23,859 research outputs found
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Structure And Properties of Nanoparticles Formed under Conditions of Wire Electrical Explosion
Structure and properties of nanoparticles formed under conditions of wire
electrical explosion were studied. It was shown that the state of WEE power
particles can be characterized as a metastable state. It leads to an increased
stability of nanopowders at normal temperatures and an increased reactivity
during heating, which is revealed in the form of threshold phenomena.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Who Contributes to the Knowledge Sharing Economy?
Information sharing dynamics of social networks rely on a small set of
influencers to effectively reach a large audience. Our recent results and
observations demonstrate that the shape and identity of this elite, especially
those contributing \emph{original} content, is difficult to predict.
Information acquisition is often cited as an example of a public good. However,
this emerging and powerful theory has yet to provably offer qualitative
insights on how specialization of users into active and passive participants
occurs.
This paper bridges, for the first time, the theory of public goods and the
analysis of diffusion in social media. We introduce a non-linear model of
\emph{perishable} public goods, leveraging new observations about sharing of
media sources. The primary contribution of this work is to show that
\emph{shelf time}, which characterizes the rate at which content get renewed,
is a critical factor in audience participation. Our model proves a fundamental
\emph{dichotomy} in information diffusion: While short-lived content has simple
and predictable diffusion, long-lived content has complex specialization. This
occurs even when all information seekers are \emph{ex ante} identical and could
be a contributing factor to the difficulty of predicting social network
participation and evolution.Comment: 15 pages in ACM Conference on Online Social Networks 201
Daytime lidar measurements of tidal winds in the mesospheric sodium layer at Urbana, Illinois
For more than 15 years lidar systems have been used to study the chemistry and dynamics of the mesospheric sodium layer. Because the layer is an excellent tracer of atmospheric wave motions, sodium lidar has proven to be particularly useful for studying the influence of gravity waves and tides on mesospheric dynamics. These waves, which originate in the troposphere and stratosphere, propagate through the mesosphere and dissipate their energy near the mesopause making important contributions to the momentum and turbulence budget in this region of the atmosphere. Recently, the sodium lidar was modified for daytime operation so that wave phenomena and chemical effects could be monitored throughout the complete diurnal cycle. The results of continuous 24 hour lidar observations of the sodium layer structure are presented alond with measurement of the semidiurnal tidal winds
Electronic structures of doped anatase : (M=Co, Mn, Fe, Ni)
We have investigated electronic structures of a room temperature diluted
magnetic semiconductor : Co-doped anatase . We have obtained the
half-metallic ground state in the local-spin-density approximation(LSDA) but
the insulating ground state in the LSDA++SO incorporating the spin-orbit
interaction. In the stoichiometric case, the low spin state of Co is realized
with the substantially large orbital moment. However, in the presence of oxygen
vacancies near Co, the spin state of Co becomes intermediate. The
ferromagnetisms in the metallic and insulating phases are accounted for by the
double-exchange-like and the superexchange mechanism, respectively. Further,
the magnetic ground states are obtained for Mn and Fe doped ,
while the paramagnetic ground state for Ni-doped .Comment: 5 pages, 4 figure
Area spectra of the rotating BTZ black hole from quasinormal modes
Following Bekenstein's suggestion that the horizon area of a black hole
should be quantized, the discrete spectrum of the horizon area has been
investigated in various ways. By considering the quasinormal mode of a black
hole, we obtain the transition frequency of the black hole, analogous to the
case of a hydrogen atom, in the semiclassical limit. According to Bohr's
correspondence principle, this transition frequency at large quantum number is
equal to classical oscillation frequency. For the corresponding classical
system of periodic motion with this oscillation frequency, an action variable
is identified and quantized via Bohr-Sommerfeld quantization, from which the
quantized spectrum of the horizon area is obtained. This method can be applied
for black holes with discrete quasinormal modes. As an example, we apply the
method for the both non-rotating and rotating BTZ black holes and obtain that
the spectrum of the horizon area is equally spaced and independent of the
cosmological constant for both cases
Area spectra versus entropy spectra in black holes in topologically massive gravity
We consider the area and entropy spectra of black holes in topologically
massive gravity with gravitational Chern-Simons term. The examples we consider
are the BTZ black hole and the warped AdS black hole. For the non-rotating BTZ
black hole, the area and entropy spectra are equally spaced and independent of
