111 research outputs found
Hepatitis B in the United States: ongoing missed opportunities for hepatitis B vaccination, evidence from the Behavioral Risk Factor Surveillance Survey, 2007
A magnetic map leads juvenile European eels to the Gulf Stream
Migration allows animals to track the environmental conditions that maximize growth, survival, and reproduction [ 1–3 ]. Improved understanding of the mechanisms underlying migrations allows for improved management of species and ecosystems [ 1–4 ]. For centuries, the catadromous European eel (Anguilla anguilla) has provided one of Europe’s most important fisheries and has sparked considerable scientific inquiry, most recently owing to the dramatic collapse of juvenile recruitment [ 5 ]. Larval eels are transported by ocean currents associated with the Gulf Stream System from Sargasso Sea breeding grounds to coastal and freshwater habitats from North Africa to Scandinavia [ 6, 7 ]. After a decade or more, maturing adults migrate back to the Sargasso Sea, spawn, and die [ 8 ]. However, the migratory mechanisms that bring juvenile eels to Europe and return adults to the Sargasso Sea remain equivocal [ 9, 10 ]. Here, we used a “magnetic displacement” experiment [ 11, 12 ] to show that the orientation of juvenile eels varies in response to subtle differences in magnetic field intensity and inclination angle along their marine migration route. Simulations using an ocean circulation model revealed that even weakly swimming in the experimentally observed directions at the locations corresponding to the magnetic displacements would increase entrainment of juvenile eels into the Gulf Stream System. These findings provide new insight into the migration ecology and recruitment dynamics of eels and suggest that an adaptive magnetic map, tuned to large-scale features of ocean circulation, facilitates the vast oceanic migrations of the Anguilla genu
Students for Health Innovation and Education (SHINE): Fostering leadership among medical students and residents
Exploring the phases of 3D artificial spin ice: From Coulomb phase to magnetic monopole crystal
Artificial spin-ices consist of lithographic arrays of single-domain magnetic
nanowires organised into frustrated lattices. These geometries are usually
two-dimensional, allowing a direct exploration of physics associated with
frustration, topology and emergence. Recently, three-dimensional geometries
have been realised, in which transport of emergent monopoles can be directly
visualised upon the surface. Here we carry out an exploration of the
three-dimensional artificial spin-ice phase diagram, whereby dipoles are placed
within a diamond-bond lattice geometry. We find a rich phase diagram,
consisting of a double-charged monopole crystal, a single-charged monopole
crystal and conventional spin-ice with pinch points associated with a Coulomb
phase. In our experimental demagnetised systems, broken symmetry forces
formation of ferromagnetic stripes upon the surface, a configuration that
forbids the formation of the lower energy double-charged monopole crystal.
Instead, we observe crystallites of single magnetic charge, superimposed upon
an ice background. The crystallites are found to form due to the intricate
distribution of magnetic charge around a three-dimensional nanostructured
vertex, which locally favours monopole formation. Our work suggests that
engineered surface energetics can be used to tune the ground state of
experimental three-dimensional ASI systems
Direct observation and control of magnetic monopole defects in an artificial spin-ice material
Magnetic monopoles have stimulated a great amount of theoretical and experimental interest since their prediction by Dirac in 1931. To date, their presence has evaded detection in high energy experiments despite intensive efforts. Recently, entities that mimic magnetic monopoles have been observed in bulk and planar frustrated materials known as spin-ice materials, and artificial spin-ice materials, respectively. In this paper we discuss the formation of these so-called monopole defects within a cobalt honeycomb artificial spin-ice lattice. Experimental results and micromagnetic simulations show that monopole defects of opposite sign are created at the boundaries of the lattice, and move in opposing directions. Discrepancies between simulations and experimental results demonstrate the importance of quenched disorder. Furthermore, we show that controlled edge nucleated monopole defect formation can be realized with the use of soft magnetic injection pads, which is a very promising development for technological applications based upon magnetic charge
A qualitative evaluation of the expanded program on immunization at Saint Mary's Hospital Lacor: Determinants of timely childhood vaccine receipt
Response to Durif et al.
