共查询到20条相似文献,搜索用时 15 毫秒
1.
B. Drossel H. Bokil M.A. Moore A.J. Bray 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):369-375
We study the link overlap between two replicas of an Ising spin glass in three dimensions using the Migdal-Kadanoff approximation
and scaling arguments based on the droplet picture. For moderate system sizes, the distribution of the link overlap shows
the asymmetric shape and large sample-to-sample variations found in Monte-Carlo simulations and usually attributed to replica
symmetry breaking. However, the scaling of the width of the distribution, and the link overlap in the presence of a weak coupling
between the two replicas are in agreement with the droplet picture. We also discuss why it is impossible to see the asymptotic
droplet-like behaviour for moderate system sizes and temperatures not too far below the critical temperature.
Received 25 May 1999 相似文献
2.
A. Montanari F. Ricci-Tersenghi 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(3):339-346
The low-temperature phase of discontinuous mean-field spin glasses is generally described by a one-step replica symmetry breaking
(1RSB) ansatz. The Gardner transition, i.e. a very-low-temperature phase transition to a full replica symmetry breaking (FRSB) phase, is often regarded as an inessential, and
somehow exotic phenomenon. In this paper we show that the metastable states which are relevant for the out-of-equilibrium
dynamics of such systems are always in a FRSB phase. The only exceptions are (to the best of our knowledge) the p-spin spherical model and the random energy model (REM). We also discuss the consequences of our results for aging dynamics
and for local search algorithms in hard combinatorial problems.
Received 10 February 2003 Published online 20 June 2003
RID="a"
ID="a"e-mail: Federico.Ricci@roma1.infn.it
RID="b"
ID="b"UMR 8549, Unité Mixte de Recherche du Centre National de la Recherche Scientifique et de l' école Normale Supérieure 相似文献
3.
R. Florian S. Galam 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):189-194
Coalition setting among a set of actors (countries, firms, individuals) is studied using concepts from the theory of spin
glasses. Given the distribution of respective bilateral propensities to either cooperation or conflict, the phenomenon of
local aggregation is modeled. In particular the number of coalitions is determined according to a minimum conflict principle.
It is found not to be always two. Along these lines, previous studies are revisited and are found not to be consistent with
their own principles. The model is then used to describe the fragmentation of former Yugoslavia. Results are compared to the
actual situation.
Received 23 February 2000 相似文献
4.
K.S.D. Beach R.J. Gooding 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(4):579-591
We examine the role of spin twists in the formation of domain walls, often called stripes, by focusing on the spin textures
found in the cluster spin glass phases of and . To this end, we derive improved analytic expressions for the spin distortions produced by a frustrating bond, both near
the core region of the bond and in the far field, and then derive an improved expression for interaction energies between
such bonds. We critique our analytical theory by comparison to numerical solutions of this problem and find excellent agreement.
By looking at collections of small numbers of such bonds localized in some region of a lattice, we demonstrate the stability
of small “clusters” of spins, each cluster having its own orientation of its antiferromagnetic order parameter. Then, we display
a domain wall corresponding to spin twists between clusters of locally ordered spins showing how spin twists can serve as
a mechanism for stripe formation. Since the charges are localized in this model, we emphasize that these domain walls are
produced in a situation for which no kinetic energy is present in the problem.
Received 10 January 2000 相似文献
5.
S. Miyashita E. Vincent 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(2):203-211
Aging in spin glasses (and in some other systems) reveals astonishing effects of `rejuvenation and memory' upon temperature
changes. In this paper, we propose microscopic mechanisms (at the scale of spin-spin interactions) which can be at the origin
of such phenomena. Firstly, we recall that, in a frustrated system, the effective average interaction between two spins may take different values (possibly with opposite signs) at different temperatures. We give simple examples
of such situations, which we compute exactly. Such mechanisms can explain why new ordering processes (rejuvenation) seem to take place in spin glasses when the temperature is lowered. Secondly, we emphasize the fact that inhomogeneous interactions do naturally lead to a wide distribution of relaxation times for thermally activated flips. `Memory spots' spontaneously
appear, in the sense that the flipping time of some spin clusters becomes extremely long when the temperature is decreased.
Such memory spots are capable of keeping the memory of previous ordering at a higher temperature while new ordering processes occur at a lower temperature. After a qualitative
discussion of these mechanisms, we show in the numerical simulation of a simplified example that this may indeed work. Our
conclusion is that certain chaos-like phenomena may show up spontaneously in any frustrated and inhomogeneous magnetic system, without impeding the occurrence of memory effects.
Received 5 February 2001 and Received in final form 27 April 2001 相似文献
6.
