共查询到20条相似文献,搜索用时 694 毫秒
1.
G. Parisi F. Tria 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):533-541
We study spin glasses on random lattices with finite connectivity. In the infinite connectivity limit they reduce to the Sherrington
Kirkpatrick model. In this paper we investigate the expansion around the high connectivity limit. Within the replica symmetry
breaking scheme at two steps, we compute the free energy at the first order in the expansion in inverse powers of the average
connectivity (z), both for the fixed connectivity and for the fluctuating connectivity random lattices. It is well known that the coefficient
of the 1/z correction for the free energy is divergent at low temperatures if computed in the one step approximation. We find that this
annoying divergence becomes much smaller if computed in the framework of the more accurate two steps breaking. Comparing the
temperature dependance of the coefficients of this divergence in the replica symmetric, one step and two steps replica symmetry
breaking, we conclude that this divergence is an artefact due to the use of a finite number of steps of replica symmetry breaking.
The 1/z expansion is well defined also in the zero temperature limit.
Received 15 July 2002 Published online 31 December 2002 相似文献
2.
A. Montanari 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(1):121-136
Gallager codes are the best error-correcting codes to date. In this paper we study them by using the tools of statistical
mechanics. The corresponding statistical mechanics model is a spin model on a sparse random graph. The model can be solved
by elementary methods (i.e. without replicas) in a large connectivity limit. For low enough temperatures it presents a completely frozen glassy phase
(q
EA = 1). The same scenario is shown to hold for finite connectivities. In this case we adopt the replica approach and exhibit
a one-step replica symmetry breaking order parameter. We argue that our ansatz yields the exact solution of the model. This
allows us to determine the whole phase diagram and to understand the performances of Gallager codes.
Received 9 April 2001 相似文献
3.
B. Canals D.A. Garanin 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(4):439-447
Thermodynamic quantities and correlation functions (CFs) of the classical antiferromagnet on the checkerboard lattice are
studied for the exactly solvable infinite-component spin-vector model, D↦∞. In contrast to conventional two-dimensional magnets with continuous symmetry showing extended short-range order at distances
smaller than the correlation length, r
ξ
c∝ exp(T
*/T), correlations in the checkerboard-lattice model decay already at the scale of the lattice spacing due to the strong degeneracy
of the ground state characterized by a macroscopic number of strongly fluctuating local degrees of freedom. At low temperatures,
spin CFs decay as <
>∝ 1/r
2 in the range a
0≪r≪ξ
c∝T
-1/2, where a0 is the lattice spacing. Analytical results for the principal thermodynamic quantities in our model are very similar with
MC simulations, exact and analytical results for the classical Heisenberg model (D = 3) on the pyrochlore lattice. This shows that the ground state of the infinite-component spin vector model on the checkerboard
lattice is a classical spin liquid.
Received 16 November 2001 and Received in final form 12 February 2002 相似文献
4.
I.I. Satija 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(2):345-348
Disordered systems exhibiting exponential localization are mapped to anisotropic spin chains with localization length being
related to the anisotropy of the spin model. This relates localization phenomenon in fermions to the rotational symmetry breaking
in the critical spin chains. One of the intriguing consequence is that the statement of Onsager universality in spin chains
implies universality of the localized fermions where the fluctuations in localized wave functions are universal. We further
show that the fluctuations about localized nonrelativistic fermions describe relativistic fermions. This provides a new approach
to understand the absence of localization in disordered Dirac fermions. We investigate how disorder affects well known universality
of the spin chains by examining the multifractal exponents. Finally, we examine the effects of correlations on the localization
characteristics of relativistic fermions.
Received 28 September 2001 / Received in final form 30 November 2001 Published online 2 October 2002
RID="a"
ID="a"e-mail: isatija@nickel.nist.gov 相似文献
5.
T. Temesvári C. De Dominicis I.R. Pimentel 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(3):361-372
Symmetry considerations and a direct, Hubbard-Stratonovich type, derivation are used to construct a replica field-theory relevant
to the study of the spin glass transition of short range models in a magnetic field. A mean-field treatment reveals that two
different types of transitions exist, whenever the replica number n is kept larger than zero. The Sherrington-Kirkpatrick critical point in zero magnetic field between the paramagnet and replica
magnet (a replica symmetric phase with a nonzero spin glass order parameter) separates from the de Almeida-Thouless line,
along which replica symmetry breaking occurs. We argue that for studying the de Almeida-Thouless transition around the upper
critical dimension d = 6, it is necessary to use the generic cubic model with all the three bare masses and eight cubic couplings. The critical
role n may play is also emphasized. To make perturbative calculations feasible, a new representation of the cubic interaction is
introduced. To illustrate the method, we compute the masses in one-loop order. Some technical details and a list of vertex
rules are presented to help future renormalisation-group calculations.
