共查询到20条相似文献,搜索用时 15 毫秒
1.
R. Cafiero A. Valleriani J.L. Vega 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):505-508
We study the behavior under perturbations of the Parallel Bak-Sneppen model (PBS) in 1+1 dimension, which has been shown to
belong to the universality class of Directed Percolation (DP) in 1+1 dimensions [#!SD96!#]. We focus our attention on the
damage-spreading features of the PBS model with both random and deterministic updating, which are studied and compared to
the known results for the extremal Bak-Sneppen model (BS) and for DP. For both random and deterministic updating, we observe
a power law growth of the Hamming distance. In addition, we compute analytically the asymptotic plateau reached by the distance
after the growing phase.
Received: 24 September 1998 / Revised: 17 November 1998 / Accepted: 19 November
1998 相似文献
2.
J.F. Nagle S. Tristram-Nagle H. Takahashi I. Hatta 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):399-400
This comment shows that data recently reported [#!ref1!#] as being seemingly in conflict with earlier data [#!ref2!#] are,
in fact, in excellent agreement. Together, both studies confirm that the kinetics of the subgel phase transformation in dipalmitoylphosphatidylcholine
(DPPC) lipid bilayers obeys Kolmogorov-Avrami (K-A) theory [#!ref3!#,#!ref4!#] with an anomalously low effective dimensionality.
Received: 11 December 1997 / Accepted: 28 January 1998 相似文献
3.
Formation of ordered micro-porous membranes 总被引:5,自引:0,他引:5
O. Pitois B. François 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):225-231
Regular micro-porous polymeric membranes have recently been discovered by rapidly evaporating a solution of CS2 containing poly(p-phenylene)-block-polystyrene [#!ref1!#]. 1,2-dichloroethane (a chlorated solvent in which polystyrene gel phase has never
been observed) is also found to produce ordered structures, which definitively excludes eventual effect of the gelation process
during the membrane formation. The observation of the solution surface during the solvent evaporation reveals the growing
of micron-sized water droplets trapped at the surface and forming compact aggregates. The study of the solution/water interface
shows that the water droplets profile is in agreement with the pore shape observed in the membranes. Moreover, the copolymer
was found to precipitate at the interface, forming a layer encapsulating the droplets and preventing their coalescence. In
that way, the final structure results from the droplets stacking under the action of large surface currents. Finally, we argue
that the decisive element in the formation of ordered structures is the ability of the polymer to precipitate at the solution/water
interface, which seems to be related the star-polymer microstructure.
Received: 18 August 1998 相似文献
4.
B. O'Shaughnessy D. Vavylonis 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):363-372
We study the reaction kinetics of end-functionalized polymer chains dispersed in an unreactive polymer melt. Starting from
an infinite hierarchy of coupled equations for many-chain correlation functions, a closed equation is derived for the 2nd
order rate constant k after postulating simple physical bounds. Our results generalize previous 2-chain treatments (valid in dilute reactants limit)
by Doi [#!doi:inter2!#], de Gennes [#!gennes:polreactionsiandii!#], and Friedman and O'Shaughnessy [#!ben:interdil_all_aip!#],
to arbitrary initial reactive group density n0 and local chemical reactivity Q. Simple mean field (MF) kinetics apply at short times, .For high Q, a transition occurs to diffusion-controlled (DC) kinetics with (where xt is rms monomer displacement in time t) leading to a density decay . If n0 exceeds the chain overlap threshold, this behavior is followed by a regime where during which k has the same power law dependence in time, , but possibly different numerical coefficient. For unentangled melts this gives while for entangled cases one or more of the successive regimes ,t
-3/8
and t
-3/4
may be realized depending on the magnitudes of Q and n0. Kinetics at times longer than the longest polymer relaxation time are always MF. If a DC regime has developed before then the long time rate constant is where R is the coil radius. We propose measuring the above kinetics in a model experiment where radical end groups are generated
by photolysis.
Received: 2 June 1998 / Revised: 9 July 1998 / Accepted: 10 July 1998 相似文献
5.
S. Franz T. Garel H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):463-468
We consider a model of two (fully) compact polymer chains, coupled through an attractive interaction. These compact chains
are represented by Hamiltonian paths (HP), and the coupling favors the existence of common bonds between the chains. We use
a (n=0 component) spin representation for these paths, and we evaluate the resulting partition function within a homogeneous saddle
point approximation. For strong coupling (i.e. at low temperature), one finds a phase transition towards a “frozen” phase where one chain is completely adsorbed onto the
other. By performing a Legendre transform, we obtain the probability distribution of overlaps. The fraction of common bonds
between two HP, i.e. their overlap q, has both lower () and upper () bounds. This means in particular that two HP with overlap greater than coincide. These results may be of interest in (bio)polymers and in optimization problems.