M. Reuhl P. Nielaba K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):225-232
,Static and dynamic properties of the Potts model on the simple cubic lattice with nearest neighbor -interaction are obtained from Monte Carlo simulations in a temperature range where full thermal equilibrium still can be
achieved (). For a lattice size L
= 16, in this range finite size effects are still negligible, but the data for the spin glass susceptibility agree with previous
extrapolations based on finite size scaling of very small lattices. While the static properties are compatible with a zero
temperature transition, they certainly do not prove it. Unlike the Ising spin glass, the decay of the time-dependent order
parameter is compatible with a simple Kohlrausch function, , while a power law prefactor cannot be distinguished. The Kohlrausch exponent y
(
T
) decreases from at [0pt] to at [0pt] however. The relaxation time is compatible with the exponential divergence postulated by McMillan for spin glasses at their lower critical dimension,
but the exponent that can be extracted still differs significantly from the theoretical value, . Thus the present results support the conclusion that the Potts spin glass in d
= 3 dimensions differs qualitatively from the Ising spin glass.
Received: 8 October 1997 / Accepted: 27 November 1997 相似文献
7.
H. Rieger R. Juhász F. Iglói 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):409-412
We study random XY and (dimerized) XX spin-1/2 quantum spin chains at their quantum phase transition driven by the anisotropy and dimerization, respectively. Using
exact expressions for magnetization, correlation functions and energy gap, obtained by the free fermion technique, the critical
and off-critical (Griffiths-McCoy) singularities are related to persistence properties of random walks. In this way we determine
exactly the decay exponents for surface and bulk transverse and longitudinal correlations, correlation length exponent and
dynamical exponent.
Received 26 September 1999 相似文献
8.
T.M. Nieuwenhuizen C.N.A. van Duin 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):191-209
In realistic spinglasses, such as , and , magnetic atoms are located at random positions. Their couplings are determined by their relative positions. For such systems
a field theory is formulated. In certain limits it reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the
Viana-Bray model. The model has a percolation transition, while for RKKY couplings the “concentration scaling” occurs. Within the Gaussian approximation the Ginzburg-Landau expansion is considered in the clusterglass phase, that is
to say, for not too small concentrations. Near special points, the prefactor of the cubic term, or the one of the replica-symmetry-breaking
quartic term, may go through zero. Around such points new spin glass phases are found.
Received: 27 April 1998 / Received in final form: 27 July 1998 / Accepted: 13 August 1998 相似文献
9.
M. Patra K. De S. Majumdar S. Giri 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(4):367-371
We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field
in case of 30% of Fe substitution in LaMnO3. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field
dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase
of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift
decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility
measurements show a sharp peak (Tp) at 51 K and a shoulder (Tf) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process,
which is in consistent with the cluster-glass compound. 相似文献
10.
R.L. Leheny Y.S. Lee G. Shirane R.J. Birgeneau 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(3):287-290
Inelastic neutron scattering with high wave-vector resolution has characterized the propagation of transverse spin wave modes
near the antiferromagnetic zone center in the metastable domain state of a random field Ising magnet. A well-defined, long
wavelength excitation is observed despite the absence of long-range magnetic order. Direct comparisons with the spin wave
dispersion in the long-range ordered antiferromagnetic state reveal no measurable effects from the domain structure. This
result recalls analogous behavior in thermally disordered anisotropic spin chains but contrasts sharply with that of the phonon
modes in relaxor ferroelectrics.
Received 2 November 2002 / Received in final form 4 February 2003 Published online 11 April 2003
RID="a"
ID="a"leheny@pha.jhu.edu 相似文献
11.
N. G. Fytas A. Malakis 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(1-2):39-43
The three-dimensional bimodal random-field Ising model is studied
via a new finite temperature numerical approach. The methods of
Wang-Landau sampling and broad histogram are implemented in a
unified algorithm by using the N-fold version of the Wang-Landau
algorithm. The simulations are performed in dominant energy
subspaces, determined by the recently developed critical minimum
energy subspace technique. The random-fields are obtained from a
bimodal distribution, that is we consider the discrete
(±Δ) case and the model is studied on cubic lattices with
sizes 4≤L ≤20. In order to extract information for the
relevant probability distributions of the specific heat and
susceptibility peaks, large samples of random-field realizations
are generated. The general aspects of the model's scaling behavior
are discussed and the process of averaging finite-size anomalies
in random systems is re-examined under the prism of the lack of
self-averaging of the specific heat and susceptibility of the
model. 相似文献
12.
F. Mila D. Dean 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):301-306
Motivated by the observation of a spin-glass transition in almost disorder-free Kagome antiferromagnets, and by the specific
form of the effective low-energy model of the S = 1/2, trimerized Kagome antiferromagnet, we investigate the possibility to obtain a spin-glass behavior in two-component,
disorder-free models. We concentrate on a toy-model, a modified Ashkin-Teller model in a magnetic field that couples only
to one species of spins, for which we prove that a dynamic spin-glass behavior occurs. The dynamics of the magnetization is
closely related to that of the underlying Ising model in zero field in which spins and pseudo-spins are intimately coupled.
