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991.
M. Eryürek M. H. Güven 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,48(2):221-228
The solid-liquid phase transitions of Lennard-Jones clusters LJN
(N=39–55) were simulated by a microcanonical molecular dynamics method using
Lennard-Jones potential, and their thermodynamic quantities were calculated.
The caloric curves of clusters (except N=42) have S-bend. To understand this
behaviour, configurational and total entropies were evaluated, and dents on
the entropy curves were taken as a sign of negative heat capacity. The heat
capacities were evaluated for N=39–55 clusters using configurational entropy
data. The potential energy distributions have bimodal behaviour for all
clusters in the given range at the melting temperature. The distinct melting
behaviour of LJ42 was explained by the topology of the potential energy
surface by examining the isomer distributions at phase transitions for
LJ39-LJ55. The isomer distributions were found to be a useful way
to interpret this behaviour and melting dynamics in general. Melting
temperature, latent heat and entropy change upon melting values were
reported and are consistent with literature values and values calculated
from bulk thermodynamic properties. The dependence of these quantities on
the size of the clusters was examined and it is found that latent heat is
the key quantity to determine the magic numbers. 相似文献
992.
5- and 6-pulse electron spin echo envelope modulation (ESEEM) of multi-nuclear spin systems 总被引:1,自引:1,他引:0
In 3-pulse ESEEM and the original 4-pulse HYSCORE, nuclei with large modulation depth (k approximately 1) suppress spectral peaks from nuclei with weak modulations (k approximately 0). This cross suppression can impede the detection of the latter nuclei, which are often the ones of interest. We show that two extended pulse sequences, 5-pulse ESEEM and 6-pulse HYSCORE, can be used as experimental alternatives that suffer less strongly from the cross suppression and allow to recover signals of k approximately 0 nuclei in the presence of k approximately 1 nuclei. In the extended sequences, modulations from k approximately 0 nuclei are strongly enhanced. In addition, multi-quantum transitions are absent which simplifies the spectra. General analytical expressions for the modulation signals in these sequences are derived and discussed. Numerical simulations and experimental spectra that demonstrate the higher sensitivity of the extended pulse sequences are presented. 相似文献
993.
Y. Safa D. Pfenniger 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(3):337-352
The equation of state and the stability of the helium-molecular hydrogen mixture at cryogenic temperature up to moderate pressure
are studied by means of current molecular physics methods and statistical mechanics perturbation theory. The phase separation,
segregation and hetero-coordination are investigated by calculating the Gibbs energy depending on the mixture composition,
pressure and temperature. Low temperature quantum effects are incorporated via cumulant approximations of the Wigner-Kirkwood
expansion. The interaction between He and H2 is determined by Double Yukawa potentials. The equation of state is derived from the hard sphere system by using the scaled
particle theory. The behavior of the mixture over a wide range of pressure is explored with the excess Gibbs energy of mixing
and the concentration fluctuations in the long wavelength limit. The theory is compared to cryogenic data and Monte-Carlo
calculation predictions. Contrary to previous similar works, the present theory retrieves the main features of the mixture
below 50 K, such as the critical point and the condensation-freezing curve, and is found to be usable well below 50 K. However,
the method does not distinguish the liquid from the solid phase. The binary mixture is found to be unstable against species
separation at low temperature and low pressure corresponding to very cold interstellar medium conditions, essentially because
H2 alone condenses at very low pressure and temperature, contrary to helium. 相似文献
994.
We present a theory for the phase behaviour of mixtures of charge-stabilised colloidal spheres plus interacting polymer chains
in good and θ -solvents within the framework of free-volume theory. We use simple but accurate combination rules for the depletion
thickness around a colloidal particle and for the osmotic pressure up to the semi-dilute concentration regime. Hence, we obtain
expressions for the free energy for mixtures of charged colloidal particles and non-adsorbing interacting polymers. From that,
we calculate the phase behaviour, and discuss its topology in dependence on the competition between the charge-induced repulsion
and the polymer-induced attraction. The homogeneous mixture of colloids and polymers becomes more stabilised against demixing
when increasing the electrostatic repulsion. This charge-induced stabilisation is strongest for small polymer-to-colloid size
ratios and is more pronounced for charged colloids mixed with polymers in a good solvent than for polymers in a θ -solvent.
For the weakly charged regime we find that the phase diagram becomes salt-concentration-independent in the protein limit for
charged colloids plus polymers in a θ -solvent. The liquid window, i.e., the concentration regimes where a colloidal liquid exists, is narrowed down upon increasing the charge-induced repulsion.
Also this effect is more pronounced when charged colloids are mixed with polymer chains in a good solvent. In summary, we
demonstrate that the solvent quality significantly influences the phase behaviour of mixtures of charged colloids plus non-adsorbing
polymers if the range of the screened electrostatic repulsion becomes of the order of the range of the depletion-induced attraction. 相似文献
995.
