共查询到20条相似文献,搜索用时 31 毫秒
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A. H. Rodríguez H. Y. Ramírez 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(2):235-238
With the help of metadynamics it is possible to calculate efficiently the free energy of systems displaying high energy barriers
as a function of few selected “collective variables”. In doing this, the contribution of all the other degrees of freedom
(“microscopic” variables) is averaged out and, thus, lost. In the following it is shown that it is possible to calculate the
thermal average of these microscopic degrees of freedom during the metadynamics, not loosing this piece of information. The
method is tested on a two-dimensional toy system and on a small molecule, that is dialanine. 相似文献
3.
J. Dietel 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(2):195-205
We reconsider energy calculations of the spin polarized ν = 1/2 Chern-Simons theory. We show that one has to be careful in
the definition of the Chern-Simons path integral in order to avoid an IR divergent magnetic ground state energy in RPA as
in [J. Dietel et al, Eur. Phys. J. B 5, 439 (1998)]. We correct the path integral and get a well behaved magnetic energy by considering the energy of the maximal
divergent graphs as well as the Hartree-Fock graphs. Furthermore, we consider the ν = 1/2 and the ν = 5/2 system with spin
degrees of freedom. In doing this we formulate a Chern-Simons theory of the ν = 5/2 system by transforming the interaction
operator to the next lower Landau level. We calculate the Coulomb energy of the spin polarized as well as the spin unpolarized
ν = 1/2 and the ν = 5/2 system as a function of the interaction strength in RPA. These energies are in good agreement with
numerical simulations of interacting electrons in the first as well as in the second Landau level. Furthermore, we calculate
the compressibility, the effective mass and the excitations of the spin polarized ν = 2 + 1/ systems where is an even number.
Received 13 June 2000 相似文献
4.
Z.F. Huang S. Solomon 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(4):601-607
We study by theoretical analysis and by direct numerical simulation the dynamics of a wide class of asynchronous stochastic
systems composed of many autocatalytic degrees of freedom. We describe the generic emergence of truncated power laws in the
size distribution of their individual elements. The exponents α of these power laws are time independent and depend only on
the way the elements with very small values are treated. These truncated power laws determine the collective time evolution
of the system. In particular the global stochastic fluctuations of the system differ from the normal Gaussian noise according
to the time and size scales at which these fluctuations are considered. We describe the ranges in which these fluctuations
are parameterized respectively by: the Lévy regime α < 2, the power law decay with large exponent ( α > 2), and the exponential
decay. Finally we relate these results to the large exponent power laws found in the actual behavior of the stock markets
and to the exponential cut-off detected in certain recent measurement.
Received 29 July 2000 and Received in final form 25 September 2000 相似文献
5.
M. Dugic D. Rakovic 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(4):781-790
We investigate the quantum-mechanical tunneling between the “patterns" of the, so-called, associative neural networks. Being
the relatively stable minima of the “configuration-energy" space of the networks, the “patterns" represent the macroscopically
distinguishable states of the neural nets. Therefore, the tunneling represents a macroscopic quantum effect, but with some
special characteristics. Particularly, we investigate the tunneling between the minima of approximately equal depth, thus
requiring no energy exchange. If there are at least a few such minima, the tunneling represents a sort of the “random walk"
process, which implies the quantum fluctuations in the system, and therefore “malfunctioning" in the information processing
of the nets. Due to the finite number of the minima, the “random walk" reduces to a dynamics modeled by the, so-called, Pauli
master equation. With some plausible assumptions, the set(s) of the Pauli master equations can be analytically solved. This
way comes the main result of this paper: the quantum fluctuations due to the quantum-mechanical tunneling can be “minimized"
if the “pattern"-formation is such that there are mutually “distant" groups of the “patterns", thus providing the “zone" structure
of the “pattern" formation. This qualitative result can be considered as a basis of the efficient deterministic functioning
of the associative neural nets.
Received 15 July 1999 相似文献
6.
P. Schwab 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(2):189-192
In a weakly disordered metal electron interactions are responsible for both decoherence of the quasi-particles as well as
for quantum corrections to thermodynamic properties. We consider electrons which are interacting with two-level-systems. We
show that the two-level-systems enhance the average equilibrium (“persistent”) current in an ensemble of mesoscopic rings.
The result supports the recent suggestion that two puzzles in mesoscopic physics may be related: The low temperature saturation
of the dephasing time and the high persistent current in rings.
