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1.
The conformational properties of charge-balanced polyampholytes described by the end-to-end distance or radius of gyration depend on parameters such as the temperature and pH as well as on the detailed charge distribution along the backbone. In this work we present a method to determine the charge distribution along a semi-stiff polyampholyte backbone which will result in a thermodynamically stable structure for the compactness of interest, from several loops to an uncoiled structure, performed in a single computer experiment. 相似文献
2.
N. Lee S. Obukhov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):371-376
We study the configurational statistics of a ring polyampholyte chain made of N randomly charged monomers with elementary charge .To a large extent, the overall structure of a polyampholyte is controlled by a total sum Q of all charges. When the total charge is smaller than , the polyampholyte has a compact globular structure. At charges larger than , the configuration has the form of a ring of small globules (beads) connected by strings. Between Q1 and Q2 we find a remarkable diversity of meta-stable configurations having the shapes of irregular clusters of small globules connected
by the strings. We estimate the number of these configurations and the energy barriers between them. Between Q1 and Q2, the minimum energy configurations are completely controlled by randomness in the charge distribution along the chain. There
are hysteresis effects in the shapes of the clusters. As the total charge increases, the linearly extended configurations
become dynamically more preferable. When the charge decreases, the circular shapes are preferred. We remark on the probable
connection with the multiple phase transitions observed in polyampholyte gels.
Received: 10 July 1997 / Accepted: 13 October 1997 相似文献
3.
E. Zhulina A.V. Dobrynin M. Rubinstein 《The European physical journal. E, Soft matter》2001,5(1):41-49
We develop a scaling theory for a single polyampholyte chain adsorbed on a charged spherical particle in a theta-solvent.
Adsorption of a polyampholyte molecule is due to its polarization in the electrostatic field of the particle. For large particles
with sizes exceeding the thickness of the adsorbed layer, the conformations of the chain are similar to the one found for
polyampholyte adsorption on charged planar surface. However, an adsorbed polyampholyte chain forms a self-similar flower-like
structure near the particles with sizes smaller than its Gaussian size. These self-similar structures result from the balance
of the polarization energy of loops and the excluded volume interactions between monomers. The structure of an adsorbed polyampholyte
in the flower-like conformation is similar to that of a neutral star polymer.
Received 3 March 2000 and Received in final form 5 July 2000 相似文献
4.
Rzehak R 《The European physical journal. E, Soft matter》2003,11(4):335-348
The decay of correlations in the conformational fluctuations of a tethered polymer subjected to a uniform flow is analyzed in terms of relaxation times and associated normal modes. These quantities are calculated numerically from Brownian dynamics simulations of several bead spring polymer models. In this way, the influence of different effects like a finite extensibility of the springs and excluded-volume as well as hydrodynamic interactions between the beads on the decay of fluctuations is identified. Moreover, by comparison of the simulation results to analytically tractable blob models with corresponding assumptions, the capability of the tensile-blob picture to predict relaxation times and modes is assessed. For excluded-volume and hydrodynamic interactions a crossover to Rouse-like behavior occurs when the flow velocity and hence the polymer deformation exceeds a certain value. For finitely extensible springs, in contrast, the relaxation times decrease monotonically with increasing polymer deformation. This latter behavior differs from assumptions often used in rheological modeling by dumbbells and is not captured by the blob model.Received: 4 April 2003, Published online: 12 August 2003PACS:
83.80.Rs Polymer solutions - 83.10.Mj Molecular dynamics, Brownian dynamics - 36.20.Ey Conformation (statistics and dynamics) - 47.50. + d Non-Newtonian fluid flows 相似文献
5.
Cocco S Marko JF Monasson R Sarkar A Yan J 《The European physical journal. E, Soft matter》2003,10(3):249-263
The elastic response of flexible polymers made of elements which can be either folded or unfolded, having different lengths
in these two states, is discussed. These situations are common for biopolymers as a result of folding interactions intrinsic
to the monomers, or as a result of binding of other smaller molecules along the polymer length. Using simple flexible-chain
models, we show that even when the energy ε associated with maintaining the folded state is comparable to k
B
T, the elastic response of such a chain can mimic usual polymer linear elasticity, but with a force scale enhanced above that
expected from the flexibility of the chain backbone. We discuss recent experiments on single-stranded DNA, chromatin fiber
and double-stranded DNA with proteins weakly absorbed along its length which show this effect. Effects of polymer semiflexiblity
and torsional stiffness relevant to experiments on proteins binding to dsDNA are analyzed. We finally discuss the competition
between electrostatic self-repulsion and folding interactions responsible for the complex elastic response of single-stranded
DNA.
