共查询到16条相似文献,搜索用时 0 毫秒
1.
2.
P. H. Chavanis 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(4):391-409
We derive the exact expression of the diffusion
coefficient of a self-gravitating Brownian gas in two
dimensions. Our formula generalizes the usual Einstein relation for
a free Brownian motion to the context of two-dimensional gravity. We
show the existence of a critical temperature Tc at which the
diffusion coefficient vanishes. For T < Tc, the diffusion
coefficient is negative and the gas undergoes gravitational
collapse. This leads to the formation of a Dirac peak concentrating
the whole mass in a finite time. We also stress that the critical
temperature Tc is different from the collapse temperature
T* at which the partition function diverges. These quantities
differ by a factor 1-1/N where N is the number of particles in
the system. We provide clear evidence of this difference by
explicitly solving the case N = 2. We also mention the analogy with
the chemotactic aggregation of bacteria in biology, the formation
of “atoms” in a two-dimensional (2D) plasma and the formation of
dipoles or “supervortices” in 2D point vortex dynamics. 相似文献
3.
The various dynamical models for fragment formation in nuclear collisions are discussed in order to bring out their relative
advantages and shortcomings. After discussing the general requirements for dynamical models that aim to describe fragment
formation, we consider the various mean-field models that incorporate fluctuations and then turn to models based on molecular
dynamics. 相似文献
4.
S.P. Avdeyev V.A. Karnaukhov W.D. Kuznetsov L.A. Petrov V.K. Rodionov A.S. Zubkevich H. Oeschler O.V. Bochkarev L.V. Chulkov E.A. Kuzmin A. Budzanovski W. Karcz M. Janicki E. Norbeck A.S. Botvina W.A. Friedman W. Nörenberg G. Papp 《The European Physical Journal A - Hadrons and Nuclei》1998,3(1):75-83
Multiple emission of intermediate-mass fragments has been studied for the collisions p + Au at 2.16, 3.6 and 8.1 GeV with
the FASA setup. The mean IMF multiplicities for events with at least one IMF are equal to 1.7, 1.9 and 2.1 (±0.2) respectively.
The multiplicity, charge distributions and kinetic energy spectra of IMF are described in the framework of a intranuclear
cascade model followed by the statistical multifragmentation model. However, between the two parts of the calculation the
excitation energies and the residual masses and charges are modified to take into account the losses during expansion. The
results support a scenario of true thermal multifragmentation of a hot and expanded target spectator.
Received: 15 December 1997 / Revised version: 24 April 1998 相似文献
5.
We present a field-theoretic Renormalization Group (RG) analysis of the statistical mechanics of long flexible, screened polyelectrolyte
chains (Debye-Hückel chains) in polar solvents where the screening length is of the order of the chain size. A systematic
analysis of the resulting field theory shows that the system is one with two length-scales requiring the calculation of scaling functions as well as exponents to fully describe its physical behaviour. This means that care
must be taken to understand the interplay of the length-scales. Using the RG we identify the relevant scaling variables and
explicitly calculate the scaling behaviour of the end-to-end distance for single chains. In addition we consider the many-chain
system and calculate the scaling behaviour of the osmotic pressure of a dilute solution of chains.
Received 16 December 1999 and Received in final form 13 December 2000 相似文献
6.
B. Bassetti C.A. Mantica P. Jona 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):359-369
A material surface of pure constituents with a flexible molecular chain (amphiphilics) is considered; thermodynamic behaviour
is studied in the chain length-temperature plane. The Hamiltonian of the system is modelled as the sum of a formation term
which refers to the polymer nature of the chain, and of a fluctuation term with a specific elastic form. For closed systems
the model exhibits phases with uniform curvature and conformational order/disorder or, alternatively, modulated phases; a
critical chain length is found for the existence of modulated phases; the dependence of transition temperature on energy parameters
is determined. A critical region is found for open systems, where conformational disorder drives spontaneous generation of
curvature; this lies above a characteristic chain length and around the shape transition temperature.
Received: 13 November 1996 / Revised: 9 May 1997 / Received in final form: 4
November 1997 / Accepted: 10 November 1997 相似文献
7.
