29,558 research outputs found
The Carriers of the Interstellar Unidentified Infrared Emission Features: Constraints from the Interstellar C-H Stretching Features at 3.2-3.5 Micrometers
The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and
11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH)
molecules, have been recently ascribed to mixed aromatic/aliphatic organic
nanoparticles. More recently, an upper limit of <9% on the aliphatic fraction
(i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers
based on the observed intensities of the 3.4 and 3.3 micrometer emission
features by attributing them to aliphatic and aromatic C-H stretching modes,
respectively, and assuming A_34./A_3.3~0.68 derived from a small set of
aliphatic and aromatic compounds, where A_3.4 and A_3.3 are respectively the
band strengths of the 3.4 micrometer aliphatic and 3.3 micrometer aromatic C-H
bonds.
To improve the estimate of the aliphatic fraction of the UIE carriers, here
we analyze 35 UIE sources which exhibit both the 3.3 and 3.4 micrometer C-H
features and determine I_3.4/I_3.3, the ratio of the power emitted from the 3.4
micrometer feature to that from the 3.3 micrometer feature. We derive the
median ratio to be ~ 0.12. We employ density functional theory
and second-order perturbation theory to compute A_3.4/A_3.3 for a range of
methyl-substituted PAHs. The resulting A_3.4/A_3.3 ratio well exceeds 1.4, with
an average ratio of ~1.76. By attributing the 3.4 micrometer
feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity
and superhydrogenation), we derive the fraction of C atoms in aliphatic form to
be ~2%. We therefore conclude that the UIE emitters are predominantly aromatic.Comment: 14 pages, 5 figures, 1 table; accepted for publication in The
Astrophysical Journa
First-principles investigation of 180-degree domain walls in BaTiO_3
We present a first-principles study of 180-degree ferroelectric domain walls
in tetragonal barium titanate. The theory is based on an effective Hamiltonian
that has previously been determined from first-principles
ultrasoft-pseudopotential calculations. Statistical properties are investigated
using Monte Carlo simulations. We compute the domain-wall energy, free energy,
and thickness, analyze the behavior of the ferroelectric order parameter in the
interior of the domain wall, and study its spatial fluctuations. An abrupt
reversal of the polarization is found, unlike the gradual rotation typical of
the ferromagnetic case.Comment: Revtex (preprint style, 13 pages) + 3 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#pad_wal
Aperiodic Quantum Random Walks
We generalize the quantum random walk protocol for a particle in a
one-dimensional chain, by using several types of biased quantum coins, arranged
in aperiodic sequences, in a manner that leads to a rich variety of possible
wave function evolutions. Quasiperiodic sequences, following the Fibonacci
prescription, are of particular interest, leading to a sub-ballistic
wavefunction spreading. In contrast, random sequences leads to diffusive
spreading, similar to the classical random walk behaviour. We also describe how
to experimentally implement these aperiodic sequences.Comment: 4 pages and 4 figure
The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
The "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6,
and 11.3 m are ubiquitously seen in various astrophysical regions. The UIE
features are characteristic of the stretching and bending vibrations of
aromatic hydrocarbons. The 3.3 m feature resulting from aromatic C--H
stretches is often accompanied by a weaker feature at 3.4 m often
attributed to aliphatic C--H stretches. The ratio of the observed intensity of
the 3.3 m aromatic C--H feature () to that of the 3.4 m
aliphatic C--H feature () allows one to estimate the aliphatic
fraction (i.e. , the number of C atoms in
aliphatic units to that in aromatic rings) of the UIE carriers, provided the
intrinsic oscillator strengths of the 3.3 m aromatic C--H stretch
() and the 3.4 m aliphatic C--H stretch () are known.
