13,315 research outputs found
Temporal disorder in up-down symmetric systems
The effect of temporal disorder on systems with up-down Z2 symmetry is
studied. In particular, we analyze two well-known families of phase
transitions: the Ising and the generalized voter universality classes, and
scrutinize the consequences of placing them under fluctuating global
conditions. We observe that variability of the control parameter induces in
both classes "Temporal Griffiths Phases" (TGP). These recently-uncovered phases
are analogous to standard Griffiths Phases appearing in systems with quenched
spatial disorder, but where the roles of space and time are exchanged. TGPs are
characterized by broad regions in parameter space in which (i) mean
first-passage times scale algebraically with system size, and (ii) the system
response (e.g. susceptibility) diverges. Our results confirm that TGPs are
quite robust and ubiquitous in the presence of temporal disorder. Possible
applications of our results to examples in ecology are discussed
Dynamical phase coexistence: A simple solution to the "savanna problem"
We introduce the concept of 'dynamical phase coexistence' to provide a simple
solution for a long-standing problem in theoretical ecology, the so-called
"savanna problem". The challenge is to understand why in savanna ecosystems
trees and grasses coexist in a robust way with large spatio-temporal
variability. We propose a simple model, a variant of the Contact Process (CP),
which includes two key extra features: varying external
(environmental/rainfall) conditions and tree age. The system fluctuates locally
between a woodland and a grassland phase, corresponding to the active and
absorbing phases of the underlying pure contact process. This leads to a highly
variable stable phase characterized by patches of the woodland and grassland
phases coexisting dynamically. We show that the mean time to tree extinction
under this model increases as a power-law of system size and can be of the
order of 10,000,000 years in even moderately sized savannas. Finally, we
demonstrate that while local interactions among trees may influence tree
spatial distribution and the order of the transition between woodland and
grassland phases, they do not affect dynamical coexistence. We expect dynamical
coexistence to be relevant in other contexts in physics, biology or the social
sciences.Comment: 8 pages, 7 figures. Accepted for publication in Journal of
Theoretical Biolog
A likely magnetic activity cycle for the exoplanet host M dwarf GJ 3512
Current radial velocity data from specialized instruments contain a large
amount of information that may pass unnoticed if their analysis is not
accurate. The joint use of Bayesian inference tools and frequency analysis has
been shown effective to reveal exoplanets but they have been used less
frequently to investigate stellar activity. We intend to use radial velocity
data of the exoplanet host star GJ 3512 to investigate its magnetic activity.
Our study includes the analysis of the photometric data available. The main
objectives of our work are to constrain the orbital parameters of the
exoplanets in the system, to determine the current level of activity of the
star and to derive an activity cycle length for it. An adaptive importance
sampling method was used to determine the parameters of the exoplanets orbit.
Generalized Lomb-Scargle periodograms were constructed with both radial
velocity curve and photometric data. A careful analysis of the harmonic
frequencies was conducted in each periodogram. Our fit to multiple Keplerian
orbits constrained the orbital parameters of two giant gas planets orbiting the
star GJ 3512. The host star showed an increase of its magnetic activity during
the last observing campaign. The accurate fit of the radial velocity curve data
to the multi-Keplerian orbit permitted to reveal the star rotation in the
residuals of the best fit and estimate an activity cycle length of ~ 14 years.Comment: Accepted for publication in the Astronomical Journal. 15 pages, 11
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Dynamics and rheology of a suspension of super-paramagnetic chains under the combined effect of a shear flow and a rotating magnetic field
This study presents an analysis of the dynamics of a single and multiple chains of spherical super-paramagnetic beads suspended in a Newtonian fluid under the combined effect of an external rotating magnetic field and a shear flow. Viscosity results depend on two main non-dimensional numbers: the ratio between the shear rate and the magnetic rotation frequency and the ratio between the hydrodynamic and magnetostatic interactions (the Mason number). When the shear rate is smaller than the magnetic frequency, the chain rotation accelerates the surrounding fluid, reducing the value of the measured suspension viscosity even below the solvent one. In this regime, shear-thickening is observed. For values of the shear rates comparable to the rotation magnetic frequency, the viscosity reaches a maximum and non-linear coupling effects come up. If the shear rate is increased to values above the rotation frequency, the viscosity decreases and a mild shear-thinning is observed. In terms of the Mason number, the suspension viscosity reduces in line with literature results reported for fixed magnetic fields, whereas the shear-rate/magnetic-frequency ratio parameters induces a shift of the viscosity curve towards larger values. Results at larger concentrations and multiple chains amplify the observed effects
Ultra Low-Power Analog Median Filters
The design and implementation of three analog median filter topologies, whose transistors operate in the deep weak-inversion region, is described. The first topology is a differential pairs array, in which drain currents are driven into two nodes in a differential fashion, while the second topology is based on a wide range OTA, which is used to maximize the dynamic range. Finally, the third topology uses three range-extended OTAs. The proposed weak-inversion filters were designed and fabricated in ON Semiconductor 0.5 micrometer technology through MOSIS. Experimental results of three-input fabricated prototypes for all three topologies are show, where power consumptions of 90nW in the first case, and 270nW in the other two cases can be noticed. A dual power supply +/-1.5 Volts were used
Magnetic Fields to Enhance Tuned Liquid Damper Performance for Vibration Control: A Review
Tuned Liquid Dampers (TLDs) are dissipative devices whose distinguished features like low cost in installation and maintenance or their multidirectional and multifrequency application to new and already existing structures make them an attractive damping option. Their working principle is similar to that of a Tuned Mass Damper but in this case the relative movement comes from a fluid that provides with mass, damping and stiffness. Moreover, TLDs can mitigate both horizontal and vertical vibrations. All these make TLDs worth deeply studying. TLD utilization in civil vibration control arose in the 1980s. From early years, different improvements have been implemented to achieve a better performance. Some of these modifications include passive variations in the geometry or the fluid. The use of smart materials applied on TLDs has also been of great interest since the 1990s and comprehends different configurations in which magnetic fields are used to passively or semi-actively improve the TLD performance. A lack of review is detected in this field. For this reason, a state-of-the-art review is presented in this paper. Its aim is to help researchers find a thorough, up-to-date classification of the different possibilities, configurations, numerical evaluation, materials used and also found limitations and future development in the application of magnetic fields on TLDs
Host genetic factors associated with hepatocellular carcinoma in patients with hepatitis C virus infection: a systematic review
Hepatitis C virus (HCV)-infected patients are at risk of developing hepatocellular carcinoma (HCC). Individuals at heightened riskcould be targeted by intensive follow-up surveillance. We have conducted a systematic review of the literature to identify host genetic predisposition to HCC in HCV-infected patients.
A comprehensive search of Medline and Embase databases was performed and the strength of evidence of associations for each gene on development of HCC was evaluated.
We identified 166 relevant studies, relating to 137 different genes, or combinations thereof. 17 genes were classified as having âgoodâ evidence of an association, a significant association was observed for 37 genes but this finding had not yet been replicated, 56 genes had mixed or limited evidence of an association, and 27 genes showed no association. IFNL3/4, TNF-α and PNPLA3 genes had the most evidence of an association. There was, however, considerable heterogeneity in study design and data quality.
In conclusion, we identified a number of genes with evidence of association with HCC, but also a need for more standardised approaches to address this clinically critical question. It is important to consider the underlying mechanism of these relationships and which are confounded by the presence of other HCC risk factors and response to therapy. We also identified many genes where the evidence of association is contradictory or requires replication, as well as a number where associations have been studied but no evidence found. These findings should help to direct future studies on host genetic predisposition to HCC in patients with HCV infection
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