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1.
Mattioni L Wittmer JP Baschnagel J Barrat JL Luijten E 《The European physical journal. E, Soft matter》2003,10(4):369-385
Correlations in the motion of reptating polymers in a melt are investigated by means of Monte Carlo simulations of the three-dimensional
slithering-snake version of the bond-fluctuation model. Surprisingly, the slithering-snake dynamics becomes inconsistent with
classical reptation predictions at high chain overlap (created either by chain length N or by the volume fraction φ of occupied lattice sites), where the relaxation times increase much faster than expected. This
is due to the anomalous curvilinear diffusion in a finite time window whose upper bound (N) is set by the density of chain ends φ/N. Density fluctuations created by passing chain ends allow a reference polymer to break out of the local cage of immobile
obstacles created by neighboring chains. The dynamics of dense solutions of “snakes” at t ≪ is identical to that of a benchmark system where all chains but one are frozen. We demonstrate that the subdiffusive dynamical
regime is caused by the slow creeping of a chain out of its correlation hole. Our results are in good qualitative agreement
with the activated-reptation scheme proposed recently by Semenov and Rubinstein (Eur. Phys. J. B, 1 (1998) 87). Additionally, we briefly comment on the relevance of local relaxation pathways within a slithering-snake scheme.
Our preliminary results suggest that a judicious choice of the ratio of local to slithering-snake moves is crucial to equilibrate
a melt of long chains efficiently.
Received: 18 December 2002 / Accepted: 3 April 2003 / Published online: 12 May 2003
RID="a"
ID="a"e-mail: jwittmer@dpm.univ-lyon1.fr
RID="b"
ID="b"Current address: University of Illinois at Urbana-Champaign. 相似文献
2.
A.N. Semenov M. Rubinstein 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):87-94
The effect of excluded-volume interactions on the reptation dynamics of long polymer chains is considered theoretically. It
is shown that interactions give rise to an exponential increase of the reptation time, , if polymer chains are long enough: , where is the number of monomers per entanglement. We propose a novel dynamical mechanism of activated reptation implying that neighboring
chains exchange conformations of their terminal fragments. It is shown that the exchange mechanism is compatible with the
equilibrium polymer chain statistics and that it provides a bridge between the previous theories.
Received: 25 July 1997 / Accepted: 8 October 1997 相似文献
3.
On the properties of small-world network models 总被引:7,自引:0,他引:7
A. Barrat M. Weigt 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):547-560
We study the small-world networks recently introduced by Watts and Strogatz [Nature 393, 440 (1998)], using analytical as well as numerical tools. We characterize the geometrical properties resulting from the
coexistence of a local structure and random long-range connections, and we examine their evolution with size and disorder
strength. We show that any finite value of the disorder is able to trigger a “small-world” behaviour as soon as the initial
lattice is big enough, and study the crossover between a regular lattice and a “small-world” one. These results are corroborated
by the investigation of an Ising model defined on the network, showing for every finite disorder fraction a crossover from
a high-temperature region dominated by the underlying one-dimensional structure to a mean-field like low-temperature region.
In particular there exists a finite-temperature ferromagnetic phase transition as soon as the disorder strength is finite.
[0.5cm]
Received 29 March 1999 and Received in final form 21 May 1999 相似文献
4.
Monte Carlo simulations are used to study the non-uniform equilibrium charge distribution along a single annealed polyelectrolyte
chain under θ solvent conditions and with added salt. Within a range of the order of the Debye length charge accumulates at
chain ends while a slight charge depletion appears in the central part of the chain. The simulation results are compared with
theoretical predictions recently given by Castelnovo et al. In the parameter range where the theory can be applied we find almost perfect quantitative agreement.
Received 7 March 2002 and Received in final form 28 May 2002 相似文献
5.
P. Jensen N. Combe H. Larralde J. L. Barrat C. Misbah A. Pimpinelli 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):497-504
We study the relaxation to equilibrium of two dimensional islands containing up to 20 000 atoms by Kinetic Monte Carlo simulations.
We find that the commonly assumed relaxation mechanism - curvature-driven relaxation via atom diffusion - cannot explain the results obtained at low temperatures, where the island edges consist in large facets.
Specifically, our simulations show that the exponent characterizing the dependence of the equilibration time on the island
size is different at high and low temperatures, in contradiction with the above cited assumptions. Instead, we propose that
- at low temperatures - the relaxation is limited by the nucleation of new atomic rows on the large facets: this allows us
to explain both the activation energy and the island size dependence of the equilibration time.
Received 7 December 1998 and Received in final form 18 March 1999 相似文献
6.
We perform Monte Carlo simulations of an existing electrophoretic microchannel device used for the size separation of large
DNA fragments. This device is normally operated with a constant (dc) driving field. In contrast, we consider the case of a
varying (ac) driving field, in the zero-frequency limit. We find that a time-asymmetric pulse can yield interesting migration
regimes, in particular bidirectional transport for different molecular sizes. We also study a spatially asymmetric version
of the device and show that it can rectify unbiased but non-equilibrium molecular motion, in agreement with previous predictions
for entropic ratchets. Finally, at finite frequency we uncover a resonance for the molecular velocity in the channel which
could lead to improved performance.