Received 26 May 2000 相似文献
7.
Within mean field approximation, a procedure is elaborated to consider noise induced phase transitions with arbitrary relations
between the noises of different degrees of freedom. The proposed approach is applied to investigate effects of cross correlation
between noises in the generalized synergetic model of Lorenz type. This cross correlation is shown to induce phase transitions
of the dynamical system under consideration. Additionally, we find the correlation between noises transforms a synergetic
behavior to a thermodynamic one.
Received 13 November 2002 Published online 11 April 2003
RID="a"
ID="a"e-mail: dikh@sumdu.edu.ua 相似文献
8.
S. Aumaître A. Alastuey S. Fauve 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,54(2):263-266
Using simple scaling arguments and two-dimensional numerical simulations of a granular gas excited by vibrating one of the
container boundaries, we study a double limit of small 1-r and large L, where r is the restitution coefficient and L the size
of the container. We show that if the particle density n0 and (1-r2)(n0 Ld) where d is the particle diameter, are kept constant and small enough, the granular temperature, i.e. the mean value of
the kinetic energy per particle, 〈E 〉/N, tends to a constant whereas the mean dissipated power per particle, 〈D 〉/N, decreases
like
when N increases, provided that (1-r2)(n0 Ld)2 < 1. The relative fluctuations of E, D and the power injected by the moving boundary, I, have simple properties in that regime.
In addition, the granular temperature can be determined from the fluctuations of the power I(t) injected by the moving boundary. 相似文献
9.
S. Kutter E.M. Terentjev 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):455-462
Twist-storing polymers respond with elastic energy penalty to coherent or random twisting along the local chain axis away
from its equilibrium, which can be straight (as in “ribbons”) or helical (as in DNA and other biopolymers). Here we study
the equilibrium conformation of such polymers, focusing on the thermodynamic balance between twist and writhe, resulting from
the competition between the random coil entropy and the potential energy stored in superhelical portions of the polymer chain.
Two macroscopic variables characterise such a chain, the end-to-end distance R and the link number Lk, which is a topological invariant of a given polymer with clamped ends. We find that with increasing link number Lk, the chain accommodates its excess twist in growing plectonemes, unless forced out of this state by stretching its end-to-end
distance R. We calculate the force-extension relation, which exhibits crossovers between different deformation regimes.
Received 16 November 2000 and Received in final form 6 February 2001 相似文献
10.
F. Volino G. Gebel H. Gérard 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):25-35
A simple lattice model based on generalised diffusion equations and Gaussian statistics, aimed at describing diffusive translational
and rotational motions, is presented. It is shown that it allows the generation of correlation functions relevant to spectroscopic
techniques that are very similar to those experimentally observed in a large variety of complex systems. For some ranges of
values of the model parameters, these functions, which can be expressed in closed mathematical forms, can be approximately
represented by the sum of two exponentials or by “stretched" exponentials.
Received 17 September 1999 and Received in final form 10 February 2000 相似文献
11.
J. Bachteler H.-R. Trebin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(3):299-306
The elastic theory of quasicrystals considers, in addition to the “normal” displacement field, three “phason” degrees of freedom.
We present an approximative solution for the elastic Green's function of icosahedral quasicrystals, assuming that the coupling
between the phonons and phasons is small.
Received: 18 December 1997 / Accepted: 6 March 1998 相似文献
12.
M. Kolář F. Slanina 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(3):379-384
We examine a model of biological evolution of Eigen's quasispecies in a so-called holey fitness landscape, where the fitness
of a site is either 0 (lethal site) or a uniform positive constant (viable site). The evolution dynamics is therefore determined
by the topology of the genome space which is modelled by the random Bethe lattice. We use the effective medium and single-defect
approximations to find the criteria under which the localized quasispecies cloud is created. We find that shorter genomes,
which are more robust to random mutations than average, represent a selective advantage which we call “topological”. A way
of assessing empirically the relative importance of reproductive success and topological advantage is suggested.
Received 9 August 2002 / Received in final form 7 November 2002 Published online 14 February 2003
RID="a"
ID="a"e-mail: slanina@fzu.cz 相似文献
13.
