495 research outputs found
Supersymmetry algebra cohomology III: Primitive elements in four and five dimensions
The primitive elements of the supersymmetry algebra cohomology as defined in
a previous paper are computed for standard supersymmetry algebras in four and
five dimensions, for all signatures of the metric and any number of
supersymmetries.Comment: v2: D=4 analysis simplified, D=5 analysis added, refs. added, typos
corrected, 32 page
Supersymmetry algebra cohomology I: Definition and general structure
The paper concerns standard supersymmetry algebras in diverse dimensions,
involving bosonic translational generators and fermionic supersymmetry
generators. A cohomology related to these supersymmetry algebras, termed
supersymmetry algebra cohomology, and corresponding "primitive elements" are
defined by means of a BRST-type coboundary operator. A method to systematically
compute this cohomology is outlined and illustrated by simple examples.Comment: v5: matches published version; 3 refs., section 5.5 and
remarks/comments in sections 1, 2.8, 3 and 7 added; minor editorial
improvements and change of titl
Surrogate Gradient Learning in Spiking Neural Networks
Spiking neural networks are nature's versatile solution to fault-tolerant and
energy efficient signal processing. To translate these benefits into hardware,
a growing number of neuromorphic spiking neural network processors attempt to
emulate biological neural networks. These developments have created an imminent
need for methods and tools to enable such systems to solve real-world signal
processing problems. Like conventional neural networks, spiking neural networks
can be trained on real, domain specific data. However, their training requires
overcoming a number of challenges linked to their binary and dynamical nature.
This article elucidates step-by-step the problems typically encountered when
training spiking neural networks, and guides the reader through the key
concepts of synaptic plasticity and data-driven learning in the spiking
setting. To that end, it gives an overview of existing approaches and provides
an introduction to surrogate gradient methods, specifically, as a particularly
flexible and efficient method to overcome the aforementioned challenges
Break up of heavy fermions at an antiferromagnetic instability
We present results of high-resolution, low-temperature measurements of the
Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2.
They support earlier conclusions of an electronic (Kondo-breakdown) quantum
critical point concurring with a field induced antiferromagnetic one. We also
discuss the detachment of the two instabilities under chemical pressure. Volume
compression/expansion (via substituting Rh by Co/Ir) results in a
stabilization/weakening of magnetic order. Moderate Ir substitution leads to a
non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor
screened by the Kondo effect. The so-derived zero-temperature global phase
diagram promises future studies to explore the nature of the Kondo breakdown
quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS
Ciguatera mini review: 21st century environmental challenges and the interdisciplinary research efforts rising to meet them
Globally, the livelihoods of over a billion people are affected by changes to marine eco-systems, both structurally and systematically. Resources and ecosystem services, provided by the marine environment, contribute nutrition, income, and health benefits for communities. One threat to these securities is ciguatera poisoning; worldwide, the most commonly reported non‐bacterial seafood‐related illness. Ciguatera is caused by the consumption of (primarily) finfish contaminated with ciguatoxins, potent neurotoxins produced by benthic single‐cell microalgae. When consumed, ciguatoxins are biotransformed and can bioaccumulate throughout the food‐web via complex path-ways. Ciguatera‐derived food insecurity is particularly extreme for small island‐nations, where fear of intoxication can lead to fishing restrictions by region, species, or size. Exacerbating these com-plexities are anthropogenic or natural changes occurring in global marine habitats, e.g., climate change, greenhouse‐gas induced physical oceanic changes, overfishing, invasive species, and even the international seafood trade. Here we provide an overview of the challenges and opportunities of the 21st century regarding the many facets of ciguatera, including the complex nature of this illness, the biological/environmental factors affecting the causative organisms, their toxins, vectors, detection methods, human‐health oriented responses, and ultimately an outlook towards the future. Ciguatera research efforts face many social and environmental challenges this century. However, several future‐oriented goals are within reach, including digital solutions for seafood supply chains, identifying novel compounds and methods with the potential for advanced diagnostics, treatments, and prediction capabilities. The advances described herein provide confidence that the tools are now available to answer many of the remaining questions surrounding ciguatera and therefore protection measures can become more accurate and routine
A yeast cell cycle model integrating stress, signaling, and physiology
The cell division cycle in eukaryotic cells is a series of highly coordinated molecular interactions that ensure that cell growth, duplication of genetic material, and actual cell division are precisely orchestrated to give rise to two viable progeny cells. Moreover, the cell cycle machinery is responsible for incorporating information about external cues or internal processes that the cell must keep track of to ensure a coordinated, timely progression of all related processes. This is most pronounced in multicellular organisms, but also a cardinal feature in model organisms such as baker's yeast. The complex and integrative behavior is difficult to grasp and requires mathematical modeling to fully understand the quantitative interplay of the single components within the entire system. Here, we present a self-oscillating mathematical model of the yeast cell cycle that comprises all major cyclins and their main regulators. Furthermore, it accounts for the regulation of the cell cycle machinery by a series of external stimuli such as mating pheromones and changes in osmotic pressure or nutrient quality. We demonstrate how the external perturbations modify the dynamics of cell cycle components and how the cell cycle resumes after adaptation to or relief from stress.Peer Reviewe
Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCuSi versus YbRhSi
In this paper the low-temperature properties of two isostructural canonical
heavy-fermion compounds are contrasted with regards to the interplay between
antiferromagnetic (AF) quantum criticality and superconductivity. For
CeCuSi, fully-gapped d-wave superconductivity forms in the vicinity of
an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum
critical point (QCP). Inelastic neutron scattering results highlight that both
quantum critical SDW fluctuations as well as Mott-type fluctuations of local
magnetic moments contribute to the formation of Cooper pairs in CeCuSi.
