193 research outputs found

### Relations for lipid bilayers. Connection of electron density profiles to other structural quantities

Three relations are derived that connect low angle diffraction/scattering results obtained from lipid bilayers to other structural quantities of interest. The first relates the area along the surface of the bilayer, the measured specific volume, and the zeroth order structure factor, F(0). The second relates the size of the trough in the center of the electron density profile, the volume of the terminal methyl groups, and the volume of the methylene groups in the fatty acid chains. The third relates the size of the headgroup electron density peak, the volume of the headgroup, and the volumes of water and hydrocarbon in the headgroup region. These relations, which are easily modified for neutron diffraction, are useful for obtaining structural quantities from electron density profiles obtained by fitting model profiles to measured low angle x-ray intensities

### Random Networks with given Rich-club Coefficient

In complex networks it is common to model a network or generate a surrogate
network based on the conservation of the network's degree distribution. We
provide an alternative network model based on the conservation of connection
density within a set of nodes. This density is measure by the rich-club
coefficient. We present a method to generate surrogates networks with a given
rich-club coefficient. We show that by choosing a suitable local linking term,
the generated random networks can reproduce the degree distribution and the
mixing pattern of real networks. The method is easy to implement and produces
good models of real networks.Comment: revised version, new figure

### Sterols sense swelling in lipid bilayers

In the mimetic membrane system of phosphatidylcholine bilayers, thickening
(pre-critical behavior, anomalous swelling) of the bilayers is observed, in the
vicinity of the main transition, which is non-linear with temperature. The
sterols cholesterol and androsten are used as sensors in a time-resolved
simultaneous small- and wide angle x-ray diffraction study to investigate the
cause of the thickening. We observe precritical behavior in the pure lipid
system, as well as with sterol concentrations less than 15%. To describe the
precritical behavior we introduce a theory of precritical phenomena.The good
temperature resolution of the data shows that a theory of the influence of
fluctuations needs modification. The main cause of the critical behavior
appears to be a changing hydration of the bilayer.Comment: 11 pages, 7 ps figures included, to appear in Phys.Rev.

### Pocket Monte Carlo algorithm for classical doped dimer models

We study the correlations of classical hardcore dimer models doped with
monomers by Monte Carlo simulation. We introduce an efficient cluster
algorithm, which is applicable in any dimension, for different lattices and
arbitrary doping. We use this algorithm for the dimer model on the square
lattice, where a finite density of monomers destroys the critical confinement
of the two-monomer problem. The monomers form a two-component plasma located in
its high-temperature phase, with the Coulomb interaction screened at finite
densities. On the triangular lattice, a single pair of monomers is not
confined. The monomer correlations are extremely short-ranged and hardly change
with doping.Comment: 6 pages, REVTeX

### Apparent phase transitions in finite one-dimensional sine-Gordon lattices

We study the one-dimensional sine-Gordon model as a prototype of roughening
phenomena. In spite of the fact that it has been recently proven that this
model can not have any phase transition [J. A. Cuesta and A. Sanchez, J. Phys.
A 35, 2373 (2002)], Langevin as well as Monte Carlo simulations strongly
suggest the existence of a finite temperature separating a flat from a rough
phase. We explain this result by means of the transfer operator formalism and
show as a consequence that sine-Gordon lattices of any practically achievable
size will exhibit this apparent phase transition at unexpectedly large
temperatures.Comment: 7 pages, 4 figure

### Dimer coverings on the Sierpinski gasket with possible vacancies on the outmost vertices

We present the number of dimers $N_d(n)$ on the Sierpinski gasket $SG_d(n)$
at stage $n$ with dimension $d$ equal to two, three, four or five, where one of
the outmost vertices is not covered when the number of vertices $v(n)$ is an
odd number. The entropy of absorption of diatomic molecules per site, defined
as $S_{SG_d}=\lim_{n \to \infty} \ln N_d(n)/v(n)$, is calculated to be
$\ln(2)/3$ exactly for $SG_2(n)$. The numbers of dimers on the generalized
Sierpinski gasket $SG_{d,b}(n)$ with $d=2$ and $b=3,4,5$ are also obtained
exactly. Their entropies are equal to $\ln(6)/7$, $\ln(28)/12$, $\ln(200)/18$,
respectively. The upper and lower bounds for the entropy are derived in terms
of the results at a certain stage for $SG_d(n)$ with $d=3,4,5$. As the
difference between these bounds converges quickly to zero as the calculated
stage increases, the numerical value of $S_{SG_d}$ with $d=3,4,5$ can be
evaluated with more than a hundred significant figures accurate.Comment: 35 pages, 20 figures and 1 tabl

### Discrete kink dynamics in hydrogen-bonded chains I: The one-component model

We study topological solitary waves (kinks and antikinks) in a nonlinear
one-dimensional Klein-Gordon chain with the on-site potential of a double-Morse
type. This chain is used to describe the collective proton dynamics in
quasi-one-dimensional networks of hydrogen bonds, where the on-site potential
plays role of the proton potential in the hydrogen bond. The system supports a
rich variety of stationary kink solutions with different symmetry properties.
We study the stability and bifurcation structure of all these stationary kink
states. An exactly solvable model with a piecewise ``parabola-constant''
approximation of the double-Morse potential is suggested and studied
analytically. The dependence of the Peierls-Nabarro potential on the system
parameters is studied. Discrete travelling-wave solutions of a narrow permanent
profile are shown to exist, depending on the anharmonicity of the Morse
potential and the cooperativity of the hydrogen bond (the coupling constant of
the interaction between nearest-neighbor protons).Comment: 12 pages, 20 figure

### Entropy Crisis, Ideal Glass Transition and Polymer Melting: Exact Solution on a Husimi Cactus

We introduce an extension of the lattice model of melting of semiflexible
polymers originally proposed by Flory. Along with a bending penalty, present in
the original model and involving three sites of the lattice, we introduce an
interaction energy that corresponds to the presence of a pair of parallel bonds
and a second interaction energy associated with the presence of a hairpin turn.
Both these new terms represent four-site interactions. The model is solved
exactly on a Husimi cactus, which approximates a square lattice. We study the
phase diagram of the system as a function of the energies. For a proper choice
of the interaction energies, the model exhibits a first-order melting
transition between a liquid and a crystalline phase. The continuation of the
liquid phase below this temperature gives rise to a supercooled liquid, which
turns continuously into a new low-temperature phase, called metastable liquid.
This liquid-liquid transition seems to have some features that are
characteristic of the critical transition predicted by the mode-coupling
theory.Comment: To be published in Physical Review E, 68 (2) (2003

- …