1,757 research outputs found
Visualizing Pure Quantum Turbulence in Superfluid He: Andreev Reflection and its Spectral Properties
Superfluid He-B in the zero-temperature limit offers a unique means of
studying quantum turbulence by the Andreev reflection of quasiparticle
excitations by the vortex flow fields. We validate the experimental
visualization of turbulence in He-B by showing the relation between the
vortex-line density and the Andreev reflectance of the vortex tangle in the
first simulations of the Andreev reflectance by a realistic 3D vortex tangle,
and comparing the results with the first experimental measurements able to
probe quantum turbulence on length scales smaller than the inter-vortex
separation.Comment: 5 pages, 4 figures, and Supplemental Material (2 pages, 2 figures
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Neoproterozoic Microfossils from the Margin of the East European Platform and the Search for a Biostratigraphic Model of Lower Ediacaran Rocks
A ca. 600 m thick siliciclastic succession in northern Russia contains abundant and diverse microfossils that document early to middle Ediacaran deposition along the northeastern margin of the East European Platform. The Vychegda Formation is poorly exposed but is well documented by a core drilled in the Timan trough region (Kel’tminskaya-1 borehole). Vychegda siliciclastics lie unconformably above Tonian to lower Cryogenian strata and below equivalents of the late Ediacaran Redkino succession that is widely distributed across the platform. The basal 10 m of the formation preserve acritarchs and fragments of problematic macrofossils known elsewhere only from pre-Sturtian successions. In contrast, the upper, nearly 400 m of the succession contains abundant and diverse large acanthomorphic acritarchs attributable to the Ediacaran Complex Acanthomorph Palynoflora (ECAP). This distinctive set of taxa is known elsewhere only from lower, but not lowermost, Ediacaran rocks. In between lies an additional assemblage of relatively simple filaments and stratigraphically long ranging sphaeromorphic acritarchs interpreted as early Ediacaran in age. Bearing in mind that knowledge of late Cryogenian (post-Strurtian/pre-Marinoan) microfossils is sparse, the Vychegda record is consistent with data from Australia and China which suggest that diverse ECAP microfossil assemblages appeared well into the Ediacaran Period. Accumulating paleontological observations underscore both the promise and challenges for the biostratigraphic characterization of the early Ediacaran Period.Organismic and Evolutionary Biolog
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Ediacaran Microfossils from the Ura Formation, Baikal-Patom Uplift, Siberia: Taxonomy and Biostratigraphic Significance
Abundant and diverse microfossils from shales of the uppermost Ura Formation, central Siberia, document early to middle Ediacaran life along the southeastern margin of the Siberian Platform. The Ura Formation is well exposed in a series of sections in the Lena River basin but the best microfossil assemblages come from a locality along the Ura River. Here, the uppermost twenty meters of the formation contain diverse microfossils exceptionally well preserved as organic compressions. Fossils include nearly two dozen morphospecies of large acanthomorphic microfossils attributable to the Ediacaran Complex Acanthomorph Palynoflora (ECAP), a distinctive assemblage known elsewhere only from lower, but not lowermost, to middle Ediacaran rocks. Discovery of ECAP in strata previously considered Mesoproterozoic through Cryogenian confirms inferences from chemostratigraphy, dramatically changing stratigraphic interpretation of sedimentary successions and Proterozoic tectonics on the Siberian Platform. Systematic paleontology is reported for 36 taxa (five described informally) assigned to 23 genera of both eukaryotic and prokaryotic microfossils. One new genus and two new species are proposed: Ancorosphaeridium magnum n. gen. n. sp. and A. minor n. gen. n. sp.Organismic and Evolutionary Biolog
Conserved Charges in the Principal Chiral Model on a Supergroup
The classical principal chiral model in 1+1 dimensions with target space a
compact Lie supergroup is investigated. It is shown how to construct a local
conserved charge given an invariant tensor of the Lie superalgebra. We
calculate the super-Poisson brackets of these currents and argue that they are
finitely generated. We show how to derive an infinite number of local charges
in involution. We demonstrate that these charges Poisson commute with the
non-local charges of the model
Casimir eigenvalues for universal Lie algebra
For two different natural definitions of Casimir operators for simple Lie
algebras we show that their eigenvalues in the adjoint representation can be
expressed polynomially in the universal Vogel's parameters and give explicit formulae for the generating functions of these
eigenvalues.Comment: Slightly revised versio
On Spin Calogero-Moser system at infinity
We present a construction of a new integrable model as an infinite limit of Calogero models of N particles with spin. It is implemented in the multicomponent Fock space. Explicit formulas for Dunkl operators, the Yangian generators in the multicomponent Fock space are presented. The classical limit of the system is examined
Vitamin D: beyond bone.
In recent years, vitamin D has been received increased attention due to the resurgence of vitamin D deficiency and rickets in developed countries and the identification of extraskeletal effects of vitamin D, suggesting unexpected benefits of vitamin D in health and disease, beyond bone health. The possibility of extraskeletal effects of vitamin D was first noted with the discovery of the vitamin D receptor (VDR) in tissues and cells that are not involved in maintaining mineral homeostasis and bone health, including skin, placenta, pancreas, breast, prostate and colon cancer cells, and activated T cells. However, the biological significance of the expression of the VDR in different tissues is not fully understood, and the role of vitamin D in extraskeletal health has been a matter of debate. This report summarizes recent research on the roles for vitamin D in cancer, immunity and autoimmune diseases, cardiovascular and respiratory health, pregnancy, obesity, erythropoiesis, diabetes, muscle function, and aging
Interaction of ballistic quasiparticles and vortex configurations in superfluid He3-B
The vortex line density of turbulent superfluid He3-B at very low temperature
is deduced by detecting the shadow of ballistic quasiparticles which are
Andreev reflected by quantized vortices. Until now the measured total shadow
has been interpreted as the sum of shadows arising from interactions of a
single quasiparticle with a single vortex. By integrating numerically the
quasi-classical Hamiltonian equations of motion of ballistic quasiparticles in
the presence of nontrivial but relatively simple vortex systems (such as
vortex-vortex and vortex-antivortex pairs and small clusters of vortices) we
show that partial screening can take place, and the total shadow is not
necessarily the sum of the shadows. We have also found that it is possible
that, upon impinging on complex vortex configurations, quasiparticles
experience multiple reflections, which can be classical, Andreev, or both.Comment: To appear in Phys Rev
Ballistic propagation of thermal excitations near a vortex in superfluid He3-B
Andreev scattering of thermal excitations is a powerful tool for studying
quantized vortices and turbulence in superfluid He3-B at very low temperatures.
We write Hamilton's equations for a quasiparticle in the presence of a vortex
line, determine its trajectory, and find under wich conditions it is Andreev
reflected. To make contact with experiments, we generalize our results to the
Onsager vortex gas, and find values of the intervortex spacing in agreement
with less rigorous estimates
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