Received 7 August 2002 and Received in final form 7 March 2003 / Published online: 15 April 2003
RID="a"
ID="a"e-mail: jmarko@uic.edu 相似文献
7.
R. Mélin K. Biljaković J.C. Lasjaunias P. Monceau 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(4):417-430
We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon.
The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that
there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective
phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain
an estimate of the glass cross-over temperature (typically T
g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving
randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and
can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising
model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T
g/T). The glass cross-over temperature Tg related to correlations among solitons is equal to the average energy barrier and scales like T
g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap.
Received 12 December 2001 相似文献
8.
Single- and double-stranded DNA and many biological and synthetic polyelectrolytes undergo two structural transitions upon
increasing the concentration of multivalent salt or molecules. First, the expanded-stretched chains in low monovalent salt
solutions collapse into nearly neutral compact structures when the density of multivalent salt approaches that of the monomers.
With further addition of multivalent salt the chains redissolve acquiring expanded-coiled conformations. We study the redissolution
transition using a two-state model (F.J. Solis, M. Olvera de la Cruz, J. Chem. Phys. 112, 2030 (2000)). The redissolution occurs when there is a high degree of screening of the electrostatic interactions between
monomers, thus reducing the energy of the expanded state. The transition is determined by the chemical potential of the multivalent
ions in the solution, μ and the inverse screening length, κ. The transition point also depends on the charge distribution
along the chain but is nearly independent of the molecular weight and degree of flexibility of the polyelectrolytes. We generate
a diagram of μversusκ2 where we find two regions of expanded conformations, one with charged chains and the other with overcharged (inverted charge)
chains, separated by a collapsed nearly neutral conformation region. The collapse and redissolution transitions occur when
the trajectory of the properties of the salt crosses the boundaries between these regions. We find that in most cases the
redissolution occurs within the same expanded branch from which the chain precipitates.
Received 15 May 2000 and Received in final form 28 June 2000 相似文献
9.
P. Cacciani J. Cosléou F. Herlemont M. Khelkhal J. Legrand 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,22(2):199-207
When a gas sample of 13CH3F is prepared with a population of isomers (ortho and para forms) far from the equilibrium given by nuclear spin statistics,
it relaxes towards this equilibrium with an exponential decay rate. This phenomenon called nuclear spin conversion is mainly
governed by intramolecular spin-spin and spin-rotation interactions. In the quantum relaxation model [P.L. Chapovsky, Phys.
Rev. A 43, 3624 (1991)], two pairs of ortho-para levels (J
= 9, K
= 3; J'
= 11, K'
= 1) and (J
= 20, K
= 3; J'
= 21, K'
= 1) are principally responsible for the conversion. The levels of the second pair are coupled by both spin-spin and spin-rotation
interactions. The application of an electric field (up to 10 kV/cm) induces a crossing of the Stark components of this pair,
which is observed for the first time. A specific experimental set-up based on an electric field of alternating triangular
shape is used, which allows the determination of the strength of both interactions via the measurement of the spin conversion decay rates. This work yields the first experimental value for the electronic contribution
to the spin-rotation interaction in 13CH3F.
Received 23 May 2002 / Received in final form 18 September 2002 Published online 21 January 2003
RID="a"
ID="a"e-mail: Patrice.Cacciani@univ-lille1.fr 相似文献
10.
