28,625 research outputs found
Polycyclic aromatic hydrocarbons and molecular hydrogen in oxygen-rich planetary nebulae: the case of NGC6720
Evolved stars are primary sources for the formation of polycyclic aromatic
hydrocarbons (PAHs) and dust grains. Their circumstellar chemistry is usually
designated as either oxygen-rich or carbon-rich, although dual-dust chemistry
objects, whose infrared spectra reveal both silicate- and carbon-dust features,
are also known. The exact origin and nature of this dual-dust chemistry is not
yet understood. Spitzer-IRS mid-infrared spectroscopic imaging of the nearby,
oxygen-rich planetary nebula NGC6720 reveals the presence of the 11.3 micron
aromatic (PAH) emission band. It is attributed to emission from neutral PAHs,
since no band is observed in the 7 to 8 micron range. The spatial distribution
of PAHs is found to closely follow that of the warm clumpy molecular hydrogen
emission. Emission from both neutral PAHs and warm H2 is likely to arise from
photo-dissociation regions associated with dense knots that are located within
the main ring. The presence of PAHs together with the previously derived high
abundance of free carbon (relative to CO) suggest that the local conditions in
an oxygen-rich environment can also become conducive to in-situ formation of
large carbonaceous molecules, such as PAHs, via a bottom-up chemical pathway.
In this scenario, the same stellar source can enrich the interstellar medium
with both oxygen-rich dust and large carbonaceous molecules.Comment: Accepted by MNRAS. 5 page
Very weak electron-phonon coupling and strong strain coupling in manganites
The coupling of the manganite stripe phase to the lattice and to strain has
been investigated via transmission electron microscopy studies of
polycrystalline and thin film manganites. In polycrystalline \PCMOfiftwo a
lockin to in a sample with has been observed for the first
time. Such a lockin has been predicted as a key part of the Landau CDW theory
of the stripe phase. Thus it is possible to constrain the size of the
electron-phonon coupling in the CDW Landau theory to between 0.04% and 0.05% of
the electron-electron coupling term. In the thin film samples, films of the
same thickness grown on two different substrates exhibited different
wavevectors. The different strains present in the films on the two substrates
can be related to the wavevector observed via Landau theory. It is demonstrated
that the the elastic term which favours an incommensurate modulation has a
similar size to the coupling between the strain and the wavevector, meaning
that the coupling of strain to the superlattice is unexpectedly strong.Comment: 6 pages, 7 figure
Reply to ``Comment on `Insulating Behavior of -DNA on the Micron Scale' "
In our experiment, we found that the resistance of vacuum-dried -DNA
exceeds at 295 K. Bechhoefer and Sen have raised a number of
objections to our conclusion. We provide counter arguments to support our
original conclusion.Comment: 1 page reply to comment, 1 figur
Solution of the Two-Channel Anderson Impurity Model - Implications for the Heavy Fermion UBe -
We solve the two-channel Anderson impurity model using the Bethe-Ansatz. We
determine the ground state and derive the thermodynamics, obtaining the
impurity entropy and specific heat over the full range of temperature. We show
that the low temperature physics is given by a line of fixed points decribing a
two-channel non Fermi liquid behavior in the integral valence regime associated
with moment formation as well as in the mixed valence regime where no moment
forms. We discuss relevance for the theory of UBe.Comment: 4 pages, 2 figures, (to be published in PRL
The structure of intercalated water in superconducting NaCoO1.37DO: Implications for the superconducting phase diagram
We have used electron and neutron powder diffraction to elucidate the
structural properties of superconducting \NaD. Our measurements show that our
superconducting sample exhbits a number of supercells ranging from
to , but the most predominant one, observed also in the neutron
data, is a double hexagonal cell with dimensions \dhx. Rietveld analysis
reveals that \deut\space is inserted between CoO sheets as to form a
layered network of NaO triangular prisms. Our model removes the need to
invoke a 5K superconducting point compound and suggests that a solid solution
of Na is possible within a constant amount of water .Comment: 4 pages, 3 figure