Received: 16 November 2001 / Accepted: 11 February 2002 / Published online: 22 April 2002 相似文献
7.
8.
A. Volmer S. Mukherji T. Nattermann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):627-645
In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction
between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [#!BlatterGeshkenbein!#] and
by Mukherji and Nattermann in the disorder dominated case [#!NattermannMukherji!#]. This attraction together with the entropic
or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of
the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of
the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal
case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction
energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting
low field phase diagram and make quantitative predictions for pure BiSCCO (Bi2Sr2CaCu2O8). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative
discussion of the low-field, low-temperature phase diagram in the presence of impurities.
Received: 9 February 1998 / Accepted: 17 April 1998 相似文献
9.
H. Kachkachi A. Ezzir M. Noguès E. Tronc 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(4):681-689
We present a microscopic model for nanoparticles, of the maghemite (-Fe2O3) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of M?ssbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core and a surface shell of constant thickness.
The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and
dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with free
boundaries at the surface. Using a surface shell of constant thickness ( nm) we vary the particle size and study the effect of surface magnetic disorder on the thermal and spatial behaviors of the
net magnetisation of the particle. We study the shift in the surface “critical region” for different surface-to-core ratios
of the exchange coupling constants. It is also shown that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is responsible for the non saturation of the magnetisation at low temperatures.
Received 1 September 1999 and Received in final form 3 November 1999 相似文献
10.
S. Kumar P. Majumdar 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(4):571-579
We suggest and implement a new Monte Carlo strategy for correlated models
involving fermions strongly coupled to classical degrees of freedom, with
accurate handling of quenched disorder as well. Current methods
iteratively diagonalise the
full Hamiltonian for a system of N sites with computation time
τN ∼N4. This limits achievable sizes to N ∼100. In
our method
the energy cost of a Monte Carlo update is computed from
the Hamiltonian of a cluster, of size Nc, constructed around the reference
site, and embedded in the larger system. As MC steps sweep over the system,
the cluster Hamiltonian also moves, being reconstructed at each site where
an update is attempted. In this method
τN,Nc
∼NNc3.
Our results are obviously exact when Nc=N,
and converge quickly to this asymptote with increasing Nc, particularly
in the presence of disorder.
We provide detailed benchmarks
on the Holstein model and the double exchange
model. The `locality' of the energy cost, as evidenced by our results,
suggests that several important but inaccessible problems can now be handled
with control. This method forms the basis of our studies in Europhys. Lett.
68, 564 (2004), Phys. Rev. Lett. 94, 136601 (2005), and
Phys. Rev. Lett. 96, 016602 (2006). 相似文献
11.
12.
Using Monte Carlo simulations, we have found that there is a surprising non-monotonic dependence of a polymer's diffusion coefficient upon the degree of disorder of the surrounding environment. Starting with a two-dimensional periodic lattice of obstacles, we randomly displace obstacles to create a quenched gel system with a tunable degree of disorder. Very small displacements increase the diffusion coefficient of polymers since they increase the width of the tube through which the polymer chains reptate. As we displace the obstacles further, however, entropic trapping is observed and the diffusion coefficient of the polymer decreases dramatically. This is a striking example of the delicate balance between entropic and frictional effects for a polymer diffusing in a dense system. 相似文献
13.
D. O. Soares-Pinto I. S. Oliveira M. S. Reis 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(3):337-340
In this work we report Monte Carlo simulations of a 2D
Ising model, in which the statistics of the Metropolis algorithm
is replaced by the nonextensive one. We compute the magnetization
and show that phase transitions are present for q ≠ 1. A
q - phase diagram (critical temperature vs. the entropic
parameter q) is built and exhibits some interesting features,
such as phases which are governed by the value of the entropic
index q. It is shown that such phases favors some energy levels
of magnetization states. It is also shown that the contribution
of the Tsallis cutoff is capital to the existence of phase
transitions. 相似文献
14.
W. Michalke S. Kreitmeier 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):469-473
The diffusion Quantum-Monte-Carlo method of solving the Schr?dinger equation is applied to the vibrational ground state of
a polyethylene molecule. The results for the ground state energy show good agreement with normal mode analysis. In addition
to stretching, bending and torsional interaction van-der-Waals interaction is applied to a single chain showing a decrease
of the energy of 5%. The decrease for a polyethylene system of 5 chains with 10 atoms per molecule at the positions of a unit
cell is determined to be 4.8% per molecule. Finally first steps towards simulating excited states were performed.
Received: 9 February 1998 / Revised: 2 April 1998 / Accepted: 23 April 1998 相似文献
15.
S. Hilgenfeldt D. Lohse M. Zomack 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):247-255
The scattering of ultrasound from bubbles of m radius, such as used in contrast enhancers for ultrasound diagnostics, is studied. We show that sound scattering and “active”
emission of sound from oscillating bubbles are not contradictory, but are just two different aspects derived from the same
physics. Treating the bubble as a nonlinear oscillator, we arrive at general formulas for scattering and absorption cross-sections.
