72,279 research outputs found
Improving the Functional Control of Aged Ferroelectrics using Insights from Atomistic Modelling
We provide a fundamental insight into the microscopic mechanisms of the
ageing processes. Using large scale molecular dynamics simulations of the
prototypical ferroelectric material PbTiO3, we demonstrate that the
experimentally observed ageing phenomena can be reproduced from intrinsic
interactions of defect-dipoles related to dopant-vacancy associates, even in
the absence of extrinsic effects. We show that variation of the dopant
concentration modifies the material's hysteretic response. We identify a
universal method to reduce loss and tune the electromechanical properties of
inexpensive ceramics for efficient technologies.Comment: 6 pages, 3 figure
Chiral Corrections to Lattice Calculations of Charge Radii
Logarithmic divergences in pion and proton charge radii associated with
chiral loops are investigated to assess systematic uncertainties in current
lattice determinations of charge radii. The chiral corrections offer a possible
solution to the long standing problem of why present lattice calculations yield
proton and pion radii which are similar in size.Comment: PostScript file only. Ten pages. Figures included. U. of MD Preprint
#92-19
Population Dynamics on Complex Food Webs
In this work we analyse the topological and dynamical properties of a simple
model of complex food webs, namely the niche model. In order to underline
competition among species, we introduce "prey" and "predators" weighted overlap
graphs derived from the niche model and compare synthetic food webs with real
data. Doing so, we find new tests for the goodness of synthetic food web models
and indicate a possible direction of improvement for existing ones. We then
exploit the weighted overlap graphs to define a competition kernel for
Lotka-Volterra population dynamics and find that for such a model the stability
of food webs decreases with its ecological complexity.Comment: 11 Pages, 5 Figures, styles enclosed in the submissio
The Delta-Delta Intermediate State in 1S0 Nucleon-Nucleon Scattering From Effective Field Theory
We examine the role of the Delta-Delta intermediate state in low energy NN
scattering using effective field theory. Theories both with and without pions
are discussed. They are regulated with dimensional regularization and MSbar
subtraction. We find that the leading effects of the Delta-Delta state can be
absorbed by a redefinition of the contact terms in a theory with nucleons only.
It does not remove the requirement of a higher dimension operator to reproduce
data out to moderate momentum. The explicit decoupling of the Delta-Delta state
is shown for the theory without pions.Comment: 16 pages, 3 figures, uses harvma
The Long and Short of Nuclear Effective Field Theory Expansions
Nonperturbative effective field theory calculations for NN scattering seem to
break down at rather low momenta. By examining several toy models, we clarify
how effective field theory expansions can in general be used to properly
separate long- and short-range effects. We find that one-pion exchange has a
large effect on the scattering phase shift near poles in the amplitude, but
otherwise can be treated perturbatively. Analysis of a toy model that
reproduces 1S0 NN scattering data rather well suggests that failures of
effective field theories for momenta above the pion mass can be due to
short-range physics rather than the treatment of pion exchange. We discuss the
implications this has for extending the applicability of effective field
theories.Comment: 22 pages, 9 figures, references corrected, minor modification
Distillation of GHZ states by selective information manipulation
Methods for distilling maximally entangled tripartite (GHZ) states from
arbitrary entangled tripartite pure states are described. These techniques work
for virtually any input state. Each technique has two stages which we call
primary and secondary distillation. Primary distillation produces a GHZ state
with some probability, so that when applied to an ensemble of systems, a
certain percentage is discarded. Secondary distillation produces further GHZs
from the discarded systems. These protocols are developed with the help of an
approach to quantum information theory based on absolutely selective
information, which has other potential applications.Comment: minor corrections, especially of some numerical values; conclusions
unaffecte
Model Independent Extraction of Without Heavy Quark Symmetry
A new method to extract is proposed based on a sum--rule for
semileptonic decays of the meson. The method relies on much weaker
assumptions than previous approaches which are based on heavy--quark symmetry.
This sum--rule only relies on the assumption that the virtual
pair content of the meson can be neglected. The extraction of the CKM
matrix element also requires that the sum--rule saturates in the kinematically
accessible region.Comment: 10 pages revtex3 manuscript. No figures, U. of MD PP #94--086. With
our apologies, some innocuous errors corrected and some references added that
had been brought to our attentio
Low Energy Theorems For Nucleon-Nucleon Scattering
Low energy theorems are derived for the coefficients of the effective range
expansion in s-wave nucleon-nucleon scattering valid to leading order in an
expansion in which both and (where is the scattering length)
are treated as small mass scales. Comparisons with phase shift data, however,
reveal a pattern of gross violations of the theorems for all coefficients in
both the and channels. Analogous theorems are developed for the
energy dependence parameter which describes mixing.
These theorems are also violated. These failures strongly suggest that the
physical value of is too large for the chiral expansion to be valid in
this context. Comparisons of with phenomenological scales known to
arise in the two-nucleon problem support this conjecture.Comment: 12 pages, 1 figure, 1 table; appendix added to discuss behavior in
chiral limit; minor revisions including revised figure reference to recent
work adde
Classical artificial two-dimensional atoms: the Thomson model
The ring configurations for classical two-dimensional atoms are calculated
within the Thomson model and compared with the results from `exact' numerical
simulations. The influence of the functional form of the confinement potential
and the repulsive interaction potential between the particles on the
configurations is investigated. We also give exact results on those eigenmodes
of the system whose frequency does not depend on the number of particles in the
system.Comment: 9 pages, RevTeX, 4 figure
Comment on `Universal relation between the Kolmogorov-Sinai entropy and the thermodynamic entropy in simple liquids'
The intriguing relations between Kolmogorov-Sinai entropy and self diffusion
coefficients and the excess (thermodynamic) entropy found by Dzugutov and
collaborators do not appear to hold for hard sphere and hard disks systems.Comment: 1 page revte
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