290 research outputs found
First-principles dynamical CPA to finite-temperature magnetism of transition metals
We present here the first-principles dynamical CPA (coherent potential
approximation) combined with the tight-binding LMTO LDA+U method towards
quantitative calculations of the electronic structure and magnetism at finite
temperatures in transition metals and compounds. The theory takes into account
the single-site dynamical charge and spin fluctuations using the functional
integral technique as well as an effective medium. Numerical results for Fe,
Co, and Ni show that the theory explains quantitatively the high-temperature
properties such as the effective Bohr magneton numbers and the excitation
spectra in the paramagnetic state, and describes the Curie temperatures
semiquantitatively.Comment: ICM'09 Proceeding
Polyelectrolyte Bundles
Using extensive Molecular Dynamics simulations we study the behavior of
polyelectrolytes with hydrophobic side chains, which are known to form
cylindrical micelles in aqueous solution. We investigate the stability of such
bundles with respect to hydrophobicity, the strength of the electrostatic
interaction, and the bundle size. We show that for the parameter range relevant
for sulfonated poly-para-phenylenes (PPP) one finds a stable finite bundle
size. In a more generic model we also show the influence of the length of the
precursor oligomer on the stability of the bundles. We also point out that our
model has close similarities to DNA solutions with added condensing agents,
hinting to the possibility that the size of DNA aggregates is under certain
circumstances thermodynamically limited.Comment: 10 pages, 8 figure
Field theoretical representation of the Hohenberg-Kohn free energy for fluids
To go beyond Gaussian approximation to the Hohenberg-Kohn free energy playing
the key role in the density functional theory (DFT), the density functional
\textit{integral} representation would be relevant, because field theoretical
approach to perturbative calculations becomes available. Then the present
letter first derives the associated Hamiltonian of density functional,
explicitly including logarithmic entropy term, from the grand partition
function expressed by configurational integrals. Moreover, two things are done
so that the efficiency of the obtained form may be revealed: to demonstrate
that this representation facilitates the field theoretical treatment of the
perturbative calculation, and further to compare our perturbative formulation
with that of the DFT.Comment: 5 pages, revtex, modified on 13 April 2000 [see eqs. (3), (6), and
(13)
The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory
We study theoretically the osmotic pressure of a suspension of charged
objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed
against an electrolyte solution using the cell model and linear
Poisson-Boltzmann (PB) theory. From the volume derivative of the grand
potential functional of linear theory we obtain two novel expressions for the
osmotic pressure in terms of the potential- or ion-profiles, neither of which
coincides with the expression known from nonlinear PB theory, namely, the
density of microions at the cell boundary. We show that the range of validity
of linearization depends strongly on the linearization point and proof that
expansion about the selfconsistently determined average potential is optimal in
several respects. For instance, screening inside the suspension is
automatically described by the actual ionic strength, resulting in the correct
asymptotics at high colloid concentration. Together with the analytical
solution of the linear PB equation for cell models of arbitrary dimension and
electrolyte composition explicit and very general formulas for the osmotic
pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis
also shows that whether or not linear theory predicts a phase separation
depends crucially on the precise definition of the pressure, showing that an
improper choice could predict an artificial phase separation in systems as
important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl
Electrolytic depletion interactions
We consider the interactions between two uncharged planar macroscopic
surfaces immersed in an electrolyte solution which are induced by interfacial
selectivity. These forces are taken into account by introducing a depletion
free-energy density functional, in addition to the usual mean-field
Poisson-Boltzmann functional. The minimization of the total free-energy
functional yields the density profiles of the microions and the electrostatic
potential. The disjoining pressure is obtained by differentiation of the total
free energy with respect to the separation of the surfaces, holding the range
and strength of the depletion forces constant. We find that the induced
interaction between the two surfaces is always repulsive for sufficiently large
separations, and becomes attractive at shorter separations. The nature of the
induced interactions changes from attractive to repulsive at a distance
corresponding to the range of the depletion forces.Comment: 17 pages, 4 Postscript figures, submitted to Physical Review
Counterion Condensation and Fluctuation-Induced Attraction
We consider an overall neutral system consisting of two similarly charged
plates and their oppositely charged counterions and analyze the electrostatic
interaction between the two surfaces beyond the mean-field Poisson-Boltzmann
approximation. Our physical picture is based on the fluctuation-driven
counterion condensation model, in which a fraction of the counterions is
allowed to ``condense'' onto the charged plates. In addition, an expression for
the pressure is derived, which includes fluctuation contributions of the whole
system. We find that for sufficiently high surface charges, the distance at
which the attraction, arising from charge fluctuations, starts to dominate can
be large compared to the Gouy-Chapmann length. We also demonstrate that
depending on the valency, the system may exhibit a novel first-order binding
transition at short distances.Comment: 15 pages, 8 figures, to appear in PR
On the fluid-fluid phase separation in charged-stabilized colloidal suspensions
We develop a thermodynamic description of particles held at a fixed surface
potential. This system is of particular interest in view of the continuing
controversy over the possibility of a fluid-fluid phase separation in aqueous
colloidal suspensions with monovalent counterions. The condition of fixed
surface potential allows in a natural way to account for the colloidal charge
renormalization. In a first approach, we assess the importance of the so called
``volume terms'', and find that in the absence of salt, charge renormalization
is sufficient to stabilize suspension against a fluid-fluid phase separation.
Presence of salt, on the other hand, is found to lead to an instability. A very
strong dependence on the approximations used, however, puts the reality of this
phase transition in a serious doubt. To further understand the nature of the
instability we next study a Jellium-like approximation, which does not lead to
a phase separation and produces a relatively accurate analytical equation of
state for a deionized suspensions of highly charged colloidal spheres. A
critical analysis of various theories of strongly asymmetric electrolytes is
presented to asses their reliability as compared to the Monte Carlo
simulations
Stick-release pattern in stretching single condensed polyelectrolyte toroids
Using Langevin dynamics simulations, we study elastic response of single
semiflexible polyelectrolytes to an external force pulling on the chain ends,
to mimic the stretching of DNA molecules by optical tweezers. The linear chains
are condensed by multivalent counterions into toroids. The force-extension
curve shows a series of sawtooth-like structure, known as the stick-release
patterns in experiments. We demonstrate that these patterns are a consequence
of the loop-by-loop unfolding of the toroidal structure. Moreover, the
dynamics, how the internal structure of chain varies under tension, is
examined. At the first stage of the stretching, the toroidal condensate
decreases its size until the loss of the first loop in the toroid and then,
oscillates around this size for the rest of the unfolding process. The normal
vector of the toroid is pulled toward the pulling-force direction and swings
back to its early direction repeatedly when the toroidal chain looses a loop.
The results provide new and valuable information concerning the elasticity and
the microscopic structure and dynamic pathway of salt-condensed DNA molecules
being stretched.Comment: 13 pages, 5 figures, accepted for publication in Macromolecule
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
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