S.V. Dordevic D.N. Basov R.C. Dynes B. Ruzicka V. Vescoli L. Degiorgi H. Berger R. Gaál L. Forró E. Bucher 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):15-23
We present a comprehensive analysis of the optical constants of the two-dimensional dichalcogenide materials 2
H
-
TaSe
2
and 2
H
-
NbSe
2
, in an attempt to address the physics of two-dimensional correlated systems. The title compounds were studied over several
decades in frequency, from the far-infrared to the ultraviolet. Measurements with linearly polarized light have allowed us
to obtain both the in-plane and out-of-plane components of the conductivity tensor. Although the electromagnetic response
of dichalcogenides is strongly anisotropic, both the in-plane and out-of-plane components of the conductivity tensor share
many common features, including the presence of a well-defined metallic component, as well as a “mid-infrared band”. We discuss
the implications of these results in the context of the spectroscopic results of other classes of low-dimensional conductors
such as the high-temperature superconducting cuprates. In particular, the analysis of the redistribution of the spectral weight
as a function of temperature, as well as the behavior of the quasiparticles relaxation rate, points to significant distinctions
between the charge dynamics of dichalcogenides and other classes of low dimensional conductors.
Received 28 October 2002 / Received in final form 10 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: degiorgi@solid.phys.ethz.ch 相似文献
11.
M. Treiber D. Helbing 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,68(4):607-618
This contribution presents a derivation of the steady-state distribution of velocities and
distances of driven particles on a onedimensional periodic ring, using a Fokker-Planck formalism. We will compare two different
situations: (i) symmetrical interaction forces fulfilling Newton’s law
of “actio = reactio” and (ii) asymmetric, forwardly directed interactions as, for example
in vehicular traffic. Surprisingly, the steady-state velocity and distance distributions
for asymmetric interactions and driving terms agree with the equilibrium distributions of
classical many-particle systems with symmetrical interactions, if the system is large enough.
This analytical result is confirmed by computer simulations and
establishes the possibility of approximating the steady state
statistics in driven many-particle systems by Hamiltonian systems. Our finding is also
useful to understand the various departure time distributions of queueing systems as a possible
effect of interactions among the elements in the respective queue [Physica A 363, 62 (2006)]. 相似文献
12.
We analyze the deviations from Maxwell-Boltzmann statistics found in recent experiments studying velocity distributions in
two-dimensional granular gases driven into a non-equilibrium stationary state by a strong vertical vibration. We show that
in its simplest version, the “stochastic thermostat” model of heated inelastic hard spheres, contrary to what has been hitherto
stated, is incompatible with the experimental data, although predicting a reminiscent high-velocity stretched-exponential
behavior with an exponent 3/2. The experimental observations lead to refine a recently proposed random restitution coefficient
model. Very good agreement is then found with experimental velocity distributions within this framework, which appears self-consistent
and further provides relevant probes to investigate the universality of the velocity statistics.
Received: 27 May 2002 / Accepted: 6 May 2003 / Published online: 27 May 2003
RID="a"
ID="a"e-mail: Alain.Barrat@th.u-psud.fr 相似文献
13.
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
14.
15.
I. Titvinidze G.I. Japaridze 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(3):383-393
The quantum phase transition in the ground state of the extended spin S
= 1/2 XY model has been studied in detail. Using the exact solution of the model the low temperature thermodynamics, as well as the
ground state phase diagram of the model in the presence of applied uniform and/or staggered magnetic field are discussed.
Received 29 November 2002 / Received in final form 24 February 2003 Published online 11 April 2003
RID="a"
ID="a"e-mail: japa@iph.hepi.edu.ge 相似文献
16.
