14,402 research outputs found
A Proposal of a Renormalization Group Transformation for Lattice Field Theories
We propose a new Real Space Renormalization Group transformation useful for
Monte Carlo calculations in theories with global or local symmetries. From
relaxation arguments we define the block-spin transformation with two tunable
free parameters, adapted to the system's action. Varying them it is possible to
place the fixed point very close to the simulation point. We show how the
method works in a simple model with global symmetry: the three dimensional XY
model.Comment: 26 pages, uuencoded compressed postscript single file, 8 figures
include
Numerical test of the Cardy-Jacobsen conjecture in the site-diluted Potts model in three dimensions
We present a microcanonical Monte Carlo simulation of the site-diluted Potts
model in three dimensions with eight internal states, partly carried out in the
citizen supercomputer Ibercivis. Upon dilution, the pure model's first-order
transition becomes of the second-order at a tricritical point. We compute
accurately the critical exponents at the tricritical point. As expected from
the Cardy-Jacobsen conjecture, they are compatible with their Random Field
Ising Model counterpart. The conclusion is further reinforced by comparison
with older data for the Potts model with four states.Comment: Final version. 9 pages, 9 figure
The out-equilibrium 2D Ising spin glass: almost, but not quite, a free-field theory
We consider the spatial correlation function of the two-dimensional Ising
spin glass under out-equilibrium conditions. We pay special attention to the
scaling limit reached upon approaching zero temperature. The field-theory of a
non-interacting field makes a surprisingly good job at describing the spatial
shape of the correlation function of the out-equilibrium Edwards-Anderson Ising
model in two dimensions.Comment: 20 pages + 5 Figure
Effect of Dilution on First Order Transitions: The Three Dimensional Three States Potts Model
We have studied numerically the effect of quenched site dilution on a first
order phase transition in three dimensions. We have simulated the site diluted
three states Potts model studying in detail the second order region of its
phase diagram. We have found that the exponent is compatible with the one
of the three dimensional diluted Ising model whereas the exponent is
definitely different.Comment: RevTex. 6 pages and 6 postscript figure
Tempering Dynamics and Relaxation Times in the Ising Model
We discuss the tempering Monte Carlo method, and its critical slowing down in
the Ising model. We show that at the tempering does not change the
critical slowing down exponent . We also discuss the exponential slowing
down for the transition from the plus to the minus state in the cold phase, and
we show that tempering reduces it to a power law slowing down. We discuss the
relation of the flip-flop rate to the surface tension for the local dynamical
schemes.Comment: 10 pages, compressed ps-file (uufiles
Finding the sweet spot for frame aggregation in 802.11 WLANs
This letter proposes an algorithm for the dynamic tuning of the maximum size of aggregated frames in 802.11 WLANs. Traffic flows with opposed requirements may coexist in these networks: traditional services as web browsing or file download that need high throughput, and services with real-time requirements that need low latency. The proposed algorithm allows the network manager to find an optimal balance (i.e. the ''sweet spot'' between throughput and latency: a ''delay budget'' can be assigned to real-time flows, with the objective of keeping the latency as close as possible to that budget, while penalizing the throughput of traditional services as little as possible. © 1997-2012 IEEE
Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions
A Microcanonical Finite Site Ansatz in terms of quantities measurable in a
Finite Lattice allows to extend phenomenological renormalization (the so called
quotients method) to the microcanonical ensemble. The Ansatz is tested
numerically in two models where the canonical specific-heat diverges at
criticality, thus implying Fisher-renormalization of the critical exponents:
the 3D ferromagnetic Ising model and the 2D four-states Potts model (where
large logarithmic corrections are known to occur in the canonical ensemble). A
recently proposed microcanonical cluster method allows to simulate systems as
large as L=1024 (Potts) or L=128 (Ising). The quotients method provides
extremely accurate determinations of the anomalous dimension and of the
(Fisher-renormalized) thermal exponent. While in the Ising model the
numerical agreement with our theoretical expectations is impressive, in the
