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1.
We revisit the problem of a two-dimensional polymer ring subject to an inflating pressure differential. The ring is modeled
as a freely jointed closed chain of N monomers. Using a Flory argument, mean-field calculation and Monte Carlo simulations, we show that at a critical pressure,
pc ∼ N-1, the ring undergoes a second-order phase transition from a crumpled, random-walk state, where its mean area scales as 〈A〉 ∼ N, to a smooth state with 〈A〉 ∼ N2. The transition belongs to the mean-field universality class. At the critical point a new state of polymer statistics is
found, in which 〈A〉 ∼ N3/2. For p ≫ pc we use a transfer-matrix calculation to derive exact expressions for the properties of the smooth state. 相似文献
2.
S. T. T. Ollila K. F. Luo T. Ala-Nissila S. -C. Ying 《The European physical journal. E, Soft matter》2009,28(4):385-393
Using Langevin dynamics simulations, we investigate the translocation dynamics of an externally driven polymer chain through
a nanopore, where a pulling force F is exerted on the first monomer whilst there is an opposing force F
E < F within the pore. Such a double-force arrangement has been proposed recently to allow better dynamical control of the translocation
process in order to sequence biopolymers. We find that in the double-force arrangement translocation becomes slower as compared
to the case under a single monomer pulling force of magnitude F - F
E , but scaling of the translocation time as a function of the chain length ∼ N
2 does not change. The waiting time (m) for monomer m to exit the pore is found to be a monotonically increasing function of the bead number almost until m
N , which indicates relatively well-defined slowing down and control of the chain velocity during translocation. We also study
the waiting time distributions for the beads in the chain, and characterize in detail fluctuations in the bead positions and
their transverse position coordinates during translocation. These data should be useful in estimating position-dependent sequencing
errors in double-force experiments. 相似文献
3.
We present an analytical approach of the dynamics of a polymer when it is quenched from a solvent into a good or bad solvent. The dynamics is studied by means of a Langevin equation, first in the absence of hydrodynamic
effect, then taking into account the hydrodynamic interactions with the solvent. The variation of the radius of gyration is
studied as a function of time. In both cases, for the first stage of collapse or swelling, the evolution is described by a
power law with a characteristic time proportional to N
4/3
(N), where N is the number of monomers, without (with) hydrodynamic interactions. At larger times, scaling laws are derived for the diffusive
relaxation time.
Received: 10 March 1998 / Received in final form: 15 September 1998 / Accepted: 25 September 1998 相似文献
4.
O.V. Bychuk B. O'Shaughnessy N.J. Turro 《The European physical journal. E, Soft matter》2001,4(3):281-291
We propose and theoretically study an experiment designed to measure short-time polymer reaction kinetics in melts or dilute
solutions. The photolysis of groups centrally located along chain backbones, one group per chain, creates pairs of spatially
highly correlated macroradicals. We calculate time-dependent rate coefficients κ(t) governing their first-order recombination kinetics, which are novel on account of the far-from-equilibrium initial conditions.
In dilute solutions (good solvents) reaction kinetics are intrinsically weak, despite the highly reactive radical groups involved.
This leads to a generalised mean-field kinetics in which the rate of radical density decay - ∼S(t), where S(t) ∼t
- (1 + g/3) is the equilibrium return probability for 2 reactive groups, given initial contact. Here g≈ 0.27 is the correlation hole exponent for self-avoiding chain ends. For times beyond the longest coil relaxation time τ,
- ∼S(t) remains true, but center of gravity coil diffusion takes over with rms displacement of reactive groups x(t) ∼t
1/2 and S(t) ∼ 1/x
3(t). At the shortest times ( t
10-6s), recombination is inhibited due to spin selection rules and we find ∼tS(t). In melts, kinetics are intrinsically diffusion-controlled, leading to entirely different rate laws. During the regime limited
by spin selection rules, the density of radicals decays linearly, n(0) - n(t) ∼t. At longer times the standard result - ∼d
3(t)/d (for randomly distributed ends) is replaced by ∼d2x
3(t)/d
2 for these correlated initial conditions. The long-time behavior, t > τ, has the same scaling form in time as for dilute solutions.
Received 18 May 2000 相似文献
5.