Received 4 December 1998 and Received in final form 10 March 1999 相似文献
6.
B. Yahiaoui A. Gharbi M. Kléman J. Malthête 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):99-110
Electron microscopy observations of replicas of freeze-fractured samples of two columnar hexagonal phases of different nature (a lyotropic one, the inverse AOT in water; a thermotropic one, ) yield very different results: most defects at microscopic scales are screw dislocations in the lyotropic phase, longitudinal
edge dislocations in the thermotropic phase. A possible way to interpret these differences is as follows: in the lyotropic
the Lamé coefficients and μ and the bend modulus K3 would not display any anomaly compared to expected values; in the thermotropic the shear modulus μ would be ten times smaller
than the compressibility modulus , while K3 would still be comparable to (but larger than) the bend modulus of a small molecules liquid crystal. We present an elementary
theoretical model of the latter case which could explain the anomalous measurements of K3 and of the longitudinal compressibility (Ref. [#!ref10!#]) without contradicting more recent measurements of (Refs. [#!ref17!#,#!ref22!#]). Essentially, the hexagonal phase would be a phase with defects (longitudinal dislocations) akin to an hexatic phase but with some differences.
Re?u : 26 mai 1997 / Révisé : 20 Janvier 1998 /
Accepté : 27 avril 1998 相似文献
7.
T. Charitat B. Fourcade 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):333-336
An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state
is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state
exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink
pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that
. This model can be generalized to the case where the bend is induced by a concentration field which follows the variations
of the curvature.
Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997 相似文献
8.
D. Sornette D. Zajdenweber 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):653-664
At what level should government or companies support research? This complex multi-faceted question encompasses such qualitative
bonus as satisfying natural human curiosity, the quest for knowledge and the impact on education and culture, but one of its
most scrutinized component reduces to the assessment of economic performance and wealth creation derived from research. Many
studies report evidences of positive economic benefits derived from basic research [#!Martin!#,#!NAS!#]. In certain areas
such as biotechnology, semi-conductor physics, optical communications [#!Ehrenreich!#], the impact of basic research is direct
while, in other disciplines, the path from discovery to applications is full of surprises. As a consequence, there are persistent
uncertainties in the quantification of the exact economic returns of public expenditure on basic research. This gives little
help to policy makers trying to determine what should be the level of funding. Here, we suggest that these uncertainties have
a fundamental origin to be found in the interplay between the intrinsic “fat tail” power law nature of the distribution of
economic returns, characterized by a mathematically diverging variance, and the stochastic character of discovery rates. In
the regime where the cumulative economic wealth derived from research is expected to exhibit a long-term positive trend, we
show that strong fluctuations blur out significantly the short-time scales: a few major unpredictable innovations may provide
a finite fraction of the total creation of wealth. In such a scenario, any attempt to assess the economic impact of research
over a finite time horizon encompassing only a small number of major discoveries is bound to be highly unreliable. New tools,
developed in the theory of self-similar and complex systems [#!Dubrulleetal!#] to tackle similar extreme fluctuations in Nature
[#!Mandelbrot!#], can be adapted to measure the economic benefits of research, which is intimately associated to this large
variability.
Received 26 October 1998 and
Received in final form 27 October 1998 相似文献
9.
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
10.
A. Albersdörfer E. Sackmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):663-672
In this paper we study the effect of monovalent and divalent ions on the swelling behavior and viscoelastic parameters of ultrathin layers of the natural polyelectrolyte hyaluronic acid
covalently coupled to glass substrates. A colloidal probe technique is applied for this purpose based on latex beads, hovering over the polymer cushion. By analyzing the vertical Brownian motion of these beads with reflection
interference contrast microscopy (RICM) we determined the equilibrium layer thickness (with 3 nm vertical resolution), the
interfacial interaction potential, and the characteristic mesh size limiting the hydrodynamic flow within the polyelectrolyte
film as a function of the ionic strength. The experimental results are interpreted in terms of three different theoretical
models: the polyelectrolyte brush approximation of Pincus [#!ref1!#], a modified polyelectrolyte brush approximation in the
high salt concentration limit of Ross and Pincus [#!ref2!#] and the simple scaling approximation for neutral adsorbed polymers
of de Gennes [#!ref3!#]. Within experimental error all of these different models fit our experimental data and yield comparable
results for the equilibrium layer thickness. Moreover we determine a thickness dependent, effective surface coverage from
both brush models. The hydrodynamic properties of the films are interpreted in terms of the Brinkmann model of elastic porous
media by assuming an effective mesh size, which depends linearly on the Debye screening length. The salt induced condensation
of the polyelectrolyte films can be described microscopically in terms of a progressive contraction of the mesh size with
increasing salt concentration.