Optical conductivity of a metal-insulator transition for the Anderson-Hubbard model in 3 dimensions away from 1/2 filling
We have completed a numerical investigation of the Anderson-Hubbard model for
three-dimensional simple cubic lattices using a real-space self-consistent
Hartree-Fock decoupling approximation for the Hubbard interaction. In this
formulation we treat the spatial disorder exactly, and therefore we account for
effects arising from localization physics. We have examined the model for
electronic densities well away 1/2 filling, thereby avoiding the physics of a
Mott insulator. Several recent studies have made clear that the combined
effects of electronic interactions and spatial disorder can give rise to a
suppression of the electronic density of states, and a subsequent
metal-insulator transition can occur. We augment such studies by calculating
the ac conductivity for such systems. Our numerical results show that weak
interactions enhance the density of states at the Fermi level and the
low-frequency conductivity, there are no local magnetic moments, and the ac
conductivity is Drude-like. However, with a large enough disorder strength and
larger interactions the density of states at the Fermi level and the
low-frequency conductivity are both suppressed, the conductivity becomes
non-Drude-like, and these phenomena are accompanied by the presence of local
magnetic moments. The low-frequency conductivity changes from a sigma-sigma_dc
omega^{1/2} behaviour in the metallic phase, to a sigma omega^2 behaviour in
the nonmetallic regime. Our numerical results show that the formation of
magnetic moments is essential to the suppression of the density of states at
the Fermi level, and therefore essential to the metal-insulator transition
Theory of One-Channel vs. Multi-Channel Kondo Effects for Ce Impurities
We introduce a model for Ce impurities in cubic metals which exhibits
competition between the Fermi-liquid fixed point of the single channel Kondo
model and the non-Fermi-liquid fixed point of the two- and three-channel Kondo
models. Using the non-crossing approximation and scaling theory, we find: (i) A
possible three-channel Kondo effect between the one- and two-channel regimes in
parameter space. (ii) The sign of the thermopower is a fixed point diagnostic.
(iii) Our results will likely survive the introduction of additional and
conduction states. We apply this model to interpret the non-Fermi liquid alloy
LaCeCuSi.Comment: 13 pages, Revtex, To appear in Phys. Rev. Let
High Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula
We report high resolution imaging of a prominent cometary globule in the
Helix nebula in the CO J=1-0 (2.6 mm) and H2 v=1-0 S(1) (2.12 micron) lines.
The observations confirm that globules consist of dense condensations of
molecular gas embedded in the ionized nebula. The head of the globule is seen
as a peak in the CO emission with an extremely narrow line width (0.5 km/s) and
is outlined by a limb-brightened surface of H2 emission facing the central star
and lying within the photo-ionized halo. The emission from both molecular
species extends into the tail region. The presence of this extended molecular
emission provides new constraints on the structure of the tails, and on the
origin and evolution of the globules.Comment: 12 pages, 3 figures. To appear in The Astrophysical Journal Letter
Co-operative Kondo Effect in the two-channel Kondo Lattice
We discuss the possibility of a co-operative Kondo effect driven by channel
interference in a Kondo lattice where local moments are coupled to a single
Fermi sea via two orthogonal scattering channels. In this situation, the
channel quantum number is not conserved. We argue that the absence of channel
conservation causes the Kondo effect in the two channels to constructively
interfere, giving rise to a superconducting condensate of composite pairs,
formed between the local moments and the conduction electrons. Our arguments
are based on the observation that a heavy Fermi surface gives rise to zero
modes for Kondo singlets to fluctuate between screening channels of different
symmetry, producing a divergent composite pair susceptibility. Secondary
screening channels couple to these divergent fluctuations, promoting an
instability into a state with long-range composite order. We present detailed a
detailed mean-field theory for this superconducting phase, and discuss the
possible implications for heavy fermion physics.Comment: 23 double column pages. 9 fig
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