5,712 research outputs found
Entropic uncertainty measure for fluctuations in two-level electron-phonon models
Two-level electron-phonon systems with reflection symmetry linearly coupled
to one or two phonon modes (exciton and E Jahn-Teller model)
exhibit strong enhancement of quantum fluctuations of the phonon coordinates
and momenta due to the complex interplay of quantum fluctuations and
nonlinearities inherent to the models. We show that for the complex correlated
quantum fluctuations of the anisotropic two-level systems the Shannon entropies
of phonon coordinate and momentum and their sum yield their proper global
description. On the other hand, the variance measures of the Heisenberg
uncertainties suffer from several shortcomings to provide proper description of
the fluctuations. Wave functions, related entropies and variances were
determined by direct numerical simulations. Illustrative variational
calculations were performed to demonstrate the effect on an analytically
tractable exciton model.Comment: 14 pages, 10 figs, published in Eur.Phys.J 38 B (2004) 25-3
A Variational Method in Out of Equilibrium Physical Systems
A variational principle is further developed for out of equilibrium dynamical
systems by using the concept of maximum entropy. With this new formulation it
is obtained a set of two first-order differential equations, revealing the same
formal symplectic structure shared by classical mechanics, fluid mechanics and
thermodynamics. In particular, it is obtained an extended equation of motion
for a rotating dynamical system, from where it emerges a kind of topological
torsion current of the form , with and
denoting components of the vector potential (gravitational or/and
electromagnetic) and is the angular velocity of the accelerated frame.
In addition, it is derived a special form of Umov-Poynting's theorem for
rotating gravito-electromagnetic systems, and obtained a general condition of
equilibrium for a rotating plasma. The variational method is then applied to
clarify the working mechanism of some particular devices, such as the Bennett
pinch and vacuum arcs, to calculate the power extraction from an hurricane, and
to discuss the effect of transport angular momentum on the radiactive heating
of planetary atmospheres. This development is seen to be advantageous and opens
options for systematic improvements.Comment: 22 pages, 1 figure, submitted to review, added one referenc
Polyelectrolyte chains in poor solvent. A variational description of necklace formation
We study the properties of polyelectrolyte chains under different solvent
conditions, using a variational technique. The free energy and the
conformational properties of a polyelectrolyte chain are studied minimizing the
free energy , depending on trial probabilities that
characterize the conformation of the chain. The Gaussian approximation is
considered for a ring of length and for an open chain of length
in poor and theta solvent conditions, including a Coulomb repulsion
between the monomers. In theta solvent conditions the blob size is measured and
found in agreement with scaling theory, including charge depletion effects,
expected for the case of an open chain. In poor solvent conditions, a globule
instability, driven by electrostatic repulsion, is observed. We notice also
inhomogeneous behavior of the monomer--monomer correlation function,
reminiscence of necklace formation in poor solvent polyelectrolyte solutions. A
global phase diagram in terms of solvent quality and inverse Bjerrum length is
presented.Comment: submitted to EPJE (soft matter
A sticky business: the status of the conjectured viscosity/entropy density bound
There have been a number of forms of a conjecture that there is a universal
lower bound on the ratio, eta/s, of the shear viscosity, eta, to entropy
density, s, with several different domains of validity. We examine the various
forms of the conjecture. We argue that a number of variants of the conjecture
are not viable due to the existence of theoretically consistent
counterexamples. We also note that much of the evidence in favor of a bound
does not apply to the variants which have not yet been ruled out.Comment: 23 pages, 4 figures, added references, corrected typos, added
subsection in response to Son's comments in arXiv:0709.465
Entanglement entropy for the long range Ising chain
We consider the Ising model in a transverse field with long-range
antiferromagnetic interactions that decay as a power law with their distance.
We study both the phase diagram and the entanglement properties as a function
of the exponent of the interaction. The phase diagram can be used as a guide
for future experiments with trapped ions. We find two gapped phases, one
dominated by the transverse field, exhibiting quasi long range order, and one
dominated by the long range interaction, with long range N\'eel ordered ground
states. We determine the location of the quantum critical points separating
those two phases. We determine their critical exponents and central-charges. In
the phase with quasi long range order the ground states exhibit exotic
corrections to the area law for the entanglement entropy coexisting with gapped
entanglement spectra.Comment: 5 pages, all comments welcom
Theory of polyzwitterion conformations
Conformational characteristics of polyzwitterionic molecules in aqueous
solutions are investigated using the variational method. Analytical relations
are derived for the radius of gyration of a single polyzwitterionic chain as a
function of the chain length, electrostatic interaction strength, added salt
concentration, dipole moment and degree of ionization of the zwitterionic
monomers. In the absence of the small ions (counterions and coions) near the
polyzwitterionic chain, attractive dipole-dipole interactions are shown to
induce a collapse of the polyzwitterionic chain. However, in the presence of
the small ions, the radius of gyration is shown to be an interplay of the
screening of the electrostatic interactions and the counterion adsorption on
the zwitterionic sites. In addition to the well-known Debye-Huckel screening of
the charge-charge interactions, screening of the charge-dipole and
dipole-dipole interactions are found to play important roles in determining the
size of the chain. Functional forms for the screened charge-dipole and
dipole-dipole interaction potentials are presented. Furthermore, counterion
adsorption on the zwitterionic monomers is predicted to be asymmetric depending
on the nature of the added salt and the zwitterionic groups. Qualitative
remarks regarding the solubility of these molecules in aqueous solutions along
with the classical "anti-polyelectrolyte" effect (increase in the solubility in
water with the addition of salt) are presented.Comment: To be appeared in J. Chem. Phy
- …