Received 10 September 1998 and Received in final form 30 November 1998 相似文献
11.
Shear break-up of reversible fractal clusters is investigated by ultrasound and multiple light scattering in the low shear
regime. We consider a dense suspension of Rayleigh scatterers (particles or clusters) with acoustic properties close to those
of the surrounding liquid so that the attenuation of the ultrasonic coherent field is weak and multiple scattering is negligible.
The concept of variance in local particle volume fraction is used to derive an original expression of the ultrasound scattering
cross-section per unit volume for Rayleigh fractal clusters. On the basis of a scaling law for the shear break-up of aggregates,
then we derive the shear stress dependence of the ultrasound scattered intensity from a suspension of reversible fractal clusters.
In a second part, we present rheo-acoustical experiments to study the shear break-up of hardened red cell aggregates in plane-plane
flow geometry and we examine both the self consistent field approximation and the scaling laws used in microrheological models.
We further compare the ability of acoustical backscattering and optical reflectometry techniques to estimate the critical
disaggregation shear stress and the particle surface adhesive energy. Finally, the microrheological model from Snabre and
Mills [#!ref5!#] based on a fractal approach is shown to describe the non Newtonian behavior of a dense distribution of hardened
red cell aggregates.
Received 12 November 1998 and Received in final form 17 May 1999 相似文献
12.
A.A. Zvyagin D.M. Apal'kov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):565-569
We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice.
We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg
antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic
field and nonzero temperature onto such a phase transition are studied.
Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998 相似文献
13.
We study the director field around a spherical particle immersed in a uniformly aligned nematic liquid crystal and assume
that the molecules prefer a homeotropic orientation at the surface of the particle. Three structures are possible: a dipole,
a Saturn-ring, and a surface-ring configuration, which we investigate by numerically minimizing the Frank free energy supplemented
by a magnetic-field and a surface term. In the dipole configuration, which is the absolutely stable structure for micron-size
particles and sufficiently strong surface anchoring, a twist transition is found and analyzed. We show that a transition from
the dipole to the Saturn ring configuration is induced by either decreasing the particle size or by applying a magnetic field.
The effect of metastability and the occurrence of hysteresis in connection with a magnetic field are discussed. The surface-ring
configuration appears when the surface-anchoring strength W is reduced. It is also favored by a large saddle-splay constant K24. A comparison with recent experiments [#!itapdb:Poulin1997!#,#!itapdb:Poulin1998!#] gives a lower bound for W, i.e., for the interface of water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium dodecyl sulfate.
Received 2 November 1998 相似文献
14.
M. Hamm M.M. Kozlov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):519-528
We present an alternative model of structure and energetics of the inverted amphiphilic mesophases. The previous studies of
the inverted hexagonal, HII, and inverted micellar cubic, QII, phases considered the amphiphilic monolayers to be homogeneously bent. In contrast, we assume a unit cell of an inverted
mesophase to consist of flat fragments of monolayer. Hence, the unit cells of the HII and QII phases are represented by a hexagonal rod and a polyhedron, respectively. Our model is motivated by Turner and Gruner's X-ray
diffraction reconstruction of structure of the HII phase. The only deformation of the amphiphilic monolayers we consider is tilt of the hydrocarbon chains with respect to the
monolayer surface, determined by the packing constraints imposed in the mesophases. Applying our recent model for the elastic
energy of tilt in liquid membranes [#!ref23!#], we show that: i) tilt accounts in a natural way for the frustration energy
of mesophases resulting from filling by the hydrocarbon chains the corners of the unit cells, ii) the energy of tilt variation
along the membrane surface is analogous to the bending energy. We compute the energetics of the HII, QIIsc and QIIfcc phases and obtain a hypothetical phase diagram in terms of the elastic constants of monolayers. Moreover, we calculate the
structural dimensions of the mesophases. We verify the model showing that the obtained phase diagram describes the recent
data for the glycolipids/water systems; the predicted dimensions of the QII phase are in accord with the measured values; the model treats quantitatively the structural features observed for the HII phase.