D.H.E. Gross E.V. Votyakov 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(1):115-126
Traditionally, phase transitions are defined in the thermodynamic limit only. We discuss how phase transitions of first order
(with phase separation and surface tension), continuous transitions and (multi)-critical points can be seen and classified
for small systems. “Small” systems are systems where the linear dimension is of the characteristic range of the interaction
between the particles; i.e. also astrophysical systems are “small” in this sense. Boltzmann defines the entropy as the logarithm of the area of the surface in the mechanical N-body phase space at total energy E. The topology of S(E,N) or more precisely, of the curvature determinant allows the classification of phase transitions without taking the thermodynamic limit. Micro-canonical thermo-statistics
and phase transitions will be discussed here for a system coupled by short range forces in another situation where entropy
is not extensive. The first calculation of the entire entropy surface S(E,N) for the diluted Potts model (ordinary (q=3)-Potts model plus vacancies) on a square lattice is shown. The regions in {E,N} where D>0 correspond to pure phases, ordered resp. disordered, and D<0 represent transitions of first order with phase separation and “surface tension”. These regions are bordered by a line
with D=0. A line of continuous transitions starts at the critical point of the ordinary (q=3)-Potts model and runs down to a branching point Pm. Along this line vanishes in the direction of the eigenvector of D with the largest eigen-value . It characterizes a maximum of the largest eigenvalue . This corresponds to a critical line where the transition is continuous and the surface tension disappears. Here the neighboring
phases are indistinguishable. The region where two or more lines with D=0 cross is the region of the (multi)-critical point. The micro-canonical ensemble allows to put these phenomena entirely
on the level of mechanics.
Received 18 October 1999 and received in final form 17 November 1999 相似文献
8.
D. Sornette J.V. Andersen 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):353-357
We revisit a simple dynamical model of rupture in random media with long-range elasticity to test whether rupture can be seen
as a first-order or a critical transition. We find a clear scaling of the macroscopic modulus as a function of time-to-rupture
and of the amplitude of the disorder, which allows us to collapse neatly the numerical simulations over more than five decades
in time and more than one decade in disorder amplitude onto a single master curve. We thus conclude that, at least in this
model, dynamical rupture in systems with long-range elasticity is a genuine critical phenomenon occurring as soon as the disorder
is non-vanishing.
Received: 11 July 1997 / Revised: 6 November 1997 / Accepted: 10 November 1997 相似文献
9.
We study the force-induced unfolding of random disordered RNA or single-stranded DNA polymers. The system undergoes a second-order
phase transition from a collapsed globular phase at low forces to an extensive necklace phase with a macroscopic end-to-end
distance at high forces. At low temperatures, the sequence inhomogeneities modify the critical behaviour. We provide numerical
evidence for the universality of the critical exponents which, by extrapolation of the scaling laws to zero force, contain
useful information on the ground-state (f = 0) properties. This provides a good method for quantitative studies of scaling exponents characterizing the collapsed globule.
In order to get rid of the blurring effect of thermal fluctuations, we restrict ourselves to the ground state at fixed external
force. We analyze the statistics of rearrangements, in particular below the critical force, and point out its implications
for force-extension experiments on single molecules.
Received 18 June 2002 and Received in final form 23 September 2002
RID="a"
ID="a"e-mail: muller@ipno.in2p3.fr 相似文献
10.
K. Boukheddaden J. Linares H. Spiering F. Varret 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(2):317-326
We investigate the dynamical properties of the 1-D Ising-like Hamiltonian taking into account short and long range interactions,
in order to predict the static and dynamic behavior of spin crossover systems. The stochastic treatment is carried out within
the frame of the local equilibrium method [1]. The calculations yield, at thermodynamic equilibrium, the exact analytic expression
previously obtained by the transfer matrix technique [2]. We mainly discuss the shape of the relaxation curves: (i) for large
(positive) values of the short range interaction parameter, a saturation of the relaxation curves is observed, reminiscent
of the behavior of the width of the static hysteresis loop [3]; (ii) a sigmoidal (self-accelerated) behavior is obtained for
large enough interactions of any type; (iii) the relaxation curves exhibit a sizeable tail (with respect to the mean-field
curves) which is clearly associated with the transient onset of first-neighbor correlations in the system, due to the effect
of short-range interactions. The case of negative short-range interaction is briefly discussed in terms of two-step properties.
Received 29 October 1999 and Received in final form 30 December 1999 相似文献
11.
D. O. Kharchenko A. V. Dvornichenko 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(1):95-103
The present study generalizes
the model of extended stochastic systems with a field-dependent kinetic coefficient [M. Ibanes, J. Garcia-Ojalvo, R. Toral,
J.M. Sancho, Phys. Rev. Lett. 87, 020601 (2001)]
to systems with symmetric and asymmetric bistable potentials.