We show that several well-known formulas are recovered in the linear limit of this ansatz. In the case of strongly nonlinear
oscillations, however, the cross-sections can be larger than those for linear response by several orders of magnitude. The
major part of the incident sound energy is then converted into emitted sound, unlike what happens in the linear case, where
the absorption cross-sections exceed the scattering cross-sections.
Received: 26 February 1998 / Revised: 13 March 1998 / Accepted: 15 March 1998 相似文献
16.
We present results of Monte Carlo simulations measuring the coherent structure function of a chain moving through an ordered lattice of fixed topological obstacles. Our computer experiments use chains up to 320 beads and cover a large range of wave vectors and a time range exceeding the reptation time. For additional information we also measured the coherent structure function of internal pieces of the chain. We compare our results i) to the predictions of the primitive chain model, ii) to an approximate form resulting from Rouse motion in a coiled tube, and iii) to our recent evaluation of the full reptation model. i) The primitive chain model can fit the data for times
, where T
2 is the Rouse time of the chain. Besides some phenomenological amplitude factor this fit involves the reptation time T
3 as a second fit parameter. For the chain lengths measured, the asymptotic behavior
is not attained. ii) The model of Rouse motion in a tube, which we have criticized before on theoretical grounds, is shown to fail also on the purely phenomenological level. iii) Our evaluation of the full reptation model yields an excellent fit to the data for both total chains and internal pieces and for all wave vectors and all times, provided specific micro-structure effects of the MC dynamics are negligible. Such micro-structure effects show up for wave vectors of the order of the inverse segment size and enforce the introduction of some phenomenological, wave-vector-dependent prefactor. For the dynamics of the total chain our data analysis based on the full reptation model shows the importance of tube length fluctuations. Universal (Rouse-type) internal relaxation, however, is unimportant. It can be observed only in the form of the diffusive motion of a short central subchain in the tube. Finally, we present a fit formula which in a large range of wave vectors and chain lengths reproduces the numerical results of our theory for the scattering from the total chain.Received: 9 July 2003, Published online: 11 November 2003PACS:
83.10.Kn Reptation and tube theories - 82.35.Lr Physical properties of polymers - 83.10.Rs Computer simulation of molecular and particle dynamics 相似文献
17.
D. Hinzke U. Nowak D.A. Garanin 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(3):435-438
We simulated the field-dependent magnetization m(H,T) and the uniform susceptibility of classical Heisenberg antiferromagnets in the chain and square-lattice geometry using Monte Carlo methods. The results
confirm the singular behavior of at small T,H: and , where D=3 is the number of spin components, J
0=zJ, and z is the number of nearest neighbors. A good agreement is achieved in a wide range of temperatures T and magnetic fields H with the first-order 1/D expansion results (D.A. Garanin, J. Stat. Phys. 83, 907 (1996)).
Received 20 March 2000 相似文献
18.
We use large-scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length l
e of isolated, uniformly charged polymers with Debye-Hückel intrachain interactions in the limit where the screening length
κ-1 exceeds the intrinsic persistence length of the chains. Our simulations cover a significantly larger part of the parameter
space than previous studies. We observe no significant deviations from the prediction l
e∝κ-2 by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theories of electrostatic bending rigidity
and electrostatically excluded volume to the stretched de Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear
or sublinear dependence of the persistence length on the screening length can be ruled out. We show that previous results
pointing into this direction are due to a combination of excluded-volume and finite chain length effects. The paper emphasizes
the role of scaling arguments in the development of useful representations for experimental and simulation data.
Received 12 February 2002 相似文献
19.
Inclusive photoproduction of mesons has been measured for photon-proton centre-of-mass energies in the range GeV and photon virtuality 1 GeV. The data sample used corresponds to an integrated luminosity of 37 pb. Total and differential cross sections as functions of the transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading
order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The measured cross sections
are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows
the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute
to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant
charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured
results in a kinematic region where the resolved photon contribution is significant.
Received: 9 July 1998 / Published online: 22 October 1998 相似文献
20.
A. Valance F. Rioual 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):543-548
Starting from the phenomenological model for sand ripple formation developed in [#!Bouchaud98!#], we proposed a new interpretation
in the light of recent experiments. Furthermore, we derive, close to the threshold of ripple instability, a nonlinear equation
for the spatio-temporal evolution of the sand bed profile, which to leading order has a quadratic nonlinearity. This equation
is identical to that derived recently on the basis of geometry and conservation law [#!Csahok98!#]. Our derivation connects
the coefficients of the nonlinear equation to the underlying physical mechanisms (reptation length...). This equation reveals
ripple structures which then undergo a coarsening process, as observed in wind tunnel experiment. Small, fast moving ripples
are absorbed by larger, slower forms resulting in a growth of the mean wavelength.
Received 5 January 1999 相似文献