Received: 9 February 1998 / Revised: 4 June 1998 / Accepted: 3 July 1998 相似文献
15.
K. Höppner A. Kaprolat W. Schülke 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):53-60
The dynamic structure factor of Li with [110] and 0.88 a.u. <q< 1.03 a.u., as measured with 1 eV resolution by means of synchrotron radiation based inelastic X-ray scattering spectroscopy
(IXSS), exhibits, in the energy loss range between 3 and 12 eV, a fine structure, which appears as a resonance around 4 eV
and an antiresonance around 8 eV, when the difference between the experimental -spectra with [110] and [111] is considered. In order to find out the origin of this fine structure we have interpreted recent TLDA (time dependent
local density approximation) calculations of the Li- [#!ref12!#], which were based on the inversion of the full dielectric matrix, by utilizing a simple two-plasmon-band model.
In this way the fine structure can be traced back to a Fano-like coupling of the discrete collective excitations (both the
regular plasmon and the so-called zone-boundary collective states (ZBCS's)) and the particle-hole excitation continuum, mediated
by the off-diagonal elements of the dielectric matrix, , where (1,1,0).
Received: 29 December 1998 / Accepted: 16 March 1998 相似文献
16.
C. Faure N. Decoster F. Argoul 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):87-97
The aggregation of 2D colloidal crystals can be performed by applying an AC field to a colloidal dispersion. This technique
is used in this work for assembling multilamellar vesicles in suspension. The dynamics of the aggregation is followed by real-time
recording of the pictures of the microsphere assembly through a phase contrast microscope. The influence of both the frequency
and the amplitude of the alternating field on the dynamical evolution of the concentration of layered particles is discussed
with respect to their size. A phenomenological model of double layer induced trapping of the particles is proposed and an
electroconvective instability of the fluid surrounding the particles is suggested from the observation of the local dynamics
of the particles, in accordance with a very recent argument of Yeh et al. [#!Yeh:97!#].
Received: 4 December 1997 / Revised: 24 March 1998 /
Accepted: 4 May 1998 相似文献
17.
P. Schlottmann A.A. Zvyagin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):325-335
We consider the one-dimensional t
-
J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J
=
t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering
matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and
the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations
diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string
hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity
depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron
and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic
susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity.
Received: 31 December 1997 / Accepted: 17 March 1998 相似文献
18.
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
19.
M. Zacchigna J. Almeida I. Vobornik G. Margaritondo D. Malterre B. Malaman M. Grioni 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):463-467
We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds.
The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations.
Received: 13 October 1997 / Accepted: 21 January 1998 相似文献
20.
J.-F. Berret D.C. Roux P. Lindner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):67-77
We have investigated the simple shear flow behavior of wormlike micelles using small-angle neutron scattering and mechanical
measurements. Ternary surfactant solutions made of cetylpyridinium chloride, hexanol and brine (0.2 M NaCl) and hereafter
abbreviated as CPCl-Hex were studied in the concentrated regime, . In a preliminary report (Berret et al. [#!ref16!#]), the discontinuity of slope observed in the shear stress versus shear rate curve was interpreted in terms of first-order phase transition between an isotropic state and a shear-induced
nematic state ( transition). At the transition rate, , the solution exhibits a macroscopic phase separation into viscous and fluid layers (inhomogeneous shear flow). Above a second
characteristic shear rate, the flow becomes homogeneous again, the sheared solution being nematic only. The neutron patterns
obtained in the two-state inhomogeneous region have been re-examined. Based on a consistent analysis of both orientational
and translational degrees of freedom related to the wormlike micelles, we emphasize new features for the transition. In the present paper, the shear rate variations of the relative proportions of each phase in the two-state region,
as well as the viscosity ratio between isotropic and nematic phases are derived. We demonstrate in addition that slightly
above the transition rate, the shear induced nematic phase is already strongly oriented, with an order parameter P
2
= 0.65. The orientational state is that of a nematic flow-oriented monodomain. Finally, from the locations of the neutron
scattering maxima for each isotropic and nematic contributions, we evaluate the concentrations for each phase and and derived a dynamical phase diagram of CPCl-Hex, in terms of the stress
versus
and . According to the classification by Schmitt et al. [#!ref22!#], the transition observed in CPCl-Hex micellar solutions could result from a positive flow-concentration coupling, in agreement
with the observed monotonically increasing shear stress in the two-phase region.
Received: 16 February 1998 / Revised: 18 February 1998 /
Accepted: 24 May 1998 相似文献