R.R. Netz H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):81-98
Variational methods are applied to a single polyelectrolyte chain. The polymer is modeled as a Gaussian chain with screened
electrostatic repulsion between all monomers. As a variational Hamiltonian, the most general Gaussian kernel, including the
possibility of a classical or mean polymer path, is employed. The resulting self-consistent equations are systematically solved
both for large and small monomer-monomer separations along the chain. In the absence of screening, the polymer is stretched
on average. It is described by a straight classical path with Gaussian fluctuations around it. If the electrostatic repulsion
is screened, the polymer is isotropically swollen for large separations, and for small separations the polymer correlation
function is calculated as an analytic expansion in terms of the monomer-monomer separation along the chain. The electrostatic
persistence length and the electrostatic blobsize are inferred from the crossover between distinct scaling ranges. We perform
a global analysis of the scaling behavior as a function of the screening length and electrostatic interaction strength , where is the Bjerrum length and A is the distance of charges along the polymer chain. We find three different scaling regimes. i) A Gaussian-persistent regime with Gaussian behavior at small, persistent behavior at intermediate, and isotropically swollen behavior at large length
scales. This regime occurs for weakly charged polymers and only for intermediate values of the screening length. The electrostatic
persistence length is defined as the crossover length between the persistent and the asymptotically swollen behavior and is given by and thus disagrees with previous (restricted) variational treatments which predict a linear dependence on the screening length
.ii) A Gaussian regime with Gaussian behavior at small and isotropically swollen behavior at large length scales. This regime occurs for weakly
charged polymers and/or strong screening, and the electrostatic repulsion between monomers only leads to subfluent corrections
to Gaussian scaling at small separations. The concept of a persistence length is without meaning in this regime. iii) A persistent regime , where the chain resembles a stretched rod on intermediate and small scales. Here the persistence length is given by the
original Odijk prediction, , if the overstretching of the chain is avoided. We also investigate the effects of a finite polymer length and of an additional
excluded-volume interaction, which modify the resultant scaling behavior. Applications to experiments and computer simulations
are discussed.
Received 24 December 1997 相似文献
17.
G. De Smedt C. Godrèche J.M. Luck 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(2):215-225
We consider a ferromagnetic Ising chain evolving under Kawasaki dynamics at zero temperature. We investigate the statistics
of the blocking time, as well as various characteristics of the metastable configurations reached by the system, including
the statistics of the final energy, the spin correlations, and the distribution of domain sizes. Results of extensive numerical
simulations are compared with analytical predictions made for the a priori ensemble of all blocked configurations with equal weights. Qualitative differences are found, e.g. in the domain sizes, which are found to be neither statistically independent nor exponentially distributed.
Received 24 October 2002 / Received in final form 13 January 2003 Published online 1st April 2003
RID="a"
ID="a"e-mail: luck@spht.saclay.cea.fr
RID="b"
ID="b"URA 2306 of CNRS 相似文献
18.
Jin Liu Moran Wang Shiyi Chen Mark O. Robbins 《Journal of computational physics》2010,229(20):7834-7847
A highly efficient molecular dynamics algorithm for micro and nanoscale electrokinetic flows is developed. The long-range Coulomb interactions are calculated using the Particle–Particle Particle–Mesh (P3M) approach. The Poisson equation for the electrostatic potential is solved in physical space using an iterative multi-grid technique. After validation, the method is used to study electroosmotic flow in nanochannels with regular or random roughness on the walls. The results show that roughness reduces the electroosmotic flow rate dramatically even though the roughness is very small compared to the channel width. The effect is much larger than for pressure driven flows because the driving force is localized near the walls where the charge distribution is high. Non-Newtonian behavior is also observed at much lower flow rates. Systematic investigation of the effect of surface charge density and random roughness will help to better understand the mechanism of electrokinetic transport in rough nanochannels and to design and optimize nanofluidic devices. 相似文献
19.
R. Citro S. Cojocaru M. Marinaro 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(2):179-185
Phonon anomalies observed in various high Tc cuprates are
analyzed theoretically within the Hubbard-Holstein model in the
limit of strong local electron correlations and in presence of
long-range Coulomb interaction. The phonon self-energy is
evaluated by taking into account the charge collective modes that
become critical upon doping approaching an instability towards an
incommensurate charge density wave (ICDW) driven by electron
correlations. The doping dependence of phonon softening features
and the highly distinctive phonon self-energy dependence on the
wave vector agree with experiments. We discuss relevance of
dynamical corrections to the density correlation function to
achieve a sizeable bond-stretching phonon softening with a
kink-like profile away from the zone boundary. 相似文献
20.
E. Buixaderas V. Porokhonskyy A. Paskhin M. Savinov J. Petzelt 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(3):319-329
Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF
coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 102-1014 Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T
c
= 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear
on cooling in the ferroelectric phase. Below T
i
the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling
for the
field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman
active B2-symmetry soft mode.
Received 24 August 2002 Published online 19 December 2002
RID="a"
ID="a"e-mail: buixader@fzu.cz 相似文献