The mean area of a two-dimensional Gaussian ring of N monomers is known to diverge when the ring is subject to a critical pressure differential, p
c ∼ N
-1. In a recent publication (Eur. Phys. J. E 19, 461 (2006)) we have shown that for an inextensible freely jointed ring this divergence turns into a second-order transition
from a crumpled state, where the mean area scales as 〈A〉 ∼ N, to a smooth state with 〈A〉 ∼ N
2. In the current work we extend these two models to the case where the swelling of the ring is caused by trapped ideal-gas
particles. The Gaussian model is solved exactly, and the freely jointed one is treated using a Flory argument, mean-field
theory, and Monte Carlo simulations. For a fixed number Q of trapped particles the criticality disappears in both models through an unusual mechanism, arising from the absence of
an area constraint. In the Gaussian case the ring swells to such a mean area, 〈A〉 ∼ NQ, that the pressure exerted by the particles is at p
c for any Q. In the freely jointed model the mean area is such that the particle pressure is always higher than p
c, and 〈A〉 consequently follows a single scaling law, 〈A〉 ∼ N
2
f (Q/N), for any Q. By contrast, when the particles are in contact with a reservoir of fixed chemical potential, the criticality is retained.
Thus, the two ensembles are manifestly inequivalent in these systems.
An erratum to this article is available at . 相似文献
6.
We consider a low-density assembly of spherical colloids, such that each is clothed by L end-grafted chemically incompatible polymer chains either of types A or B. These are assumed to be dissolved in a good common solvent. We assume that colloids are of small size to be considered as
star-polymers. Two adjacent star-polymers A and B interact through a force F originating from both excluded-volume effects and chemical mismatch between unlike monomers. Using a method developed by
Witten and Pincus (Macromolecules 19, 2509 (1986)) in the context of star-polymers of the same chemical nature, we determine exactly the force F as a function of the center-to-center distance h. We find that this force is the sum of two contributions F
e and F
s. The former, that results from the excluded volume, decays as F
e∼A
L
h
-1, with the L -dependent universal amplitude A
L∼L
3/2. While the second, which comes from the chemical mismatch, decays more slowly as F
s∼χB
L
h
-1 - τ, where τ is a critical exponent whose value is found to be τ 0.40, and χ is the standard Flory interaction parameter. We find that the corresponding L-dependent universal amplitude is B
L∼L
3 + τ
/2. Theses forces are comparable near the cores of two adjacent star-polymers, i.e. for h∼h
c∼a
(a is the monomer size). Finally, for two star-polymers of the same chemical nature (A or B), the force F that simply results from excluded-volume effects coincides exactly with F
e, and then the known result is recovered.
Received 2 October 2000 and Received in final form 24 January 2001 相似文献
7.
8.
Marko JF 《Journal of statistical physics》2011,142(6):1353-1370
Scaling laws for Gauss linking number Ca and writhing number Wr for spherically confined flexible polymers with thermally
fluctuating topology are analyzed. For ideal (phantom) polymers each of N segments of length unity confined to a spherical pore of radius R there are two scaling regimes: for sufficiently weak confinement (R≫N
1/3) each chain has |Wr|≈N
1/2, and each pair of chains has average |Ca|≈N/R
3/2; alternately for sufficiently tight confinement (N
1/3≫R), |Wr|≈|Ca|≈N/R
3/2. Adding segment-segment avoidance modifies this result: for n chains with excluded volume interactions |Ca|≈(N/n)1/2
f(φ) where f is a scaling function that depends approximately linearly on the segment concentration φ=nN/R
3. Scaling results for writhe are used to estimate the maximum writhe of a polymer; this is demonstrated to be realizable through
a writhing instability that occurs for a polymer which is able to change knotting topology and which is subject to an applied
torque. Finally, scaling results for linking are used to estimate bounds on the entanglement complexity of long chromosomal
DNA molecules inside cells, and to show how “lengthwise” chromosome condensation can suppress DNA entanglement. 相似文献
9.
Loktionov E. Yu. Ovchinnikov A. V. Protasov Yu. S. Protasov Yu. Yu. Sitnikov D. S. 《Optics and Spectroscopy》2012,112(4):631-637
The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy
efficiency, of the laser ablation of flat polymer targets ((C2F4)
n
, (CH2O)
n
) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs)
of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I
0 up to 1015 W/cm2) under normal atmospheric and vacuum (p ∼ 10−4 mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation. 相似文献
10.