The spin-glass like history dependence of the magnetization is a consequence of the ageing of the underlying Ising model.
Received 21 September 2001 and Received in final form 16 January 2002 相似文献
13.
A simple model for systems of dipolarly interacting single-domain ultrafine ferromagnetic particles is studied by Monte Carlo
simulations of zero field cooling and field cooling as well as relaxation experiments. By investigating systems characterized
by an identical moderate concentration but different types of particle positions' disorder, it is shown that the positional
disorder has a crucial influence on the magnetic behavior of the system. For extreme values of positional disorder, the interplay
between spatial disorder and dipolar interaction can even lead to a cooperative freezing at low temperatures.
Received 28 November 2001 相似文献
14.
N. Douarche F. Calvo P. J. Jensen G. M. Pastor 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):77-80
The properties of two-dimensional ensembles of magnetic
nanoparticles that interact by magnetic dipole coupling are
investigated. The low-temperature magnetic arrangements, the
average binding energy E
dip due to dipolar interactions, and its
scaling behavior with respect to the particle density
C are calculated for
different types of structural disorder and particle-size
distributions. Many different metastable magnetic states are
obtained, which exhibit strong noncollinearities and are
reminiscent of a spin-glass behavior. For a given
C, |E
dip| increases with increasing disorder
of the particle positions. For random distributions at low
particle densities C0.2,
E
dip is dominated by the contributions of
short interparticle distances. Thus, it scales as |
E
dip| C
with an unusually small exponent
= 0.85–1. The straightforward scaling of the dipole interaction,
3/2, is obtained only for C0.5 or for nearly periodic ensembles.
The finite temperature behavior of these disordered interacting
nanomagnets is explored. The specific heat and magnetic
susceptibility are calculated by performing Monte Carlo
simulations. The onset of long-range magnetic order is
discussed. In addition we determine hysteresis loops at finite
temperatures and compare the results for different degrees of
disorder. 相似文献
15.
K. Jonason P. Nordblad 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):23-28
Dynamic magnetic properties and ageing phenomena of the re-entrant ferromagnet (Fe0.20Ni0.80)75P16B6Al3 are investigated by time dependent zero field cooled magnetic relaxation, m(t), measurements. The influence of a temperature cycling (perturbation), ,(prior the field application) on the relaxation rate is investigated both in the low temperature re-entrant spin glass “phase”
and in the ferromagnetic phase. In the ferromagnetic phase the influence of a positive and a negative temperature cycle (of
equal magnitude) on the response is almost the same (symmetric response). The result at lower temperatures, in the RSG “phase”
is asymmetric, with a strongly affected response for positive, and hardly no influence on the response for negative temperature
cycles. The behaviour at low temperatures is similar to what is observed in ordinary spin glasses.
Received 20 August 1998 相似文献
16.
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
17.
18.
C. Djurberg K. Jonason P. Nordblad 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):15-21
Experiments on the temperature and time dependence of the response function and the field cooled magnetisation of a Cu(Mn)
spin glass at temperatures below the zero field spin glass temperature are used to explore the non-equilibrium nature of the
underlying spin configuration. The results imply that a certain spin configuration is imprinted on the system as the temperature
is decreased at a constant cooling rate. The cooling rate governs the magnitude of the FC magnetisation ((H,T)). Any intermittent halt at a constant temperature, , imprints an extended spin configuration, a process that is reflected e.g. in a downward relaxation of . On continued cooling at the same rate, the magnitude of (T) remains at a lower level than that of a continuous cooling curve. These results are put into the context of the corresponding
behaviour of the response function as observed in measurements of the relaxation of the zero field cooled magnetisation.
Received 27 October 1998 and Received in final form 30 November 1998 相似文献
19.
L. Seetha Lakshmi K. Dörr K. Nenkov A. Handstein V. S. Sastry K.-H. Müller 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(2):115-121
We report the charge state modification effects at the Mn site on the ground
state properties of colossal magnetoresistive manganites. Ta5+
substitution results in an appreciable increase in the lattice parameters
and unit cell volume due to increased Mn3+ concentration. The
ferromagnetic-metallic ground state modifies to a cluster glass insulator
for
. The reduction in the transition temperatures with
increasing x is ∼39 K/at.%. Besides the modification of majority carrier concentration due to increased Mn3+ concentration and enhanced local structural effects, the local
electrostatic potential of the substituent seems to contribute to the
unusually strong reduction of the transition temperatures of the compounds.
Thermo magnetic irreversibility just below Curie temperature (Tc),
non-saturation of magnetization, two distinct magnetic transitions in ac
susceptibility in an appropriate static field: close to Tc and other at
low temperature (the spin freezing temperature (Tg)) and non-stationary
dynamics with a characteristic maximum in the magnetic viscosity close to
Tg confirm a cluster glass state for
. These results
find additional support from a linear low temperature magnetic specific heat
of x = 0.10 with a characteristic broad maximum close to Tg. 相似文献