14,247 research outputs found
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
Exact Solution of Return Hysteresis Loops in One Dimensional Random Field Ising Model at Zero Temperature
Minor hysteresis loops within the main loop are obtained analytically and
exactly in the one-dimensional ferromagnetic random field Ising-model at zero
temperature. Numerical simulations of the model show excellent agreement with
the analytical results
Ion-acoustic solitary waves and shocks in a collisional dusty negative ion plasma
We study the effects of ion-dust collisions and ion kinematic viscosities on
the linear ion-acoustic instability as well as the nonlinear propagation of
small amplitude solitary waves and shocks (SWS) in a negative ion plasma with
immobile charged dusts. {The existence of two linear ion modes, namely the
`fast' and `slow' waves is shown, and their properties are analyzed in the
collisional negative ion plasma.} {Using the standard reductive perturbation
technique, we derive a modified Korteweg-de Vries-Burger (KdVB) equation which
describes the evolution of small amplitude SWS.} {The profiles of the latter
are numerically examined with parameters relevant for laboratory and space
plasmas where charged dusts may be positively or negatively charged.} It is
found that negative ion plasmas containing positively charged dusts support the
propagation of SWS with negative potential. However, the perturbations with
both positive and negative potentials may exist when dusts are negatively
charged. The results may be useful for the excitation of SWS in laboratory
negative ion plasmas as well as for observation in space plasmas where charged
dusts may be positively or negatively charged.Comment: 13 pages, 9 figures; To appear in Physical Review
Ab initio Hartree-Fock Born effective charges of LiH, LiF, LiCl, NaF, and NaCl
We use the Berry-phase-based theory of macroscopic polarization of dielectric
crystals formulated in terms of Wannier functions, and state-of-the-art
Gaussian basis functions, to obtain benchmark ab initio Hartree-Fock values of
the Born effective charges of ionic compounds LiH, LiF, LiCl, NaF, and NaCl. We
find excellent agreement with the experimental values for all the compounds
except LiCl and NaCl, for which the disagreement with the experiments is close
to 10% and 16%, respectively. This may imply the importance of many-body
effects in those systems.Comment: 11 pages, Revtex, 2 figures (included), to appear in Phys. Rev. B
April 15, 200
A study of the dynamics of droughts in Northern Brazil: Observations, theory, and numerical experiments with a global atmospheric circulation model
The monthly mean sea surface temperature anomalies over tropical Altantic and rainfall anomalies over two selected stations for 25 years (1948-1972) were examined. It is found that the most severe drought events are associated with the simultaneous occurrence of warm sea surface temperature anomalies over north and cold sea surface temperature anomalies over south tropical Atlantic. Simultaneous occurrences of warm sea surface temperature anomaly at 15 deg N, 45 deg W and cold sea surface temperature anomaly at 15 deg S, 5 deg W were always associated with negative anomalies of rainfall, and vice versa. A simple primitive equation model is used to calculate the frictionally controlled and thermally driven circulation due to a prescribed heating function in a resting atmosphere
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Nonlinear Interactions Between Gravitational Radiation and Modified Alfven Modes in Astrophysical Dusty Plasmas
We present an investigation of nonlinear interactions between Gravitational
Radiation and modified Alfv\'{e}n modes in astrophysical dusty plasmas.
Assuming that stationary charged dust grains form neutralizing background in an
electron-ion-dust plasma, we obtain the three wave coupling coefficients, and
calculate the growth rates for parametrically coupled gravitational radiation
and modified Alfv\'{e}n-Rao modes. The threshold value of the gravitational
wave amplitude associated with convective stabilization is particularly small
if the gravitational frequency is close to twice the modified Alfv\'en
wave-frequency. The implication of our results to astrophysical dusty plasmas
is discussed.Comment: A few typos corrected. Published in Phys. Rev.
Dynamics of spin 1/2 quantum plasmas
The fully nonlinear governing equations for spin 1/2 quantum plasmas are
presented. Starting from the Pauli equation, the relevant plasma equations are
derived, and it is shown that nontrivial quantum spin couplings arise, enabling
studies of the combined collective and spin dynamics. The linear response of
the quantum plasma in an electron--ion system is obtained and analyzed.
Applications of the theory to solid state and astrophysical systems as well as
dusty plasmas are pointed out.Comment: 4 pages, 2 figures, to appear in Physical Review Letter
Return to return point memory
We describe a new class of systems exhibiting return point memory (RPM) that
are different from those discussed before in the context of ferromagnets. We
show numerically that one dimensional random Ising antiferromagnets have RPM,
when configurations evolve from a large field. However, RPM is violated when
started from some stable configurations at finite field unlike in the
ferromagnetic case. This implies that the standard approach to understanding
ferromagnetic RPM systems will fail for this case. We also demonstrate RPM with
a set of variables that keep track of spin flips at each site. Conventional RPM
for the spin configuration is a projection of this result, suggesting that spin
flip variables might be a more fundamental representation of the dynamics. We
also present a mapping that embeds the antiferromagnetic chain in a two
dimensional ferromagnetic model, and prove RPM for spin exchange dynamics in
the interior of the chain with this mapping
Tool support for statistical testing of software components
We describe the "STSC" prototype tool that supports the statistical testing of software components. The tool supports a wide range of operational profiles and test oracles for test case generation and output evaluation. The tool also generates appropriate values for different types of input parameters of operations. STSC automatically generates a test driver from an operational profile. This test driver invokes a test oracle that is implemented as a behaviour-checking version of the implementation. To evaluate the flexibility and usability of the tool, it has been applied to several case studies using different types of operational profiles and test oracles
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