discrete (±Δ) distribution,
and we study the model for various values of the disorder strength
Δ, Δ=0.5,1,1.5 and 2, on cubic lattices with
linear sizes L=4–24. We extract information for the probability
distributions of the specific heat peaks over samples of random
fields. This permits us to obtain the phase diagram and present
the finite-size behavior of the specific heat. The question of
saturation of the specific heat is re-examined and it is shown
that the open problem of universality for the random-field Ising
model is strongly influenced by the lack of self-averaging of the
model. This property appears to be substantially depended on the
disorder strength. 相似文献
2.
N. G. Fytas A. Malakis 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(1):111-120
The one-parametric Wang-Landau (WL) method is
implemented together with an extrapolation scheme to yield
approximations of the two-dimensional (exchange-energy,
field-energy) density of states (DOS) of the 3D bimodal
random-field Ising model (RFIM). The present approach generalizes
our earlier WL implementations, by handling the final stage of the
WL process as an entropic sampling scheme, appropriate for the
recording of the required two-parametric histograms. We test the
accuracy of the proposed extrapolation scheme and then apply it to
study the size-shift behavior of the phase diagram of the 3D
bimodal RFIM. We present a finite-size converging approach and a
well-behaved sequence of estimates for the critical disorder
strength. Their asymptotic shift-behavior yields the critical
disorder strength and the associated correlation length's
exponent, in agreement with previous estimates from ground-state
studies of the model. 相似文献
3.
A.K. Hartmann U. Nowak 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(1):105-109
We investigate the critical behavior of three-dimensional random-field Ising systems with both Gauss and bimodal distribution
of random fields and additional the three-dimensional diluted Ising antiferromagnet in an external field. These models are
expected to be in the same universality class. We use exact ground-state calculations with an integer optimization algorithm
and by a finite-size scaling analysis we calculate the critical exponents , , and . While the random-field model with Gauss distribution of random fields and the diluted antiferromagnet appear to be in same
universality class, the critical exponents of the random-field model with bimodal distribution of random fields seem to be
significantly different.
Received: 9 July 1998 / Received in final form: 15 July 1998 / Accepted: 20 July
1998 相似文献
4.
The influence of random site dilution on the critical properties of the two-dimensional Ising model on a square lattice was explored by Monte Carlo simulations with the Wang-Landau sampling. The lattice linear size was L=20-120 and the concentration of diluted sites q=0.1,0.2,0.3. Its pure version displays a second-order phase transition with a vanishing specific heat critical exponent α, thus, the Harris criterion is inconclusive, in that disorder is a relevant or irrelevant perturbation for the critical behaviour of the pure system. The main effort was focused on the specific heat and magnetic susceptibility. We have also looked at the probability distribution of susceptibility, pseudocritical temperatures and specific heat for assessing self-averaging. The study was carried out in appropriate restricted but dominant energy subspaces. By applying the finite-size scaling analysis, the correlation length exponent ν was found to be greater than one, whereas the ratio of the critical exponents (α/ν) is negative and (γ/ν) retains its pure Ising model value supporting weak universality. 相似文献
5.
The random field Ising model is studied numerically at both zero and positive temperature. Ground states are mapped out in a region of random field and external field strength. Thermal states and thermodynamic properties are obtained for all temperatures using the Wang-Landau algorithm. The specific heat and susceptibility typically display sharp peaks in the critical region for large systems and strong disorder. These sharp peaks result from large domains flipping. For a given realization of disorder, ground states and thermal states near the critical line are found to be strongly correlated--a concrete manifestation of the zero temperature fixed point scenario. 相似文献
6.
The density of states of long-range Blume-Emery-Griffiths (BEG) and short-range Ising models are obtained by using Wang-Landau sampling with adaptive windows in energy and magnetization space. With accurate density of states, we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al. in the regions of positive and negative temperature. The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane. However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart. 相似文献
7.
The density of states of long-range Blume-Emery-Griffiths(BEG) and short-range Ising models are obtained by using Wang-Landau sampling with adaptive windows in energy and magnetization space.With accurate density of states,we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al.in the regions of positive and negative temperature.The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane.However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart. 相似文献
8.
