5,480 research outputs found
Improving the conductance of carbon nanotube networks through resonant momentum exchange
We present a mechanism to improve the conductivity of carbon nanotube (CNT)
networks by improving the conductance between CNTs of different chirality. We
argue generally that a weak perturbation can greatly improve the inter-tube
conductance by allowing momentum-conserving tunnelling. The mechanism is
verified with a tight-binding model, allowing an investigation of its impact
for a network containing a range of chiralities. We discuss practical
implementations, and conclude that it may be effected by weak physical
interactions, and therefore does not require chemical bonding to the CNTs.Comment: 6 pages, 4 figure
Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13
An analysis of the CN and CH band strengths measured in a large sample of M71
and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic
spectra to quantify the underlying C and N abundances. In the case of M71 it is
found that the observed CN and CH band strengths are best matched by the
{\it{identical}} C/N/O abundances which fit the bright giants, implying: 1)
little if any mixing is taking place during red giant branch ascent in M71, and
2) a substantial component of the C and N abundance inhomogeneities is in place
before the main sequence turn-off. The unlikelihood of mixing while on the main
sequence requires an explanation for the abundance variations which lies
outside the present stars (primordial inhomogeneities or intra-cluster self
enrichment). For M13 it is shown that the 3883\AA CN bands are too weak to be
measured in the spectra for any reasonable set of expected compositions. A
similar situation exists for CH as well. However, two of the more luminous
program stars do appear to have C abundances considerably greater than those
found among the bright giants thereby suggesting deep mixing has taken place on
the M13 red giant branch.Comment: 14 pages, 4 figures, accepted for publication by A
The 67 Hz Feature in the Black Hole Candidate GRS 1915+105 as a Possible ``Diskoseismic'' Mode
The Rossi X-ray Timing Explorer (RXTE) has made feasible for the first time
the search for high-frequency (~ 100 Hz) periodic features in black hole
candidate (BHC) systems. Such a feature, with a 67 Hz frequency, recently has
been discovered in the BHC GRS 1915+105 (Morgan, Remillard, & Greiner). This
feature is weak (rms variability ~0.3%-1.6%), stable in frequency (to within ~2
Hz) despite appreciable luminosity fluctuations, and narrow (quality factor Q ~
20). Several of these properties are what one expects for a ``diskoseismic''
g-mode in an accretion disk about a 10.6 M_sun (nonrotating) - 36.3 M_sun
(maximally rotating) black hole (if we are observing the fundamental mode
frequency). We explore this possibility by considering the expected luminosity
modulation, as well as possible excitation and growth mechanisms---including
turbulent excitation, damping, and ``negative'' radiation damping. We conclude
that a diskoseismic interpretation of the observations is viable.Comment: 4 Pages, Latex (emulateapj.sty included), to Appear in ApJ Letters,
Vol. 477, Final Version with Updated Reference
Rietveld refinement of the crystal structures of Rb2X Si5O12 (X = Ni, Mn)
The synthetic leucite silicate framework mineral analogues Rb2XSi5O12 {X = Ni
[dirubidium nickel(II) pentasilicate] and Mn [dirubidium manganese(II)
pentasilicate]} have been prepared by high-temperature solid-state synthesis.
The results of Rietveld refinements, using X-ray powder diffraction data
collected using Cu K[alpha] X-rays, show that the title compounds crystallize in the
space group Pbca and adopt the cation-ordered structure of Cs2CdSi5O12 and
other leucites. The structures consist of tetrahedral SiO4 and XO4 units sharing
corners to form a partially substituted silicate framework. Extraframework Rb+
cations sit in channels in the framework. All atoms occupy the 8c general
position for this space group. In these refined structures, silicon and X atoms are
ordered onto separate tetrahedrally coordinated sites (T-sites). However, the Ni
displacement parameter and the Ni—O bond lengths suggest that for the X = Ni
sample, there may actually be some T-site cation disorder
Production of multipartite entanglement for electron spins in quantum dots
We propose how to generate genuine multipartite entanglement of electron spin
qubits in a chain of quantum dots using the naturally available single-qubit
rotations and two-qubit Heisenberg exchange interaction in the system. We show
that the minimum number of required operations to generate entangled states of
the GHZ-, cluster and W-type scales linearly with the number of qubits and
estimate the fidelities of the generated entangled cluster states. As the
required single and two-qubit operations have recently been realized, our
proposed scheme opens the way for experimental investigation of multipartite
entanglement with electron spin qubits.Comment: 8 pages, 2 Figure
Rietveld refinements of the crystal structures of Rb2XSi5O12 (X = Mn, Ni)
Poster Number: CCG06
Synthetic analogues of the silicate framework mineral leucite (KAlSi2O6) with the stoichiometry
Rb2XSi5O12 (X = Mn, Ni) have been prepared by high temperature solid-state synthesis. Ambient temperature X-ray powder diffraction data have been collected on these samples. Analysis of
these powder diffraction data show that these samples both consist of single phases [1] isostructural with the Pbca cation-ordered framework leucite structure of Cs2CdSi5O12 [2].
Rietveld refinement [3] shows that for X = Mn this crystal structure has complete Mn and Si cation order over the tetrahredrally coordinated sites (T-sites) in the silicate framework. However, for X = Ni, Rietveld refinement suggests that there may be some Ni and Si cation T-site cation disorder
C and N Abundances in Stars At the Base of the Red Giant Branch in M5
We present an analysis of a large sample of moderate resolution Keck LRIS
spectra of subgiant (V \sim 17.2) and fainter stars in the Galactic globular
cluster M5 (NGC 5904) with the goal of deriving C and N abundances.
Star-to-star stochastic variations with significant range in both [C/Fe] and
[N/Fe] are found at all luminosities extending to the bottom of the RGB at M_V
\sim +3. Similar variations in CH appear to be present in the main sequence
turnoff spectra. There is no sign of a change in the behavior of C and N with
evolutionary stage over the full range in luminosity of the RGB and SGB. The C
and N abundances appear strongly anti-correlated, as would be expected from the
CN-cycle processing of stellar material. Yet the present stars are considerably
fainter than the RGB bump, the point at which deep mixing is believed to set
in. On this basis, while the observed abundance pattern is consistent with
proton capture nucleosynthesis, we infer that the site of the reactions is
likely not within the present sample, but rather in a population of more
massive (2 -- 5 M(Sun)) now defunct stars. The range of variation of the N
abundances is very large and the sum of C+N increases as C decreases. To
reproduce this requires the incorporation not only of CN but also of
ON-processed material. Furthermore, the existence of this correlation is quite
difficult to reproduce with an external mechanism such as ``pollution'' with
material processed in a more massive AGB star, which mechanism is fundamentally
stochastic in nature. We therefore suggest that although the internal mixing
hypothesis has serious flaws,new theoretical insights are needed and it should
not be ruled out yet. (abridged)Comment: Slightly updated version to conform to that accepted by the A
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Synthesis and Variable Temperature Electrical Conductivity Studies of Highly Ordered TiO2 Nanotubes
Rafts of aligned, high aspect ratio TiO2 nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225-400 °C. Length, outer diameter and wall thickness of the nanotubes were approximately 60-80 µm, 160 nm and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO2 nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment attemperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250 - 350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.Engineering and Applied Science
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