96,494 research outputs found
Binding energies and electronic structures of adsorbed titanium chains on carbon nanotubes
We have studied the binding energies and electronic structures of metal (Ti,
Al, Au) chains adsorbed on single-wall carbon nanotubes (SWNT) using first
principles methods. Our calculations have shown that titanium is much more
favored energetically over gold and aluminum to form a continuous chain on a
variety of SWNTs. The interaction between titanium and carbon nanotube
significantly modifies the electronic structures around Fermi energy for both
zigzag and armchair tubes. The delocalized 3d electrons from the titanium chain
generate additional states in the band gap regions of the semiconducting tubes,
transforming them into metals.Comment: 4 pages, 3 figure
Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation
The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth
in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by
muon-spin rotation. A pronounced OIE on was observed
with a relative isotope shift of
=-5(2)% for x =0.3 and -9(2)% for
x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane
effective mass . The OIE exponents of T_{c} and of
exhibit a relation that appears to be generic for
cuprate superconductors.Comment: 4 pages, 4 eps figures, RevTex
Modelling of Electroluminescence in Polymers Using a Bipolar Charge Transport Model
Electroluminescence (EL) in polymeric materials is thought to occur due to the energy dissipation process from the recombination of opposite polarity charge carriers. It is considered as an indication of storage and transport of charge carriers in cable insulation subject to electrical stresses and may indicate the change in charge movement due to aging or degradation processes. Under ac electric fields, the interaction of opposite polarity charge carriers at the interface of polymer/conductor is enhanced compared with dc conditions, and seems to contribute a lot to the electroluminescence rather than the charge behaviours in the bulk of polymers. The dynamics of charge carriers both at the interface of polymer/conductor and in the bulk of polymers is investigated through a simulation work using a bipolar charge transport model. Figure 1 compares experimental electroluminescence results with simulated data from the recombination of injected charge carriers. The paper will give more details on EL model and comparison under various waveforms and frequencies
A Radial Velocity Survey for LMC Microlensed Sources
We propose a radial velocity survey with the aim to resolve the current
dispute on the LMC lensing: in the pro-macho hypothesis the lenses are halo
white dwarfs or machos in general; in the pro-star hypothesis both the lenses
and the sources are stars in various observed or hypothesized structures of the
Magellanic Clouds and the Galaxy. Star-star lensing should prefer sources at
the backside or behind the LMC disc because lensing is most efficient if the
source is located a few kpc behind a dense screen of stars, here the thin disc
of the LMC. This signature of self-lensing can be looked for by a radial
velocity survey since kinematics of the stars at the back can be markedly
different from that of the majority of stars in the cold, rapidly rotating disc
of the LMC. Detailed simulations of effect together with optimal strategies of
carrying out the proposed survey are reported here. Assuming that the existing
30 or so alerted stars in the LMC are truely microlensed stars, their
kinematics can test the two lensing scenarios; the confidence level varies with
the still very uncertain structure of the LMC. Spectroscopy of the existing
sample and future events requires about two or three good-seeing nights per
year at a 4m-8m class southern telescope, either during the amplification phase
or long after.Comment: minor changes of text, ApJ accepte
A Model for Abundances in Metal-Poor Stars
It is argued that the abundances of r-process related elements in stars with
-3<[Fe/H]<-1 can be explained by the contributions of three sources. The
sources are: the first generations of very massive (>100 solar masses) stars
that are formed from Big Bang debris and are distinct from SNII, and two types
of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events
are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe
(presumably leaving behind black holes). The L events are of low frequency and
produce Fe and dominantly light (A<130) r-elements (essentially none above Ba).
By using the observed abundances in two ultra-metal-poor stars and the solar
r-abundances, the initial or prompt inventory of elements produced by the first
generations of very massive stars and the yields of H and L events can be
determined. The abundances of a large number of elements in a star can then be
calculated from the model by using only the observed Eu and Fe abundances. To
match the model results and the observational data for stars with -3<[Fe/H]<-1
requires that the solar r-abundances for Sr, Y, Zr, and Ba must be
significantly increased from the standard values. Whether the solar
r-components of these elements used here to obtain a fit to the stellar data
can be reconciled with those obtained from solar abundances by subtracting the
s-components calculated from models is not clear.Comment: 47 pages, 19 figures, to appear in Ap
Analytical considerations of flow boiling heat transfer in metal-foam filled tubes
Flow boiling in metal-foam filled tube was analytically investigated based on a modified microstructure model, an original boiling heat transfer model and fin analysis for metal foams. Microstructure model of metal foams was established, by which fiber diameter and surface area density were precisely predicted. The heat transfer model for flow boiling in metal foams was based on annular pattern, in which two phase fluid was composed by vapor region in the center of the tube and liquid region near the wall. However, it was assumed that nucleate boiling performed only in the liquid region. Fin analysis and heat transfer network for metal foams were integrated to obtain the convective heat transfer coefficient at interface. The analytical solution was verified by its good agreement with experimental data. The parametric study on heat transfer coefficient and boiling mechanism was also carried out
Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood
Two distinct halo populations were found in the solar neighborhood by a
series of works. They can be clearly separated by [alpha\Fe] and several other
elemental abundance ratios including [Cu/Fe]. Very recently, a non-local
thermodynamic equilibrium (non-LTE) study revealed that relatively large
departures exist between LTE and non-LTE results in copper abundance analysis.
We aim to derive the copper abundances for the stars from the sample of Nissen
et al (2010) with both LTE and non-LTE calculations. Based on our results, we
study the non-LTE effects of copper and investigate whether the high-alpha
population can still be distinguished from the low-alpha population in the
non-LTE [Cu/Fe] results. Our differential abundance ratios are derived from the
high-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.
Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesis
method, we derive the non-LTE copper abundances based on the new atomic model
with current atomic data obtained from both laboratory and theoretical
calculations. The copper abundances determined from non-LTE calculations are
increased by 0.01 to 0.2 dex depending on the stellar parameters compared with
the LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one in
the metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects into
consideration, the high- and low-alpha stars still show distinguishable copper
abundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus
[Fe/H]. The non-LTE effects are strong for copper, especially in metal-poor
stars. Our results confirmed that there are two distinct halo populations in
the solar neighborhood. The dichotomy in copper abundance is a peculiar feature
of each population, suggesting that they formed in different environments and
evolved obeying diverse scenarios.Comment: 9 pages, 7 figures, 2 table
Learning-aided Stochastic Network Optimization with Imperfect State Prediction
We investigate the problem of stochastic network optimization in the presence
of imperfect state prediction and non-stationarity. Based on a novel
distribution-accuracy curve prediction model, we develop the predictive
learning-aided control (PLC) algorithm, which jointly utilizes historic and
predicted network state information for decision making. PLC is an online
algorithm that requires zero a-prior system statistical information, and
consists of three key components, namely sequential distribution estimation and
change detection, dual learning, and online queue-based control.
Specifically, we show that PLC simultaneously achieves good long-term
performance, short-term queue size reduction, accurate change detection, and
fast algorithm convergence. In particular, for stationary networks, PLC
achieves a near-optimal , utility-delay
tradeoff. For non-stationary networks, \plc{} obtains an
utility-backlog tradeoff for distributions that last
time, where
is the prediction accuracy and is a constant (the
Backpressue algorithm \cite{neelynowbook} requires an length
for the same utility performance with a larger backlog). Moreover, PLC detects
distribution change slots faster with high probability ( is the
prediction size) and achieves an convergence time. Our results demonstrate
that state prediction (even imperfect) can help (i) achieve faster detection
and convergence, and (ii) obtain better utility-delay tradeoffs
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