Received 9 October 2001 相似文献
6.
M. Picco F. Ritort M. Sales 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(4):565-582
The use of parameters measuring order-parameter fluctuations (OPF) has been encouraged by the recent results reported in referenece
[2,3] which show that two of these parameters, G and G
c, take universal values in the . In this paper we present a detailed study of parameters measuring OPF for two mean-field models with and without time-reversal
symmetry which exhibit different patterns of replica symmetry breaking below the transition: the Sherrington-Kirkpatrick model
with and without a field and the Ising p-spin glass (p = 3). We give numerical results and analyze the consequences which replica equivalence imposes on these models in the infinite
volume limit. We give evidence for the transition in each system and discuss the character of finite-size effects. Furthermore,
a comparative study between this new family of parameters and the usual Binder cumulant analysis shows what kind of new information
can be extracted from the finite T behavior of these quantities. The two main outcomes of this work are: 1) Parameters measuring OPF give better estimates than
the Binder cumulant for T
c and even for very small systems they give evidence for the transition. 2) For systems with no time-reversal symmetry, parameters
defined in terms of connected quantities are the proper ones to look at.
Received 20 September 2000 and Received in final form 10 January 2001 相似文献
7.
M.A. Garcia-Bach 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):439-448
For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range spin pairing order can be identified which enables the
separation of the space spanned by finite-range (covalent) valence-bond configurations into w
+1 subspaces. Since every subspace has an equivalent counter subspace connected by translational symmetry, twofold degeneracy,
breaking translational symmetry is found except for the subspace where the ground state of w = even belongs to. In terms of energy ordering, (non)degeneracy and the discontinuities introduced in the long-range spin
pairing order by topological spin defects, the differences between even and odd ladders are explained in a general and systematic
way.
Received 19 July 1999 and Received in final form 8 October 1999 相似文献
8.
Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites
A. Weiße H. Fehske 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):487-494
We derive a realistic microscopic model for doped colossal magnetoresistance manganites, which includes the dynamics of charge,
spin, orbital and lattice degrees of freedom on a quantum mechanical level. The model respects the SU(2) spin symmetry and the full multiplet structure of the manganese ions within the cubic lattice. Concentrating on the hole
doped domain ( 0≤x≤0.5) we study the influence of the electron-lattice interaction on spin and orbital correlations by means of exact diagonalisation
techniques. We find that the lattice can cause a considerable suppression of the coupling between spin and orbital degrees
of freedom and show how changes in the magnetic correlations are reflected in dynamic phonon correlations. In addition, our
calculation gives detailed insights into orbital correlations and demonstrates the possibility of complex orbital states.
Received 4 September 2002 / Received in final form 8 November 2002 Published online 31 December 2002 相似文献
9.
S. Boettcher 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(1):29-39
The average ground state energy and entropy for ±J spin glasses on Bethe lattices of connectivities k + 1 = 3..., 26 at T = 0 are approximated numerically. To obtain sufficient accuracy for large system sizes (up to n = 212), the Extremal Optimization heuristic is employed which provides high-quality results not only for the ground state energies
per spin ek+1 but also for their entropies sk+1. The results indicate sizable differences between lattices of even and odd connectivities. The extrapolated ground state
energies compare very well with recent one-step replica symmetry breaking calculations. These energies can be scaled for all
even connectivities k + 1 to within a fraction of a percent onto a simple functional form, e
k + 1 = E
SK
- (2E
SK + )/, where E
SK = - 0.7633 is the ground state energy for the broken replica symmetry in the Sherrington-Kirkpatrick model. But this form
is in conflict with perturbative calculations at large k + 1, which do not distinguish between even and odd connectivities. We also find non-zero entropies per spin sk+1 at small connectivities. While sk+1 seems to vanish asymptotically with 1/(k + 1) for even connectivities, it is numerically indistinguishable from zero already for odd k + 1 ≥ 9.
Received 9 August 2002 Published online 27 January 2003
RID="a"
ID="a"e-mail: sboettc@emory.edu www.physics.emory.edu/faculty/boettcher 相似文献
10.
