21,923 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
Polarity-induced oxygen vacancies at LaAlO3|SrTiO3 interfaces
Using first-principles density functional theory calculations, we find a
strong position and thickness dependence of the formation energy of oxygen
vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an
analytical capacitor model. Oxygen vacancies are preferentially formed at
p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons
introduced by the oxygen vacancies reduce their energy by moving to the n-type
interface. This asymmetric behavior makes an important contribution to the
conducting (insulating) nature of n-type (p-type) interfaces while providing a
natural explanation for the failure to detect evidence for the polar
catastrophe in the form of core level shifts
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
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
Thermodynamics with density and temperature dependent particle masses and properties of bulk strange quark matter and strangelets
Thermodynamic formulas for investigating systems with density and/or
temperature dependent particle masses are generally derived from the
fundamental derivation equality of thermodynamics. Various problems in the
previous treatments are discussed and modified. Properties of strange quark
matter in bulk and strangelets at both zero and finite temperature are then
calculated based on the new thermodynamic formulas with a new quark mass
scaling, which indicates that low mass strangelets near beta equilibrium are
multi-quark states with an anti-strange quark, such as the pentaquark
(u^2d^2\bar{s}) for baryon nmber 1 and the octaquark (u^4d^3\bar{s}) for
dibaryon etc.Comment: 14 pages, 12 figures, Revtex4 styl
- …