Our recent study [1] in Current Biology used a magnetic displacement experiment and simulations in an ocean circulation model to provide evidence that young European eels possess a ‘magnetic map’ that can aid their marine migration. Our results support two major conclusions: first, young eels distinguish among magnetic fields corresponding to locations across their marine range; second, for the fields that elicited significantly non-random orientation, swimming in the experimentally observed direction from the corresponding locations would increase entrainment in the Gulf Stream system. In their critique, Durif et al. [2] seem to conflate the separate and potentially independent ‘map step’ and ‘compass step’ of animal navigation. In the map step, an animal derives positional information to select a direction, whereas in the compass step the animal maintains that heading 3, 4. Our experiment was designed such that differences in eel orientation among treatments would indicate an ability to use the magnetic field as a map; the compass cue(s) used by eels was not investigated
Dynamics of artificial spin ice: continuous honeycomb network
We model the dynamics of magnetization in an artificial analog of spin ice
specializing to the case of a honeycomb network of connected magnetic
nanowires. The inherently dissipative dynamics is mediated by the emission,
propagation and absorption of domain walls in the links of the lattice. These
domain walls carry two natural units of magnetic charge, whereas sites of the
lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between
these charges play a major role in the physics of the system, as does quenched
disorder caused by imperfections of the lattice. We identify and describe
different regimes of magnetization reversal in an applied magnetic field
determined by the orientation of the applied field with respect to the initial
magnetization. One of the regimes is characterized by magnetic avalanches with
a 1/n distribution of lengths.Comment: 19 pages, focus issue of New J. Phys. on artificial frustrated
systems, minor clarifications requested by refere
Topology by Design in Magnetic nano-Materials: Artificial Spin Ice
Artificial Spin Ices are two dimensional arrays of magnetic, interacting
nano-structures whose geometry can be chosen at will, and whose elementary
degrees of freedom can be characterized directly. They were introduced at first
to study frustration in a controllable setting, to mimic the behavior of spin
ice rare earth pyrochlores, but at more useful temperature and field ranges and
with direct characterization, and to provide practical implementation to
celebrated, exactly solvable models of statistical mechanics previously devised
to gain an understanding of degenerate ensembles with residual entropy. With
the evolution of nano--fabrication and of experimental protocols it is now
possible to characterize the material in real-time, real-space, and to realize
virtually any geometry, for direct control over the collective dynamics. This
has recently opened a path toward the deliberate design of novel, exotic
states, not found in natural materials, and often characterized by topological
properties. Without any pretense of exhaustiveness, we will provide an
introduction to the material, the early works, and then, by reporting on more
recent results, we will proceed to describe the new direction, which includes
the design of desired topological states and their implications to kinetics.Comment: 29 pages, 13 figures, 116 references, Book Chapte
Extensive degeneracy, Coulomb phase and magnetic monopoles in an artificial realization of the square ice model
Artificial spin ice systems have been introduced as a possible mean to
investigate frustration effects in a well-controlled manner by fabricating
lithographically-patterned two-dimensional arrangements of interacting magnetic
nanostructures. This approach offers the opportunity to visualize
unconventional states of matter, directly in real space, and triggered a wealth
of studies at the frontier between nanomagnetism, statistical thermodynamics
and condensed matter physics. Despite the strong efforts made these last ten
years to provide an artificial realization of the celebrated square ice model,
no simple geometry based on arrays of nanomagnets succeeded to capture the
macroscopically degenerate ground state manifold of the corresponding model.
Instead, in all works reported so far, square lattices of nanomagnets are
characterized by a magnetically ordered ground state consisting of local
flux-closure configurations with alternating chirality. Here, we show
experimentally and theoretically, that all the characteristics of the square
ice model can be observed if the artificial square lattice is properly
designed. The spin configurations we image after demagnetizing our arrays
reveal unambiguous signatures of an algebraic spin liquid state characterized
by the presence of pinch points in the associated magnetic structure factor.
Local excitations, i.e. classical analogues of magnetic monopoles, are found to
be free to evolve in a massively degenerated, divergence-free vacuum. We thus
provide the first lab-on-chip platform allowing the investigation of collective
phenomena, including Coulomb phases and ice-like physics.Comment: 26 pages, 10 figure
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