M. Cosentino Lagomarsino B. Bassetti P. Jona 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(1):81-88
We introduce a model system of stochastic entities, called rowers which include some essentials of the behavior of real cilia. We introduce and discuss the problem of symmetry breaking for
these objects and its connection with the onset of macroscopic, directed flow in the fluid. We perform a mean field-like calculation
showing that hydrodynamic interaction may provide for the symmetry breaking mechanism and the onset of fluid flow. Finally,
we discuss the problem of the metachronal wave in a stochastic context through an analytical calculation based on a path integral
representation of our model equation.
Received 12 June 2001 and Received in final form 9 January 2002 相似文献
11.
H. Laftchiev D. Samsœn P. Quentin J. Piperova 《The European Physical Journal A - Hadrons and Nuclei》2001,12(2):155-159
We report on Hartree-Fock-Bogoliubov (HFB) calculations of the ground-state rotational band of the heavy nucleus 254No recently observed experimentally. The calculated quadrupole deformation is consistent with the experimental value of β
= 0.27 and is almost constant over the whole band. We also reproduce fairly well the excitation spectra and moments of inertia
of this isotope up to the maximal experimentally observed state of spin 20. The rather high stability of this nucleus against
fission is illustrated by the deformation energy curve providing very high fission barriers at zero spin within the HFB and
HFB plus Lipkin-Nogami formalisms. The variation of these barriers with increased angular velocities is also studied.
Received: 23 November 2000 / Accepted: 24 October 2001 相似文献
12.
H. Weigel 《The European Physical Journal A - Hadrons and Nuclei》1998,2(4):391-402
Abstact: Within an extended Skyrme soliton model for baryons the interplay between the collective radial motion and the SU(3)–flavor–rotations
is investigated. The coupling between these modes is mediated by flavor symmetry breaking. Collective coordinates which describe
the corresponding large amplitude fluctuations are introduced and treated canonically. When diagonalizing the resulting Hamiltonian
flavor symmetry breaking is fully taken into consideration. As eigenstates not only the low–lying (1/2)+ and (3/2)+ baryons but also their radial excitations are obtained and compared to the empirical data. In particular the relevance of
radial excitations for the penta–quark baryon Z
+ (Y=2, I=0, J
π=(1/2)+) is discussed. In this approach its mass is predicted to be 1.58 GeV. Furthermore the widths for various hadronic decays
are estimated which, for example, yields Γ(Z
+→NK) ∼ 100 MeV for the only permissible decay process of the Z
+.
Received: 20 April 1998 / Revised version: 29 May 1998 相似文献
13.
F. Cinti M. Affronte A.G.M. Jansen 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):461-468
The level crossing mechanism between the ground and the first excited state of Na:Fe6 antiferromagnetically coupled iron rings is studied by torque magnetometry down to 40 mK and in magnetic fields up to 28
T. The step width at the crossing field Bc assumes a finite value at the lowest temperatures. This fact is ascribed to the presence of level anticrossing, not expected
for a ring with axial, i.e. S6 point group, symmetry. Assuming a reduced symmetry, we revised the model Hamiltonian of such a spin system by introducing
a Dzyaloshinsky-Moriya (DM) term and we show, by exact diagonalization, that DM term can account for the mixing of states with different parity.
In particular, analytical as well numerical analysis show that the introduction of the DM term may contribute to the broadening
of the torque step as well as for the finite energy gap at Bc observed by heat capacity in a similar ring Li:Fe6 as previously reported [#!aclbg!#].
Received 3 September 2002 Published online 31 December 2002 相似文献
14.
A. Benyoussef D. Dohmi A. El Kenz L. Peliti 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):503-510
Using a replica formalism, a generalization of a recent mean field model corresponding to the observed wrinkling transition
in randomly polymerized membranes is presented. In this model we study the effects of global fluctuations of the surface normals
to the flat membrane, which can be introduced by a random local field. In absence of these global fluctuations, we show that,
the model exhibits both continuous and discontinuous transitions between flat and wrinkled phases, contrary to what has been
predicted by Bensimon et al. and Attal et al. Phase diagrams both in replica symmetry and in breaking of replica symmetry in sense of Almeida and Thouless are given. We
have also investigated the effects of global fluctuations on the replica symmetry phase diagram. We show that, the wrinkled
phase is favored and the flat phase is unstable. For large global fluctuations, the transition between wrinkled and flat phases
becomes first order.
Received: 3 December 1997 / Revised: 31 March 1998 / Accepted: 3 August 1998 相似文献
15.