Potts case we need to carefully incorporate logarithmic corrections to the
microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure
IoTsafe, Decoupling Security from Applications for a Safer IoT
The use of robust security solutions is a must for the Internet of Things (IoT) devices and their applications: regulators in different countries are creating frameworks for certifying those devices with an acceptable security level. However, even for already certified devices, security protocols have to be updated when a breach is found or a certain version becomes obsolete. Many approaches for securing IoT applications are nowadays based on the integration of a security layer [e.g., using transport layer security, (TLS)], but this may result in difficulties when upgrading the security algorithms, as the whole application has to be updated. This fact may shorten the life of IoT devices. As a way to overcome these difficulties, this paper presents IoTsafe, a novel approach relying on secure socket shell (SSH), a feasible alternative to secure communications in IoT applications based on hypertext transfer protocol (HTTP and HTTP/2). In order to illustrate its advantages, a comparison between the traditional approach (HTTP with TLS) and our scheme (HTTP with SSH) is performed over low-power wireless personal area networks (6loWPAN) through 802.15.4 interfaces. The results show that the proposed approach not only provides a more robust and easy-To-update solution, but it also brings an improvement to the overall performance in terms of goodput and energy consumption. Core server stress tests are also presented, and the server performance is also analyzed in terms of RAM consumption and escalation strategies
Comment on "Evidence of Non-Mean-Field-Like Low-Temperature Behavior in the Edwards-Anderson Spin-Glass Model"
A recent interesting paper [Yucesoy et al. Phys. Rev. Lett. 109, 177204
(2012), arXiv:1206:0783] compares the low-temperature phase of the 3D
Edwards-Anderson (EA) model to its mean-field counterpart, the
Sherrington-Kirkpatrick (SK) model. The authors study the overlap distributions
P_J(q) and conclude that the two models behave differently. Here we notice that
a similar analysis using state-of-the-art, larger data sets for the EA model
(generated with the Janus computer) leads to a very clear interpretation of the
results of Yucesoy et al., showing that the EA model behaves as predicted by
the replica symmetry breaking (RSB) theory.Comment: Version accepted for publication in PRL. 1 page, 1 figur
Myristic acid potentiates palmitic acid-induced lipotoxicity and steatohepatitis associated with lipodystrophy by sustaning de novo ceramide synthesis.
Palmitic acid (PA) induces hepatocyte apoptosis and fuels de novo ceramide synthesis in the endoplasmic reticulum (ER). Myristic acid (MA), a free fatty acid highly abundant in copra/palmist oils, is a predictor of nonalcoholic steatohepatitis (NASH) and stimulates ceramide synthesis. Here we investigated the synergism between MA and PA in ceramide synthesis, ER stress, lipotoxicity and NASH. Unlike PA, MA is not lipotoxic but potentiated PA-mediated lipoapoptosis, ER stress, caspase-3 activation and cytochrome c release in primary mouse hepatocytes (PMH). Moreover, MA kinetically sustained PA-induced total ceramide content by stimulating dehydroceramide desaturase and switched the ceramide profile from decreased to increased ceramide 14:0/ceramide16:0, without changing medium and long-chain ceramide species. PMH were more sensitive to equimolar ceramide14:0/ceramide16:0 exposure, which mimics the outcome of PA plus MA treatment on ceramide homeostasis, than to either ceramide alone. Treatment with myriocin to inhibit ceramide synthesis and tauroursodeoxycholic acid to prevent ER stress ameliorated PA plus MA induced apoptosis, similar to the protection afforded by the antioxidant BHA, the pan-caspase inhibitor z-VAD-Fmk and JNK inhibition. Moreover, ruthenium red protected PMH against PA and MA-induced cell death. Recapitulating in vitro findings, mice fed a diet enriched in PA plus MA exhibited lipodystrophy, hepatosplenomegaly, increased liver ceramide content and cholesterol levels, ER stress, liver damage, inflammation and fibrosis compared to mice fed diets enriched in PA or MA alone. The deleterious effects of PA plus MA-enriched diet were largely prevented by in vivo myriocin treatment. These findings indicate a causal link between ceramide synthesis and ER stress in lipotoxicity, and imply that the consumption of diets enriched in MA and PA can cause NASH associated with lipodystrophy
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