Signature of chaos in power spectrum 总被引:1,自引:0,他引:1
We investigate the nature of the numerically computed power spectral densityP(f, N, τ) of a discrete (sampling time interval,τ) and finite (length,N) scalar time series extracted from a continuous time chaotic dynamical system. We highlight howP(f, N, τ) differs from the true power spectrum and from the power spectrum of a general stochastic process. Non-zeroτ leads to aliasing;P(f, N, τ) decays at high frequencies as [πτ/sinπτf]2, which is an aliased form of the 1/f
2 decay. This power law tail seems to be a characteristic feature of all continuous time dynamical systems, chaotic or otherwise.
Also the tail vanishes in the limit ofN → ∞, implying that the true power spectral density must be band width limited. In striking contrast the power spectrum of
a stochastic process is dominated by a term independent of the length of the time series at all frequencies. 相似文献
11.
Plasma parameters in the upgraded Trimyx-M Galathea 总被引:1,自引:0,他引:1
A. I. Morozov A. I. Bugrova A. M. Bishaev A. S. Lipatov M. V. Kozintseva 《Technical Physics》2007,52(12):1546-1551
Results are presented from measurements of the plasma parameters in the upgraded Trimyx-M Galathea. After the barrier magnetic
field and the energy of the injected hydrogen plasma bunch were increased to B
bar ∼ 0.1 T and W
0 ≈ 200 J, respectively, the following plasma parameters were achieved: the density n ∼ 5 × 1013 cm−3, the plasma confinement time τ* = 800–900 μs, the elergy of the confined plasma W
1 ∼ 100 J, the ratio of the plasma pressure to the barrier magnetic pressure β
0 ∼ 0.2, the electron temperature T
e
∼ 20 eV, and the ion temperature T
i
∼ 2T
e
. The maximum time during which the plasma density decreased e-fold, τ
p
, was found to be 300 μs at B
bar = 0.1 T, which agrees with the classical transport model. 相似文献
12.
A microscopic theory of the Efetov supermatrix sigma-model type is constructed for the low-lying electron states in a mixed
superconductive-normal system with disorder. This technique is used for the study of the localized states in the core of a
vortex in a moderately clean superconductor with τ
−1≫ω
0∼Δ2/E
F
. At low energies ε≪ω
Th∼ (ω
0/τ)1/2, the energy level statistics is described by the “zero-dimensional” limit of this supermatrix theory, and the result for
the density of states is equivalent to that obtained within Altland-Zirnbauer random matrix model. Nonzero modes of the sigma
model increase the mean interlevel distance by the relative amount [2 ln (1/ω
0
τ)]−1.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 78–83 (10 July 1998)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
13.
Studies of a classical III–V semiconductor (InSb) doped with 3d magnetic ions (Mn2+, having a localized spin S=55/2) reveal some unexpected transport properties. It is found that the transition from the metallic to the low-temperature
insulator phase occurs at an impurity concentration N
Mn∼N
cr=2× 1017 cm−3 and a temperature T<T
cr∼1 K. Under these conditions a giant negative magnetoresistance arises. The experimental results can be explained in terms
of the onset of a hard Mott-Hubbard gap Δ in the impurity band formed by the shallow manganese acceptor in InSb at N
Mn∼N
cr. A model describing the gap formation is proposed.
Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 358–362 (10 March 1999)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
14.
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
15.
The electrical and luminescent characteristics of a barrier-discharge lamp filled with a Kr/CCl4 (150: 1) mixture are experimentally studied versus the value of pd, which varies in the range (7.6–14) × 103 Pa cm. When simulating the gas discharge using similarity parameters, the following relationships are fulfilled: for pd = const (p is the pressure, d is the interelectrode distance), the pulse duration and the mean current density are τ
j
∼ 1/p and 〈j〉 ∼ p; the surface charge density on the electrodes, σ ∼ const; the duration of the UV radiation pulse and the efficiency of UV
radiation due to a KrCl* (222 nm) exciplex, τrad ∼ 1/p and η ∼ p
2. The maximal radiation efficiency achieved in the experiments is about 13%. Deviations from the similarity laws for the gas
discharge are related to the filamentary form of the observed discharge. Qualitative analysis indicates that similarity laws
may be fulfilled for such a form of discharge as well but locally, within a single filament. 相似文献
16.