In YbRhSi, superconductivity appears to be suppressed at
mK by AF order ( = 70 mK). Ultra-low temperature measurements reveal a
hybrid order between nuclear and 4f-electronic spins, which is dominated by the
Yb-derived nuclear spins, to develop at slightly above 2 mK. The hybrid
order turns out to strongly compete with the primary 4f-electronic order and to
push the material towards its QCP. Apparently, this paves the way for
heavy-fermion superconductivity to form at = 2 mK. Like the pressure -
induced QCP in CeRhIn, the magnetic field - induced one in YbRhSi
is of the local Kondo-destroying variety which corresponds to a Mott-type
transition at zero temperature. Therefore, these materials form the link
between the large family of about fifty low- unconventional heavy - fermion
superconductors and other families of unconventional superconductors with
higher s, notably the doped Mott insulators of the cuprates, organic
charge-transfer salts and some of the Fe-based superconductors. Our study
suggests that heavy-fermion superconductivity near an AF QCP is a robust
phenomenon.Comment: 30 pages, 7 Figures, Accepted for publication in Philosophical
Magazin
Body-part-specific Representations of Semantic Noun Categories.
Word meaning processing in the brain involves ventrolateral temporal cortex, but a semantic contribution of the dorsal stream, especially frontocentral sensorimotor areas, has been controversial. We here examine brain activation during passive reading of object-related nouns from different semantic categories, notably animal, food, and tool words, matched for a range of psycholinguistic features. Results show ventral stream activation in temporal cortex along with category-specific activation patterns in both ventral and dorsal streams, including sensorimotor systems and adjacent pFC. Precentral activation reflected action-related semantic features of the word categories. Cortical regions implicated in mouth and face movements were sparked by food words, and hand area activation was seen for tool words, consistent with the actions implicated by the objects the words are used to speak about. Furthermore, tool words specifically activated the right cerebellum, and food words activated the left orbito-frontal and fusiform areas. We discuss our results in the context of category-specific semantic deficits in the processing of words and concepts, along with previous neuroimaging research, and conclude that specific dorsal and ventral areas in frontocentral and temporal cortex index visual and affective–emotional semantic attributes of object-related nouns and action-related affordances of their referent objects
High-Level Magnetic Activity on a Low-Mass Close Binary: GSC 02038-0293
Taking into account results obtained from light-curve analysis and
out-of-eclipse analyses, we discuss the nature of GSC 02038-00293 and also its
magnetic activity behaviour. We obtained light curves of the system during
observing seasons 2007, 2008 and 2011. We obtained its secondary minimum
clearly in I-band observations in 2008 for the first time. Analysing this light
curve, we found the physical parameters of the components. The light-curve
analysis indicates that the possible mass ratio of the system is 0.35. We
obtained the remaining V-band light curves, extracting the eclipses. We
modelled these remaining curves using the spotmodel program and found possible
spot configurations of the magnetically active component for each observing
season. The models demonstrated that there are two active longitudes for the
active component. The models reveal that both active longitudes migrate in the
direction of decreasing longitude. We also examined the light curves in
out-of-eclipse phases with respect to minimum and maximum brightness,
amplitude, etc. The amplitude of the curves during out-of-eclipse phases varies
in a sinusoidal way with a period of ~8.9yr the mean brightness of the system
is dramatically decreasing. The phases of the deeper minimum during
out-of-eclipse periods exhibit a migration toward decreasing phase.Comment: 15 pages, 8 figures, 5 table
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