23,979 research outputs found
Influence of Dzyaloshinskii-Moriya interactions on magnetic structure of a spin-1/2 deformed kagome lattice antiferromagnet
Motivated by the recent neutron scattering experiment on Rb2Cu3SnF12 [Nat.
Phys. 6, 865 (2010)], we investigate the effect of Dzyaloshinskii-Moriya
interactions in a theoretical model for the magnetic structure of this
material. Considering the valence bond solid ground state, which has a 12-site
unit cell, we develop the bond operator mean-field theory. It is shown that the
Dzyaloshinskii-Moriya interactions significantly modify the triplon dispersions
around the Gamma point and cause a shift of the spin gap (the minimum triplon
gap) position from the K to Gamma point in the first Brilloin zone. The spin
gap is also evaluated in exact diagonalization studies on a 24-site cluster. We
discuss a magnetic transition induced by the Dzyaloshinskii-Moriya interactions
in the bond operator framework. Moreover, the magnetization process under
external magnetic fields is studied within the exact diagonalization and strong
coupling expansion approaches. We find that the results of all above approaches
are consistent with the experimental findings.Comment: 14 pages, 10 figures; typos corrected, and acknowledgements and
references adde
An Adaptive Algorithm to Optimize the Dynamics of IEEE 802.15.4 Networks
Presentado en ICST 2013IEEE 802.15.4 standard is becoming one of the most popular technologies for the deployment of low rate Wireless Personal Area Networks with strong power constraints. In order to reduce the energy consumption, beacon-enabled networks with long network inactive periods can be employed. However, the duration of these inactivity periods, as some other configuration parameters, are conventionally set to default values and remain fixed during the whole network operation. This implies that if they are misconfigured the network will not adapt to changes in the conditions of the environment, particularly to the most determining one, i.e. the traffic load. This paper proposes a simple procedure for the dynamic adaptation of several key parameters of IEEE 802.15.4 networks. Under this procedure, the 802.15.4 parameters are modified as a function of the existing traffic conditions.Spanish National Project No.TEC2009-13763-C02-01
Albumin-heparin microspheres as carriers for cytostatic agents
Much work has been done on adriamycin-loaded albumin microspheres (Alb-MS) for chemoembolization [1–4], the rationale being that site-specific drug delivery may increase the therapeutic efficacy of the drug. Alb-Ms are being investigated because of their biocompatibility and because the degradation products of these microspheres are non-toxic. However, these microspheres have some disadvantages (i.e. drug loading during the microsphere preparation, low payloads, large burst effects). These disadvantages can be overcome by the incorporation of heparin (a highly negatively charged mucopolysaccharide). Albumin-heparin microspheres were prepared (i) by crosslinking of soluble albumin and heparin first using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and subsequently glutaraldehyde (Alb-Hep-MS) and (ii) by crosslinking a preformed soluble conjugate of heparin and albumin with glutaraldehyde (Alb-Hep-Conj-MS). Albumin-heparin microspheres could be loaded with adriamycin after microsphere preparation giving payloads of 15–30%. Preliminary in vitro adriamycin release experiments showed that Alb-Hep-Conj-MS exhibit sustained release properties. Furthermore ion-exchange properties could be observed both with Alb-Hep-MS and Alb-Hep-Conj-MS. In vitro and in vivo toxicity experiments with Alb-Hep-MS showed no adverse effects
Adriamycin loading and release characteristics of albumin-heparin conjugate microspheres
Biodegradable ion-exchange microspheres, prepared from a prefabricated conjugate of albumin and heparin were investigated as carriers for adriamycin. The ion-exchange microspheres could be loaded with adriamycin giving payloads up to 33% w/w, depending on the heparin content of the conjugate. In vitro adriamycin release depended on the ionic strength of the release medium. In ion containing media, for instance saline, 90% of the drug was released within 45 min, whereas in non-ionic media, such as distilled water, only 30% was released. Drug release profiles could be modelled by combining ion-exchange kinetics and diffusion controlled drug release models
Simulation of iced wing aerodynamics
The sectional and total aerodynamic load characteristics of moderate aspect ratio wings with and without simulated glaze leading edge ice were studied both computationally, using a three dimensional, compressible Navier-Stokes solver, and experimentally. The wing has an untwisted, untapered planform shape with NACA 0012 airfoil section. The wing has an unswept and swept configuration with aspect ratios of 4.06 and 5.0. Comparisons of computed surface pressures and sectional loads with experimental data for identical configurations are given. The abrupt decrease in stall angle of attack for the wing, as a result of the leading edge ice formation, was demonstrated numerically and experimentally
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
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
Critical renormalized coupling constants in the symmetric phase of the Ising models
Using a novel finite size scaling Monte Carlo method, we calculate the four,
six and eight point renormalized coupling constants defined at zero momentum in
the symmetric phase of the three dimensional Ising system. The results of the
2D Ising system that were directly measured are also reported. Our values of
the six and eight point coupling constants are significantly different from
those obtained from other methods.Comment: 7 pages, 2 figure
- …