In numerical simulations of nonabelian plasma instabilities in the hard-loop approximation, a turbulent spectrum has been
observed that is characterized by a phase-space density of particles n(p)∼p
−ν
with exponent ν≃2, which is larger than expected from relativistic 2↔2 scatterings. Using the approach of Zakharov, L’vov and Falkovich,
we analyze possible Kolmogorov coefficients for relativistic (m≥4)-particle processes, which give at most ν=5/3 perturbatively for an energy cascade. We discuss non-perturbative scenarios which lead to larger values. As an extreme
limit we find the result ν=5 generically in an inherently non-perturbative effective field theory situation, which coincides with results obtained by
Berges et al. in large-N scalar field theory. If we instead assume that scaling behavior is determined by Schwinger–Dyson resummations such that the
different scaling of bare and dressed vertices matters, we find that intermediate values are possible. We present one simple
scenario, which would single out ν=2. 相似文献
17.
The dynamics of a flat isotropic brane Universe with two-component matter source —perfect fluid with the equation of statep = (γ − 1)ρ and a scalar field with a power-law potentialV ∼ φα is investigated. We describe solutions for which the scalar field energy density scales as a power-law of the scale factor.
We also describe solutions existing in regions of the parameter space where these scaling solutions are unstable or do not
exist. 相似文献
18.
Using Brownian hydrodynamic simulation techniques, we study single polymers in shear. We investigate the effects of hydrodynamic
interactions, excluded volume, chain extensibility, chain length and semiflexibility. The well-known stretching behavior with
increasing shear rate [(g)\dot] \dot{{\gamma}} is only observed for low shear [(g)\dot] \dot{{\gamma}} < [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} , where [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} is the shear rate at maximum polymer extension. For intermediate shear rates [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} < [(g)\dot] \dot{{\gamma}} < [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} the radius of gyration decreases with increasing shear with minimum chain extension at [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} . For even higher shear [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} < [(g)\dot] \dot{{\gamma}} the chain exhibits again shear stretching. This non-monotonic stretching behavior is obtained in the presence of excluded-volume
and hydrodynamic interactions for sufficiently long and inextensible flexible polymers, while it is completely absent for
Gaussian extensible chains. We establish the heuristic scaling laws [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} ∼ N
-1.4 and [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} ∼ N
0.7 as a function of chain length N , which implies that the regime of shear-induced chain compression widens with increasing chain length. These scaling laws
also imply that the chain response at high shear rates is not a universal function of the Weissenberg number Wi = [(g)\dot] \dot{{\gamma}}
t \tau anymore, where t \tau is the equilibrium relaxation time. For semiflexible polymers a similar non-monotonic stretching response is obtained. By
extrapolating the simulation results to lengths corresponding to experimentally studied DNA molecules, we find that the shear
rate [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} to reach the compression regime is experimentally realizable. 相似文献
19.
S. N. Nakamura D. Tomono Y. Matsuda G. Mason M. Iwasaki K. Ishida T. Matsuzaki I. Watanabe S. Sakamoto K. Nagamine 《Hyperfine Interactions》2001,138(1-4):445-450
The lifetime of the positive muon (τμ
+) can be directly associated with the Fermi Coupling Constant (G
F
), which is one of the most basic parameters of the Standard Model. However, the current experimental accuracy of the τμ
+ is ∼30 ppm and it has not been improved for more than 15 years. We propose a new experiment for a pulsed muon facility such
as RIKEN-RAL to measure the muon lifetime with multi-decay per one time window method. The advantage of our setup, no time
window limitation, enables us to test the exponential decay law (EDL) in the long decay time region at the same time. The
preliminary analysis set a new upperlimit for the EDL deviation in the muon decay. We accumulated ∼1010 muon decays and analysis is in progress.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
The scaling of the energy of ion beams (beamlets) in resonance regions of the low-altitude plasma sheet boundary layer has
been analyzed using the measurements made on the Interball-2 and Cluster satellites at distances of 3.0 to 6.0 Earth’s radii
and numerical simulations of the acceleration of ions in the current sheet of the Earth’s magnetotail. The experimental test
of the previously theoretically predicted scaling W
N
∼ N
A
(where W
N
is the energy at the Nth resonance and A ∼ 1.33) shows that the real scaling of resonance energies varies in a wide range A ∈ [0.61, 1.75] and is independent of the geomagnetic indices K
p
and AE. Model calculations with allowance for an electric field E
z
perpendicular to the current sheet are in good agreement with the experimental data. They indicate that the scaling increases
in the case of the dominance of the ion current and decreases in the case of the dominance of the electron current (A > 1.33 and A < 1.33, respectively). 相似文献