In this article we summarize the computational results on and
and their implications for the aromaticity and aliphaticity of the
UIE carriers. We use density functional theory and second-order perturbation
theory to derive and from the infrared vibrational spectra
of seven PAHs with various aliphatic substituents (e.g., methyl-, dimethyl-,
ethyl-, propyl-, butyl-PAHs, and PAHs with unsaturated alkyl-chains). The mean
band strengths of the aromatic () and aliphatic () C--H
stretches are derived and then employed to estimate the aliphatic fraction of
the UIE carriers by comparing / with /. We
conclude that the UIE emitters are predominantly aromatic, as revealed by the
observationally-derived ratio ~ 0.12 and the
computationally-derived ratio ~ 1.76 which suggest an
upper limit of ~ 0.02 for the aliphatic
fraction of the UIE carriers.Comment: 67 pages, 18 figures, 8 tables; invited article accepted for
publication in "New Astronomy Review"; a considerable fraction of this
article is concerned with the computational techniques and results, readers
who are mainly interested in astrophysics may wish to only read
"Introduction", and "Astrophysical Implications
Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3
Using path-integral Monte Carol simulations and an ab initio effective
Hamiltonian, we study the effects of quantum fluctuations on structural phase
transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find
quantum fluctuations affect ferroelectric (FE) transitions more strongly than
antiferrodistortive (AFD) ones, even though the effective mass of a single FE
local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress
the FE transition completely, and reduce the AFD transition temperature from
130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the
transition, but do reduce the transition temperature by 35-50 K. The
implications of the calculations are discussed.Comment: Revtex (preprint style, 14 pages) + 2 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#wz_qs
A Semi-parametric Technique for the Quantitative Analysis of Dynamic Contrast-enhanced MR Images Based on Bayesian P-splines
Dynamic Contrast-enhanced Magnetic Resonance Imaging (DCE-MRI) is an
important tool for detecting subtle kinetic changes in cancerous tissue.
Quantitative analysis of DCE-MRI typically involves the convolution of an
arterial input function (AIF) with a nonlinear pharmacokinetic model of the
contrast agent concentration. Parameters of the kinetic model are biologically
meaningful, but the optimization of the non-linear model has significant
computational issues. In practice, convergence of the optimization algorithm is
not guaranteed and the accuracy of the model fitting may be compromised. To
overcome this problems, this paper proposes a semi-parametric penalized spline
smoothing approach, with which the AIF is convolved with a set of B-splines to
produce a design matrix using locally adaptive smoothing parameters based on
Bayesian penalized spline models (P-splines). It has been shown that kinetic
parameter estimation can be obtained from the resulting deconvolved response
function, which also includes the onset of contrast enhancement. Detailed
validation of the method, both with simulated and in vivo data, is provided
Generation adequacy assessment for power systems with wind turbine and energy storage
Wind power has been considered as an environmental friendly electrical generation resource; however, the high wind power penetration can lead to high-risk levels in power system reliability. Energy storage system (ESS) is a promising means to smooth variations of wind power and improve the system reliability. Simulation models for assessing generation adequacies of power systems with wind power generation system (WPGS) and ESS are presented in this paper. The impacts of different wind power penetration levels on the reliability benefits from ESS are analyzed in the first case-study, when the WPGS is utilized to replace the conventional generators with same total rated power capacity. In the second case-study, the WPGS and ESS are installed to meet the annual growth of load demand and maintain the generation adequacy levels. The Monte Carlo simulation is used to simulate the operation of generating units and ESS, considering the force outage rates (FOR) of generators and random fluctuations of wind speeds. ©2010 IEEE.published_or_final_versionThe Innovative Smart Grid Technologies (ISGT) Conference, Gaithersburg, MD., 19-21 January 2010. In Proceedings of ISGT, 2010, p. 1-
A New Supersymmetric and Exactly Solvable Model of Correlated Electrons
A new lattice model is presented for correlated electrons on the unrestricted
-dimensional electronic Hilbert space (where
is the lattice length). It is a supersymmetric generalization of the
Hubbard model, but differs from the extended Hubbard model proposed by Essler,
Korepin and Schoutens. The supersymmetry algebra of the new model is
superalgebra . The model contains one symmetry-preserving free real
parameter which is the Hubbard interaction parameter , and has its origin
here in the one-parameter family of inequivalent typical 4-dimensional irreps
of . On a one-dimensional lattice, the model is exactly solvable by
the Bethe ansatz.Comment: 10 pages, LaTex. (final version to appear in Phys.Rev.Lett.
A New Solution of the Yang-Baxter Equation Related to the Adjoint Representation of
A new solution of the Yang-Baxter equation, that is related to the adjoint
representation of the quantum enveloping algebra , is obtained by
fusion formulas from a non-standard solution.Comment: 16 pages (Latex), Preprint BIHEP-TH-93-3
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