We consider a crosslinked polymer blend that may undergo a microphase separation. When the temperature is changed from an
initial value towards a final one very close to the spinodal point, the mixture is out equilibrium. The aim is the study of
dynamics at a given time t, before the system reaches its final equilibrium state. The dynamics is investigated through the structure factor, S(q, t), which is a function of the wave vector q, temperature T, time t, and reticulation dose D. To determine the phase behavior of this dynamic structure factor, we start from a generalized Langevin equation (model C) solved by the time composition fluctuation. Beside the standard de Gennes Hamiltonian, this equation incorporates a Gaussian
local noise, ζ. First, by averaging over ζ, we get an effective Hamiltonian. Second, we renormalize this dynamic field theory and write a Renormalization-Group equation
for the dynamic structure factor. Third, solving this equation yields the behavior of S(q, t), in space of relevant parameters. As result, S(q, t) depends on three kinds of lengths, which are the wavelength q
−1, a time length scale R(t) ∼ t
1/z
, and the mesh size ξ
*. The scale R(t) is interpreted as the size of growing microdomains at time t. When R(t) becomes of the order of ξ
*, the dynamics is stopped. The final time, t
*, then scales as t
* ∼ ξ
*
z, with the dynamic exponent z = 6−η. Here, η is the usual Ising critical exponent. Since the final size of microdomains ξ
* is very small (few nanometers), the dynamics is of short time. Finally, all these results we obtained from renormalization
theory are compared to those we stated in some recent work using a scaling argument. 相似文献
996.
P. M.C. de Oliveira S. Moss de Oliveira D. Stauffer S. Cebrat A. Pękalski 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(2):245-254
We discovered a dynamic phase transition induced by sexual
reproduction. The dynamics is a pure Darwinian rule applied to diploid
bit-strings with both fundamental ingredients to drive Darwin's
evolution: (1) random mutations and crossings which act in the sense of
increasing the entropy (or diversity); and (2) selection which acts in
the opposite sense by limiting the entropy explosion. Selection wins
this competition if mutations performed at birth are few enough, and
thus the wild genotype dominates the steady-state population. By slowly
increasing the average number m of mutations, however, the
population suddenly undergoes a mutational degradation precisely at a
transition point mc. Above this point, the “bad”
alleles (represented by 1-bits) spread over the genetic pool of the
population, overcoming the selection pressure. Individuals become
selectively alike, and evolution stops. Only below this point, m < mc, evolutionary life is possible.
The finite-size-scaling behaviour of this transition is
exhibited for large enough “chromosome” lengths L, through
lengthy computer simulations. One important and surprising observation
is the L-independence of the transition curves, for large L.
They are also independent on the population size. Another is that mc is near unity, i.e. life cannot be stable with much more
than one mutation per diploid genome, independent of the chromosome
length, in agreement with reality. One possible consequence is that an
eventual evolutionary jump towards larger L enabling the storage
of more genetic information would demand an improved DNA copying
machinery in order to keep the same total number of mutations per
offspring. 相似文献
997.
998.
T. Bodineau B. Derrida V. Lecomte F. van Wijland 《Journal of statistical physics》2008,133(6):1013-1031
We obtain explicit expressions for the long range correlations in the ABC model and in diffusive models conditioned to produce
an atypical current of particles. In both cases, the two-point correlation functions allow one to detect the occurrence of
a phase transition as they become singular when the system approaches the transition. 相似文献
999.
The problem of Group Testing is to identify defective items out of a set of objects by means of pool queries of the form “Does the pool contain at least a defective?”. The aim is of course to perform detection with the fewest possible queries, a problem which has relevant practical applications in different fields including molecular biology and computer science. Here we study GT in the probabilistic setting focusing on the regime of small defective probability and large number of objects, p→0 and N→∞. We construct and analyze one-stage algorithms for which we establish the occurrence of a non-detection/detection phase transition resulting in a sharp threshold, \(\overline{M}\), for the number of tests. By optimizing the pool design we construct algorithms whose detection threshold follows the optimal scaling \(\overline{M}\propto Np|\log p|\). Then we consider two-stages algorithms and analyze their performance for different choices of the first stage pools. In particular, via a proper random choice of the pools, we construct algorithms which attain the optimal value (previously determined in (Mézard and Toninelli, arXiv:0706.3104)) for the mean number of tests required for complete detection. We finally discuss the optimal pool design in the case of finite p. 相似文献
1000.
A methodology is proposed for studying rare events in stochastic partial differential equations in systems that are so large
that standard large deviation theory does not apply. The idea is to deduce the behavior of the original model by breaking
the system into appropriately scaled subsystems that are sufficiently small for large deviation theory to apply but sufficiently
large to be asymptotically independent from one another. The methodology is illustrated in the context of a simple one-dimensional
stochastic partial differential equation. The application reveals a connection between the dynamics of the partial differential
equation and the classical Johnson–Mehl–Avrami–Kolmogorov nucleation and growth model. It also illustrates that rare events
are much more likely and predictable in large systems than in small ones due to the extra entropy provided by space. 相似文献