G.L.S. Paula W. Figueiredo 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):519-522
The stationary states of the random-field Ising model are determined through the master equation approach, where the contact
with the heat bath is simulated by the Glauber stochastic dynamics. The phase diagram of the model is constructed from the
stationary values of the magnetization as a function of temperature and field amplitude. The continuous phase transitions
coincide with the equilibrium ones, while the first-order transitions occur at fields larger than the corresponding values
at equilibrium. The difference between the fields at the limit of stability of the ordered phase and that of the equilibrium
is maximum at zero temperature and vanishes at the tricritical point. We also find the mean field time auto-correlation function
at the stationary states of the model.
Received: 4 June 1997 / Revised: 5 August 1997 / Accepted: 10 November 1997 相似文献
9.
Mixed crystals of betaine, phosphite and betaine phosphate have been investigated using broadband di-electric spectroscopy
at frequencies 10−1Hz≤ν≤109Hz and temperatures 1.5 K≤T≤300 K for several betaine phosphate concentrationsx. For 0.2≤x≤0.65 an orientational glass state is found at low temperatures. The broad susceptibility spectra were analyzed using the
concepts of distributions of relaxation times and of distributions of energy barriers. A critical comparison of the different
approaches is given. In the mixed crystals that show antiferroelectric order at low temperatures, charge transport phenomena
are studied. The static permittivity of the proton glass-forming crystals is analyzed, in terms of effectively one- and three-dimensional
Ising models that incorporate random fields and random bonds. 相似文献
10.
Effect of heavy cation doping (Ca2+ at the A-site) on the thermal properties of perovskite LaMnO3 has been investigated using the Rigid Ion Model (RIM)). As strong electron-phonon interactions are present in these compounds,
the lattice part of the specific heat deserves proper attention. The specific heat of magnetoresistance compound La0.25Ca0.75MnO3 as a function of temperature (10 K ≤ T ≤ 300 K) is reported. Our results on specific heat are in good agreement with the measured values of specific heat at lower
temperatures. In addition, the results on the cohesive energy (ϕ), molecular force constant (f), Restrahalen frequency (ν
0), Debye temperature (Θ
D) and Gruneisen parameter (γ) are also discussed.
相似文献
11.
N. G. Fytas A. Malakis 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,79(1):13-20
We investigate, by means of extensive Monte Carlo
simulations, the magnetic critical behavior of the
three-dimensional bimodal random-field Ising model at the strong
disorder regime. We present results in favor of the two-exponent
scaling scenario, [`(h)]\bar{\eta} = 2η, where η and
[`(h)]\bar{\eta} are the critical exponents describing the power-law
decay of the connected and disconnected correlation functions and
we illustrate, using various finite-size measures and properly
defined noise to signal ratios, the strong violation of
self-averaging of the model in the ordered phase. 相似文献
12.
B. Lobe W. Janke K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):283-291
We analyze recently extended high-temperature series expansions for the “Edwards-Anderson” spin-glass susceptibility of the
p-state Potts glass model on d-dimensional hypercubic lattices for the case of a symmetric bimodal distribution of ferro- and antiferromagnetic nearest-neighbor
couplings . In these star-graph expansions up to order 22 in the inverse temperature , the number of Potts states p and the dimension d are kept as free parameters which can take any value. By applying several series analysis techniques to the new series expansions,
this enabled us to determine the critical coupling Kc and the critical exponent of the spin-glass susceptibility in a large region of the two-dimensional (p,d)-parameter space. We discuss the thus obtained information with emphasis on the lower and upper critical dimensions of the
model and present a careful comparison with previous estimates for special values of p and d.
Received: 25 May 1998 / Revised and Accepted: 11 August 1998 相似文献
13.
A modified model of preequilibrium decay is considered. This model makes it possible to calculate cross sections for multiparticle
photonucleon reactions with allowance for the impact of collective isospin effects. Features of the photodisintegration of
101 ≤ A ≤ 135 tin isotopes in the energy range 2 ≤ E
γ
≤ 140 MeV are studied on the basis of this model. 相似文献
14.
We simulated the van Hemmen spin glass model by multicanonical algorithm. The exact results for this mean-field model are reproduced. Physical quantities such as energy density, specific heat, susceptibility and order parameters are evaluated at all temperatures. We also studied an alternate model with short range interactions, which displays the many-valley picture in 2D for random variables having values ±1.On leave of the absence from Department of Physics Engineering, Hacettepe University, Ankara, Turkey. 相似文献
15.