S.G. Magalh aes A.A. Schmidt A. Theumann B. Coqblin 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):419-425
The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non
zero mean J0 and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where
the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry
ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions
among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J0, J and its relation with the Kondo interaction coupling JK. These results could be used to address part of the experimental data for the CeNi
1 - x
Cu
x compound, when x ⩽ 0.8.
Received 24 June 2002 Published online 31 December 2002 相似文献
16.
J. Schnack H.-J. Schmidt J. Richter J. Schulenburg 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):475-481
For many spin systems with constant isotropic antiferromagnetic next-neighbour Heisenberg coupling the minimal energies E
min(S) form a rotational band, i.e. depend approximately quadratically on the total spin quantum number S, a property which is also known as Landé interval rule. However, we find that for certain coupling topologies, including
recently synthesised icosidodecahedral structures this rule is violated for high total spins. Instead the minimal energies
are a linear function of total spin. This anomaly results in a corresponding jump of the magnetisation curve which otherwise
would be a regular staircase.
Received 27 August 2001 and Received in final form 18 October 2001 相似文献
17.
B. Binz D. Baeriswyl B. Douçot 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(1):69-87
The weak coupling instabilities of a two dimensional Fermi system are investigated for the case of a square lattice using
a Wilson renormalization group scheme to one loop order. We focus on a situation where the Fermi surface passes through two
saddle points of the single particle dispersion. In the case of perfect nesting, the dominant instability is a spin density
wave but d-wave superconductivity as well as charge or spin flux phases are also obtained in certain regions in the space of coupling
parameters. The low energy regime in the vicinity of these instabilities can be studied analytically. Although saddle points
play a major role (through their large contribution to the single particle density of states), the presence of low energy
excitations along the Fermi surface rather than at isolated points is crucial and leads to an asymptotic decoupling of the
various instabilities. This suggests a more mean-field like picture of these instabilities, than the one recently established
by numerical studies using discretized Fermi surfaces.
Received 11 April 2001 and Received in final form 6 September 2001 相似文献
18.
We study a chiral spin liquid wave function defined as a Gutzwiller projected BCS state with a complex pairing function. After projection, spontaneous dimerization is found for any odd but finite number of chains, thus satisfying the Lieb-Schultz-Mattis theorem, whereas for an even number of chains there is no dimerization. The two-dimensional thermodynamic limit is consistently reached for a large number of chains since the dimer order parameter vanishes in this limit. This property clearly supports the possibility of a spin liquid ground state in two dimensions with a gap to all physical excitations and with no broken translation symmetry. 相似文献
19.
I. Bâldea H. Köppel L.S. Cederbaum 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(2):289-299
We present a new effect that is possible for strongly correlated electrons in commensurate mesoscopic rings: the collective tunneling of electrons between classically equivalent configurations, corresponding to ordered states possessing charge and
spin density waves (CDW, SDW) and charge separation (CS). Within an extended Hubbard model at half filling studied by exact
numerical diagonalization, we demonstrate that the ground state phase diagram comprises, besides conventional critical lines
separating states characterized by different orderings (e.g. CDW, SDW, CS), critical lines separating phases with the same ordering (e.g. CDW-CDW) but with different symmetries. While the former also exist in infinite systems, the latter are specific for mesoscopic systems and directly related to a collective tunnel effect. We emphasize that, in order to construct correctly
a phase diagram for mesoscopic rings, the examination of CDW, SDW and CS correlation functions alone is not sufficient, and
one should also consider the symmetry of the wave function that cannot be broken. We present examples demonstrating that the
jumps in relevant physical properties at the conventional and new critical lines are of comparable magnitude. These transitions
could be studied experimentally e.g. by optical absorption in mesoscopic systems. Possible candidates are cyclic molecules and ring-like nanostructures of quantum
dots.
Received 27 November 2000 相似文献
20.
Two-level systems (TLS) interacting with conduction electrons are possibly described by the two-channel Kondo Hamiltonian. In this case the channel degeneracy is due to the real spin of the electrons. The possibility of breaking that degeneracy (conservation) has interest on his own. In fact, we show that the interaction of the conduction electrons with a spin-orbit scatterer nearby the TLS leads to the breaking of the channel degeneracy (conservation) only in the case of electron-hole symmetry breaking. The generated channel symmetry breaking TLS-electron couplings are, however, too weak to result in any observable effects.Received: 21 October 2003, Published online: 8 December 2003PACS:
72.15.Cz Electrical and thermal conduction in amorphous and liquid metals and alloys - 72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect) - 71.55.-i Impurity and defect levels 相似文献