332 research outputs found
Perturbation theory for the one-dimensional optical polaron
The one-dimensional optical polaron is treated on the basis of the
perturbation theory in the weak coupling limit. A special matrix diagrammatic
technique is developed. It is shown how to evaluate all terms of the
perturbation theory for the ground-state energy of a polaron to any order by
means of this technique. The ground-state energy is calculated up to the eighth
order of the perturbation theory. The effective mass of an electron is obtained
up to the sixth order of the perturbation theory. The radius of convergence of
the obtained series is estimated. The obtained results are compared with the
results from the Feynman polaron theory.Comment: 9 pages, 2 figures, RevTeX, to be published in Phys. Rev. B (2001)
Ap
Structure of Dipalmitoylphosphatidylcholine/Cholesterol Bilayer at Low and High Cholesterol Concentrations: Molecular Dynamics Simulation
By using molecular dynamics simulation technique we studied the changes occurring in membranes constructed of dipalmitoylphosphatidylcholine (DPPC) and cholesterol at 8:1 and 1:1 ratios. We tested two different initial arrangements of cholesterol molecules for a 1:1 ratio. The main difference between two initial structures is the average number of nearest-neighbor DPPC molecules around the cholesterol molecule. Our simulations were performed at constant temperature (T = 50 degrees C) and pressure (P = 0 atm). Durations of the runs were 2 ns. The structure of the DPPC/cholesterol membrane was characterized by calculating the order parameter profiles for the hydrocarbon chains, atom distributions, average number of gauche defects, and membrane dipole potentials. We found that adding cholesterol to membranes results in a condensing effect: the average area of membrane becomes smaller, hydrocarbon chains of DPPC have higher order, and the probability of gauche defects in DPPC tails is lower. Our results are in agreement with the data available from experiments
Analyticity of The Ground State Energy For Massless Nelson Models
We show that the ground state energy of the translationally invariant Nelson
model, describing a particle coupled to a relativistic field of massless
bosons, is an analytic function of the coupling constant and the total
momentum. We derive an explicit expression for the ground state energy which is
used to determine the effective mass.Comment: 33 pages, 1 figure, added a section on the calculation of the
effective mas
Variational Interpolation Algorithm between Weak- and Strong-Coupling Expansions
For many physical quantities, theory supplies weak- and strong-coupling
expansions of the types and \alpha ^p\sum b_n
(\alpha^{-2/q) ^n, respectively. Either or both of these may have a zero
radius of convergence. We present a simple interpolation algorithm which
rapidly converges for an increasing number of known expansion coefficients. The
accuracy is illustrated by calculating the ground state energies of the
anharmonic oscillator using only the leading large-order coefficient
(apart from the trivial expansion coefficent ). The errors are less
than 0.5 for all g. The algorithm is applied to find energy and mass of the
Fr\"ohlich-Feynman polaron. Our mass is quite different from Feynman's
variational approach.Comment: PostScript, http://www.physik.fu-berlin.de/kleinert.htm
Impact of Cholesterol on Voids in Phospholipid Membranes
Free volume pockets or voids are important to many biological processes in
cell membranes. Free volume fluctuations are a prerequisite for diffusion of
lipids and other macromolecules in lipid bilayers. Permeation of small solutes
across a membrane, as well as diffusion of solutes in the membrane interior are
further examples of phenomena where voids and their properties play a central
role. Cholesterol has been suggested to change the structure and function of
membranes by altering their free volume properties. We study the effect of
cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC)
bilayers by means of atomistic molecular dynamics simulations. We find that an
increasing cholesterol concentration reduces the total amount of free volume in
a bilayer. The effect of cholesterol on individual voids is most prominent in
the region where the steroid ring structures of cholesterol molecules are
located. Here a growing cholesterol content reduces the number of voids,
completely removing voids of the size of a cholesterol molecule. The voids also
become more elongated. The broad orientational distribution of voids observed
in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a
distribution where orientation along the bilayer normal is favored. Our results
suggest that instead of being uniformly distributed to the whole bilayer, these
effects are localized to the close vicinity of cholesterol molecules
Coarse-Grained Models of Biological Membranes within the Single Chain Mean Field Theory
The Single Chain Mean Field theory is used to simulate the equilibrium
structure of phospholipid membranes at the molecular level. Three levels of
coarse-graining of DMPC phospholipid surfactants are present: the detailed
44-beads double tails model, the 10-beads double tails model and the minimal
3-beads model. We show that all three models are able to reproduce the
essential equilibrium properties of the phospholipid bilayer, while the
simplest 3-beads model is the fastest model which can describe adequately the
thickness of the layer, the area per lipid and the rigidity of the membrane.
The accuracy of the method in description of equilibrium structures of
membranes compete with Monte Carlo simulations while the speed of computation
and the mean field nature of the approach allows for straightforward
applications to systems with great complexity.Comment: Accepted for publication in Soft Matte
Binding of Polarons and Atoms at Threshold
If the polaron coupling constant is large enough, bipolarons or
multi-polarons will form. When passing through the critical from
above, does the radius of the system simply get arbitrarily large or does it
reach a maximum and then explodes? We prove that it is always the latter. We
also prove the analogous statement for the Pekar-Tomasevich (PT) approximation
to the energy, in which case there is a solution to the PT equation at
. Similarly, we show that the same phenomenon occurs for atoms, e.g.,
helium, at the critical value of the nuclear charge. Our proofs rely only on
energy estimates, not on a detailed analysis of the Schr\"odinger equation, and
are very general. They use the fact that the Coulomb repulsion decays like
, while `uncertainty principle' localization energies decay more rapidly,
as .Comment: 19 page
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