Ph. Tordjeman C. Robert G. Marin P. Gerard 《The European physical journal. E, Soft matter》2001,4(4):459-465
In order to study the effect of the α,β crystalline structure of polypropylene (PP) on its mechanical properties, it is necessary
to prepare samples with variable α/β-phase content but with constant crystallinity and constant spherulite size. With this
objective, heat treatment was first defined to be applied to an isotactic PP containing a β nucleating agent in order to achieve
these conditions. Then study of the effect of the β-phase content on the tensile properties and fracture behaviour has been
done at room temperature. The mechanical properties at fracture were assessed by three-point bending tests and were analysed
on the basis of the “Essential Work of Fracture” (EWF). The results show that the elongation at fracture under tensile stress
and the “near” Plane-Strain Essential Work of Fracture, w
Ie, increase substantially with the β-phase content. Besides, Young's modulus and the yield stress in tensile tests decrease
slowly with the β-phase content. Finally, these results are analysed taking account the differences in structure of the α
and β spherulites.
Received 18 September 2000 and Received in final form 19 December 2000 相似文献
14.
B. Örçal A. Erzan 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):721-728
We study the temporal and spatial correlations in a one-dimensional model of a heterogeneous fault zone, in the presence of
viscoelastic effects. As a function of dynamical weakening and of dissipation, the system exhibits three different “phases":
one in which there are no time correlations between the events, a second, in which there are “Omori's law” type temporal correlations,
and a third, runaway phase with quasiperiodic system size events.
Received 30 May 2000 and Received in final form 7 September 2000 相似文献
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R. Folman J. Schmiedmayer H. Ritsch D. Vitali 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,13(1):93-107
Following almost a century of debate on possible “independent of measurement" elements of reality, or “induced" elements of
reality - originally invoked as an ad-hoc collapse postulate, we propose a novel line of interference experiments which may be able to examine the regime of induced
elements of reality. At the basis of the proposed experiment, lies the hypothesis that models of “induced" elements of reality
should exhibit symmetry breaking within quantum evolution. The described symmetry experiment is thus aimed at being able to detect and resolve spatial symmetry breaking signatures. The proposed experiment stands at
the edge of present day technological abilities and will be, so we believe, realizable in the near future.
Received 2 December 1999 and Received in final form 6 April 2000 相似文献
17.
X.H. Yan J.W. Ding Q.B. Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(2):157-163
Developing a renormalization group approach, we study the hopping conductivity of nanocrystalline chains with different site
energies. Exact calculations show that many parameters including nano-sizes, randomness of grain distributions, lattice distortions,
site energies, transition rates, Fermi energy, and temperature influence the conductivity. Some new singular features, for
example the frequency shift, the amplitude fluctuations, and the interchange between “peak” and “valley” behavior of the imaginary
part of the conductivity can be caused by certain parameters mentioned above, while the interface distortions modulate mainly
the overall amplitudes of the conductivity at the whole frequency region.
Received 13 January 2000 and Received in final form 12 September 2000 相似文献
18.
Thermal convection in a rotating layer of a magnetic fluid 总被引:1,自引:0,他引:1
G.K. Auernhammer H.R. Brand 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):157-168
We consider Brownian particles with the ability to take up energy from the environment, to store it in an internal depot,
and to convert internal energy into kinetic energy of motion. Provided a supercritical supply of energy, these particles are
able to move in a “high velocity” or active mode, which allows them to move also against the gradient of an external potential.
We investigate the critical energetic conditions of this self-driven motion for the case of a linear potential and a ratchet
potential. In the latter case, we are able to find two different critical conversion rates for the internal energy, which
describe the onset of a directed net current into the two different directions. The results of computer simulations are confirmed
by analytical expressions for the critical parameters and the average velocity of the net current. Further, we investigate
the influence of the asymmetry of the ratchet potential on the net current and estimate a critical value for the asymmetry
in order to obtain a positive or negative net current.
Received 20 September 1999 相似文献
19.
E. Rasel F. Pereira Dos Santos F. Saverio Pavone F. Perales C.S. Unnikrishnan M. Leduc 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,7(3):311-316
We report a study of transverse laser cooling on a metastable helium beam using spectrally broadened diode lasers (“white
light") to increase its flux. For this purpose, beam profile and atomic flux versus laser power and other parameters have been characterized. We have performed experiments to compare this technique with other
transverse cooling methods using monochromatic light. Best results are obtained with a “ziz-zag" configuration using “white
light".
Received 21 December 1998 and Received in final form 27 May 1999 相似文献