S. Garcia L. Ghivelder S. Soriano I. Felner 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):307-313
The magnetic properties of the superconducting ruthenate-cuprate RuSr2Eu
1.4Ce0.6Cu2O10-δ (Ru-1222) have been studied by a
scaling analysis of the dc magnetization and ac susceptibility measurements.
Non-linear M(H) curves reveal the presence of nano-size clusters with a
net magnetic moment of ∼102 μB at 180 K, near the
deviation from a Curie-Weiss behavior. On cooling, no scaling was observed
down to 90 K, discarding the possibility of magnetic phase separation of
collinear ferromagnetic particles. We explain this result in terms of a
variable number of contributing particles, with a temperature dependent net
magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the
emergence of a system of non-interacting particles, which couple on further
cooling. The observed cluster-glass features are preserved down to the
lowest measured temperature (10 K); no signature of long-range order was
detected. The frequency shift of the peak in the real part of the ac
susceptibility does not follow the Vogel-Fulcher type dependence, as
previously reported. The puzzling temperature dependence of the coercive
field, HC(T), is correlated with the changes in the scaling factors. 相似文献
16.
Microscopic calculations of weak interaction rates of nuclei in stellar environment for A = 18 to 100 总被引:2,自引:0,他引:2
J.-U. Nabi H.V. Klapdor-Kleingrothaus 《The European Physical Journal A - Hadrons and Nuclei》1999,5(3):337-339
We report here the microscopic calculation of weak interaction rates in stellar matter for 709 nuclei with A = 18 to 100
using a generalized form of proton-neutron quasiparticle RPA model with separable Gamow-Teller forces. This is the first ever
extensive microscopic calculation of weak rates calculated over a wide temperature-density grid which includes 107≤ T(K) ≤ 30 × 109 and 10 ≤ρ Ye (gcm−3) ≤ 1011, and over a larger mass range. Particle emission processes from excited states, previously ignored, are taken into account,
and are found to significantly affect some β decay rates. The calculated capture and decay rates take into consideration the
latest experimental energy levels and ft value compilations. Our calculation of electron capture and β-decay rates, in the
fp-shell, show considerable differences with a recently reported shell model diagonalization approach calculation.
Received: 16 April 1999 相似文献
17.
Jonas Johansson 《Physics letters. A》2008,372(42):6301-6304
Finite size effects on the phase transition in the 2D Potts model with open boundary conditions are studied with Wang-Landau Monte Carlo simulations. We show the lattice size dependent cross-over from first order to continuous phase transition and discuss it in terms of surface induced disorder and size dependence of the latent heat. 相似文献
18.
Suman Sinha 《Physics letters. A》2009,373(3):308-314
Performance of Wang-Landau (WL) algorithm in two continuous spin models is tested by determining the fluctuations in energy histogram. Finite size scaling is performed on a modified XY-model using different WL sampling schemes. Difficulties faced in simulating relatively large continuous systems using WL algorithm are discussed. 相似文献
19.
The phenomenological amplitude for the reaction πN→ππN fixed by fittings to the experimental data in the energy region 0.300 ≤P
Lab≤ 500 MeV/c is used for modelling the Chew–Low extrapolation and Olsson–Turner threshold approach. It is shown that the uncritical
application of the former results in enermous theoretical errors, the extracted values being in fact random numbers. The results
of the Olsson–Turner method are characterized by significant systematic errors coming from unknown details of the isobar physics.
Received: 10 December 1997 相似文献
20.
Y.Q. Ma W. Figueiredo 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):103-106
By introducing boson operators, a quantum spherical XY model in the presence of a random field has been studied by the coherent state path integral approach. The phase diagram
is obtained, and the effects of the random-field fluctuations on the possibilities of the existence of a ferromagnetic phase are discussed. At the critical point, , the order parameter M describing the ordered ferromagnetic phase disappears as .Since the model is equivalent to a Bose system, we also show that the phase transition at zero temperature between the superfluid
and the disordered Mott insulator phases occurs at the chemical potential , where J0 is the strength of the exchange interaction. As the temperature T goes to zero, the asymptotic behavior of the entropy and the specific heat are and , respectively.
Received: 20 May 1997 / Accepted: 20 October 1997 相似文献