182 research outputs found
Calculation of Screening Masses in a Chiral Quark Model
We consider a simple model for the coordinate-space vacuum polarization
function which is often parametrized in terms of a screening mass. We discuss
the circumstances in which the standard result for the screening mass,
, is obtained. In the model considered here, that result is
obtained when the momenta in the relevant vacuum polarization integral are
small with respect to the first Matsubara frequency.Comment: 6 pages, 2 figure
Theory of nonlinear optical properties of phenyl-substituted polyacetylenes
In this paper we present a theoretical study of the third-order nonlinear
optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the
third-harmonic-generation (THG) process. We study the aforesaid process in
PDPA's using both the independent electron Hueckel model, as well as
correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based
calculations were performed using various configuration interaction (CI)
methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the
quadruples-CI (QCI) methods, and the both longitudinal and the transverse
components of third-order susceptibilities were computed. The Hueckel model
calculations were performed on oligo-PDPA's containing up to fifty repeat
units, while correlated calculations were performed for oligomers containing up
to ten unit cells. At all levels of theory, the material exhibits highly
anisotropic nonlinear optical response, in keeping with its structural
anisotropy. We argue that the aforesaid anisotropy can be divided over two
natural energy scales: (a) the low-energy response is predominantly
longitudinal and is qualitatively similar to that of polyenes, while (b) the
high-energy response is mainly transverse, and is qualitatively similar to that
of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April
15, 2004
Voice protection module
The development of digitalization is inextricably linked to the protection
of information which is one of the most important assets of any organization. In this regard
one of the problems of information security is the leakage of information through the acoustic
channel
Excitonic Strings in one dimensional organic compounds
Important questions concern the existence of excitonic strings in organic
compounds and their signatures in the photophysics of these systems. A model in
terms of Hard Core Bosons is proposed to study this problem in one dimension.
Mainly the cases with two and three particles are studied for finite and
infinite lattices, where analytical results are accessible. It is shown that if
bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at
least in a certain range of parameters, will exist. Moreover, the behaviour of
the transitions from one exciton to the biexciton is fully clarified. The
results are in agreement with exact finite cluster diagonalizations of several
model Hamiltonians.Comment: 36 pages, 4 eps figs. to appear in Phys. Rev.
Why pair production cures covariance in the light-front?
We show that the light-front vaccum is not trivial, and the Fock space for
positive energy quanta solutions is not complete. As an example of this non
triviality we have calculated the electromagnetic current for scalar bosons in
the background field method were the covariance is restored through considering
the complete Fock space of solutions. We also show thus that the method of
"dislocating the integration pole" is nothing more than a particular case of
this, so that such an "ad hoc" prescription can be dispensed altogether if we
deal with the whole Fock space. In this work we construct the electromagnetic
current operator for a system composed of two free bosons. The technique
employed to deduce these operators is through the definition of global
propagators in the light front when a background electromagnetic field acts on
one of the particles.Comment: 11 pages, 2 figure
Spin-order-induced multiferroicity in LiCuFe2(VO4)3 and disorder effects in NaCuFe2(VO4)3
Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically
ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further
transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in
magnetic susceptibility chi and specific heat Cp measurements. While no anomaly
has been detected in dielectric property of NaCuFe2(VO4)3, the step-like
feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3.
These data suggest the spin-order-induced ferroelectricity in Li compound and
no such thing in Na compound. On the contrary, the Moessbauer spectroscopy
study suggests similarly wide distribution of hyperfine field in between T* and
TN for both the compounds. The first principles calculations also provide
similar values for magnetic exchange interaction parameters in both compounds.
These observations lead us to conclude on the crucial role of alkali metals
mobility within the channels of the crystal structure needed to be considered
in explaining the improper multiferroicity in one compound and its absence in
other.Comment: 9 pages, 5 figure
A nonlinear approach to NN interactions using self-interacting meson fields
Motivated by the success of models based on chiral symmetry in NN
interactions we investigate self-interacting scalar, pseudoscalar and vector
meson fields and their impact for NN forces. We parametrize the corresponding
nonlinear field equations and get analytic wavelike solutions. A probability
amplitude for the propagation of particle states is calculated and applied in
the framework of a boson-exchange NN potential. Using a proper normalization of
the meson fields makes all self-scattering amplitudes finite. The same
normalization is able to substitute for the phenomenological form factors used
in conventional boson exchange potentials and thus yields an phenomenological
understanding of this part of the NN interaction. We find an empirical scaling
law which relates the meson self-interaction couplings to the pion mass and
self-interaction coupling constant. Our model yields np phase shifts comparable
to the Bonn B potential results and deuteron properties, in excellent agreement
with experimental data.Comment: Reviewed version, 25 pages REVTeX, more info at
http://i04ktha.desy.d
Comment on Intrinsic and dynamically generated scalar meson states
The scalar-meson assignments of Shakin and Wang in a generalized
Nambu--Jona-Lasinio model are contradicted by recent experimental information.
Also the strict distinction made by these authors between ``intrinsic'' and
``dynamically generated'' states is contested, as well as a number of other
statements.Comment: 7 pages, (v2 cosmetics, v3 corrections in one reference), to be
published in Phys.Rev.
Consistent description of NN and pi-N interactions using the solitary boson exchange potential
A unified description of NN and pi-N elastic scattering is presented in the
framework of the one solitary boson exchange potential (OSBEP). This model
already successfully applied to analyze NN scattering is now extended to
describe pi-N scattering while also improving its accuracy in the NN domain. We
demonstrate the importance of regularization of pi-N scattering amplitudes
involving Delta isobars and derivative meson-nucleon couplings, as this model
always yields finite amplitudes without recourse to phenomenological form
factors. We find an empirical scaling relation of the meson self interaction
coupling constants consistent with that previously found in the study of NN
scattering. Finally, we demonstrate that the OSBEP model does not contradict
the soft-pion theorems of pi-N scattering.Comment: 29 pages RevTeX, submitted to Phys. Rev. C, further information at
http://i04ktha.desy.d
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