It is found that in systems with a relaxational flow and a symmetric local potential, reentrant phase transitions can be observed.
In the case of an asymmetric local potential, a hysteresis-like behaviour in the order parameter appears. It is shown that
such phase transitions can be controlled by the constant that governs relaxation flow, noise intensity and spatial coupling
intensity. 相似文献
12.
Spontaneous-search method and short-time dynamics: applications to the
Domany-Kinzel cellular automaton 总被引:1,自引:0,他引:1
S. D. da Cunha U. L. Fulco L. R. da Silva F. D. Nobre 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(1):93-100
The one-dimensional Domany-Kinzel cellular automaton is investigated by two
numerical approaches: (i) the spontaneous-search method, which is a method
appropriated for a search of criticality; (ii) short-time dynamics. Both
critical frontiers of the system are investigated, namely, the one
separating the frozen and active phases, as well as the critical line
determined by damage spreading between two cellular automata, that splits
the active phase into the nonchaotic and chaotic phases.
The efficiency of the spontaneous-search method is established herein
through a precise estimate of both critical frontiers, and in addition to
that, it is shown that this method may also be used in the determination
of the critical exponent ν⊥. Using the critical frontiers
obtained, other exponents are estimated through short-time dynamics. It is
verified that the critical exponents of both critical frontiers fall in the
universality class of directed percolation. 相似文献
13.
A large deviation technique is used to calculate the microcanonical entropy function s(v,m) of the mean-field ϕ4-model as a function of the potential energy v and the magnetization m. As in the canonical ensemble, a continuous phase transition
is found. An analytical expression is obtained for the critical energy vc(J) as a function of the coupling parameter J. 相似文献
14.
H. Behringer M. Pleimling A. Hüller 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(1):81-93
Cluster molecular field approximations represent a substantial progress over the simple Weiss theory where only one spin is
considered in the molecular field resulting from all the other spins. In this work we discuss a systematic way of improving
the molecular field approximation by inserting spin clusters of variable sizes into a homogeneously magnetised background.
The density of states of these spin clusters is then computed exactly. We show that the true non-classical critical exponents
can be extracted from spin clusters treated in such a manner. For this purpose a molecular field finite size scaling theory
is discussed and effective critical exponents are analysed. Reliable values of critical quantities of various Ising and Potts
models are extracted from very small system sizes.
Received 30 September 2002 / Received in final form 25 November 2002 Published online 27 January 2003
RID="a"
ID="a"e-mail: pleim@theorie1.physik.uni-erlangen.de 相似文献
15.
H. Chang N.X. Chen J.K. Liang G.H. Rao 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):55-60
The structural stability of La2Co17-xMx (M = Mn, Mo, Nb, Ti, V, Al, Cr, Ni and Si) based on the interatomic potential has been studied. The calculated site preference
of the third element M is found to be the 6c site, which is in agreement with the experiments. In the calculations, if the
crystal cohesive energy of La2Co16Mn is taken as the highest one in the crystallization of La2Co17-xMx, the lowest content x of the third element M (M = Mn, Mo, Nb, Ti and V) required to stabilize La2Co17-xMx, is near that found in the experiments. The differences of the cell parameters between the calculated and the experimental
values are less than 0.4%. The differences of the atomic parameters for Co (or M) between the calculated and the experimental
values are about or even smaller than 1%, and that of La is about 3%. Because the energies of La(Co1-xAlx)13 are lower than those of La2(Co1-xAlx)17, La2(Co1-xAlx)17 could not be formed in the experiments. In the calculations, with either a range of deformation of the structure or the reconstruction
of the initial structure La2Co17 from LaCo5, the same results including the cohesive energy curves and the crystallographic parameters can be retrieved after the action
of the interatomic potentials.
Received 1st November 2002 / Received in final form 17 February 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: hchang@aphy.iphy.ac.cn 相似文献
16.
We investigate the delocalization transition appearing in an exclusion process with two internal states, respectively on two
parallel lanes. At the transition, delocalized domain walls form in the density profiles of both internal states, in agreement
with a mean-field approach. Remarkably, the topology of the system’s phase diagram allows for the delocalization of a (localized)
domain wall when approaching the transition. We quantify the domain wall’s delocalization close to the transition by analytic
results obtained within the framework of the domain wall picture. Power law dependences of the domain wall width on the distance
to the delocalization transition as well as on the system size are uncovered, they agree with numerical results. 相似文献