the coupling constant \v of the Chern-Simons term. For the rotating BTZ black
hole case, the spectra of the inner and outer horizon areas are not equally
spaced in general and dependent of the coupling constant \v. However the
entropy spectrum is equally spaced and independent of the coupling constant
\v. For the warped AdS black holes for by using the quasinormal modes
obtained without imposing the boundary condition at radial infinity we find
again that the entropy spectrum is equally spaced and independent of the
coupling constant \v, while the spectra of the inner and outer horizon areas
are not equally spaced and dependent of the coupling constant \v. Our result
implies that the entropy spectrum has a universal behavior regardless of the
presence of the gravitational Chern-Simons term, and therefore it implies that
the entropy is more `fundamental' than the horizon area.Comment: 16 page
Quantifying signals with power-law correlations: A comparative study of detrended fluctuation analysis and detrended moving average techniques
Detrended fluctuation analysis (DFA) and detrended moving average (DMA) are
two scaling analysis methods designed to quantify correlations in noisy
non-stationary signals. We systematically study the performance of different
variants of the DMA method when applied to artificially generated long-range
power-law correlated signals with an {\it a-priori} known scaling exponent
and compare them with the DFA method. We find that the scaling
results obtained from different variants of the DMA method strongly depend on
the type of the moving average filter. Further, we investigate the optimal
scaling regime where the DFA and DMA methods accurately quantify the scaling
exponent , and how this regime depends on the correlations in the
signal. Finally, we develop a three-dimensional representation to determine how
the stability of the scaling curves obtained from the DFA and DMA methods
depends on the scale of analysis, the order of detrending, and the order of the
moving average we use, as well as on the type of correlations in the signal.Comment: 15 pages, 16 figure
On the Ground State of Electron Gases at Negative Compressibility
Two- and three-dimensional electron gases with a uniform neutralizing
background are studied at negative compressibility. Parametrized expressions
for the dielectric function are used to access this strong-coupling regime,
where the screened Coulomb potential becomes overall attractive for like
charges. Closely examining these expressions reveals that the ground state with
a periodic modulation of the charge density, albeit exponentially damped,
replaces the homogeneous one at positive compressibility. The wavevector
characterizing the new ground state depends on the density and is complex,
having a positive imaginary part, as does the homogeneous ground state, and
real part, as does the genuine charge density wave.Comment: 6 double-column pages, 2 figures. 2nd version is an extension of the
1st one, giving more detail
Dynamic ploidy changes drive fluconazole resistance in human cryptococcal meningitis.
BACKGROUND: Cryptococcal meningitis (CM) causes an estimated 180,000 deaths annually, predominantly in sub-Saharan Africa, where most patients receive fluconazole (FLC) monotherapy. While relapse after FLC monotherapy with resistant strains is frequently observed, the mechanisms and impact of emergence of FLC resistance in human CM are poorly understood. Heteroresistance (HetR) - a resistant subpopulation within a susceptible strain - is a recently described phenomenon in Cryptococcus neoformans (Cn) and Cryptococcus gattii (Cg), the significance of which has not previously been studied in humans. METHODS: A cohort of 20 patients with HIV-associated CM in Tanzania was prospectively observed during therapy with either FLC monotherapy or in combination with flucytosine (5FC). Total and resistant subpopulations of Cryptococcus spp. were quantified directly from patient cerebrospinal fluid (CSF). Stored isolates underwent whole genome sequencing and phenotypic characterization. RESULTS: Heteroresistance was detectable in Cryptococcus spp. in the CSF of all patients at baseline (i.e., prior to initiation of therapy). During FLC monotherapy, the proportion of resistant colonies in the CSF increased during the first 2 weeks of treatment. In contrast, no resistant subpopulation was detectable in CSF by day 14 in those receiving a combination of FLC and 5FC. Genomic analysis revealed high rates of aneuploidy in heteroresistant colonies as well as in relapse isolates, with chromosome 1 (Chr1) disomy predominating. This is apparently due to the presence on Chr1 of ERG11, which is the FLC drug target, and AFR1, which encodes a drug efflux pump. In vitro efflux levels positively correlated with the level of heteroresistance. CONCLUSION: Our findings demonstrate for what we believe is the first time the presence and emergence of aneuploidy-driven FLC heteroresistance in human CM, association of efflux levels with heteroresistance, and the successful suppression of heteroresistance with 5FC/FLC combination therapy. FUNDING: This work was supported by the Wellcome Trust Strategic Award for Medical Mycology and Fungal Immunology 097377/Z/11/Z and the Daniel Turnberg Travel Fellowship
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