82 research outputs found
Atomistic perspective of long lifetimes of small skyrmions at room temperature
The current development to employ magnetic skyrmions in novel spintronic
device designs has led to a demand for room temperature-stable skyrmions of
ever smaller size. We present extensive studies on skyrmion stability in
atomistic magnetic systems in two- and three-dimensional geometries. We show
that for materials described by the same micromagnetic parameters, the
variation of the atomistic exchange between different neighbors, the stacking
order, and the number of layers of the atomic lattice can significantly
influence the rate of the thermally activated decay of a skyrmion. These
factors alone are important considerations, but it is shown that their
combination can open up novel avenues of materials design in the search for
sub-10nm skyrmions, as their lifetime can be extended by several orders of
magnitude
Antiskyrmions stabilized at interfaces by anisotropic Dzyaloshinskii-Moriya interaction
Chiral magnets are an emerging class of topological matter harbouring
localized and topologically protected vortex-like magnetic textures called
skyrmions, which are currently under intense scrutiny as a new entity for
information storage and processing. Here, on the level of micromagnetics we
rigorously show that chiral magnets cannot only host skyrmions but also
antiskyrmions as least-energy configurations over all non-trivial homotopy
classes. We derive practical criteria for their occurrence and coexistence with
skyrmions that can be fulfilled by (110)-oriented interfaces in dependence on
the electronic structure. Relating the electronic structure to an atomistic
spin-lattice model by means of density-functional calculations and minimizing
the energy on a mesoscopic scale applying spin-relaxation methods, we propose a
double layer of Fe grown on a W(110) substrate as a practical example. We
conjecture that ultrathin magnetic films grown on semiconductor or heavy metal
substrates with symmetry are prototype classes of materials hosting
magnetic antiskyrmions.Comment: 20 pages (11 pages + 9 pages supplementary material
Spirit: Multifunctional Framework for Atomistic Spin Simulations
The \textit{Spirit} framework is designed for atomic scale spin simulations
of magnetic systems of arbitrary geometry and magnetic structure, providing a
graphical user interface with powerful visualizations and an easy to use
scripting interface. An extended Heisenberg type spin-lattice Hamiltonian
including competing exchange interactions between neighbors at arbitrary
distance, higher-order exchange, Dzyaloshinskii-Moriya and dipole-dipole
interactions is used to describe the energetics of a system of classical spins
localised at atom positions. A variety of common simulations methods are
implemented including Monte Carlo and various time evolution algorithms based
on the Landau-Lifshitz-Gilbert equation of motion, which can be used to
determine static ground state and metastable spin configurations, sample
equilibrium and finite temperature thermodynamical properties of magnetic
materials and nanostructures or calculate dynamical trajectories including spin
torques induced by stochastic temperature or electric current. Methods for
finding the mechanism and rate of thermally assisted transitions include the
geodesic nudged elastic band method, which can be applied when both initial and
final states are specified, and the minimum mode following method when only the
initial state is given. The lifetime of magnetic states and rate of transitions
can be evaluated within the harmonic approximation of transition-state theory.
The framework offers performant CPU and GPU parallelizations. All methods are
verified and applications to several systems, such as vortices, domain walls,
skyrmions and bobbers are described
Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum
We report a first measurement for ultra-high energy cosmic rays of the
correlation between the depth of shower maximum and the signal in the water
Cherenkov stations of air-showers registered simultaneously by the fluorescence
and the surface detectors of the Pierre Auger Observatory. Such a correlation
measurement is a unique feature of a hybrid air-shower observatory with
sensitivity to both the electromagnetic and muonic components. It allows an
accurate determination of the spread of primary masses in the cosmic-ray flux.
Up till now, constraints on the spread of primary masses have been dominated by
systematic uncertainties. The present correlation measurement is not affected
by systematics in the measurement of the depth of shower maximum or the signal
in the water Cherenkov stations. The analysis relies on general characteristics
of air showers and is thus robust also with respect to uncertainties in
hadronic event generators. The observed correlation in the energy range around
the `ankle' at differs significantly from
expectations for pure primary cosmic-ray compositions. A light composition made
up of proton and helium only is equally inconsistent with observations. The
data are explained well by a mixed composition including nuclei with mass . Scenarios such as the proton dip model, with almost pure compositions, are
thus disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray
flux at Earth.Comment: Published version. Added journal reference and DOI. Added Report
Numbe
Modelling the relationship between 231Pa/230Th distribution in North Atlantic sediment and Atlantic Meridional Overturning Circulation
Down-core variations in North Atlantic 231Paxs/230Thxs have been interpreted as changes in the strength of
the Atlantic meridional overturning circulation (AMOC). This modeling study confirms that hypothetical
changes in the AMOC would indeed be recorded as changes in the distribution of sedimentary 231Paxs/230Thxs.
At different sites in the North Atlantic the changes in sedimentary 231Pa/230Th that we simulate are diverse and
do not reflect a simple tendency for 231Paxs/230Thxs to increase toward the production ratio (0.093) when the
AMOC strength reduces but instead are moderated by the particle flux. In its collapsed or reduced state the
AMOC does not remove 231Pa from the North Atlantic: Instead, 231Pa is scavenged to the North Atlantic
sediment in areas of high particle flux. In this way the North Atlantic 231Paxs/230Thxs during AMOC shutdown
follows the same pattern as 231Paxs/230Thxs in modern ocean basins with reduced rates of meridional overturning
(i.e., Pacific or Indian oceans). We suggest that mapping the spatial distribution of 231Paxs/230Thxs across several
key points in the North Atlantic is an achievable and practical qualitative indicator of the AMOC strength in the
short term. Our results indicate that additional North Atlantic sites where down-core observations of
231Paxs/230Thxs would be useful coincide with locations which were maxima in the vertical particle flux during
these periods. Reliable estimates of the North Atlantic mean 231Paxs/230Thxs should remain a goal in the longer
term. Our results hint at a possible ‘‘seesaw-like’’ behavior in 231Pa/230Th in the South Atlantic
Party rules, party resources, and the politics of parliamentary democracies: how parties organize in the 21st Century
This article introduces the first findings of the Political Party Database (PPDB) project, a major survey of party organizations in parliamentary and semi-presidential democracies. The project’s first round of data covers 122
parties in 19 countries. In this paper we describe the scope of the database, then investigate what it tells us about contemporary party organization in these countries, focussing on parties’ resources, structures and internal decision-making. We examine organizational patterns by
country and party family, and where possible we make temporal comparisons with older datasets. Our analyses suggest a remarkable coexistence of uniformity and diversity. In terms of the major organizational resources on which parties can draw, such as members, staff and finance, the new evidence largely confirms the continuation of trends identified in previous research: i.e., declining membership, but enhanced financial resources and more paid staff. We also find remarkable uniformity regarding the core architecture of party organizations. At the same time, however, we find substantial variation between countries and
party families in terms of their internal processes, with particular regard to how internally democratic they are, and in the forms that this democratization takes
The molecular basis of polysaccharide cleavage by lytic polysaccharide monooxygenases.
Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that oxidatively break down recalcitrant polysaccharides such as cellulose and chitin. Since their discovery, LPMOs have become integral factors in the industrial utilization of biomass, especially in the sustainable generation of cellulosic bioethanol. We report here a structural determination of an LPMO-oligosaccharide complex, yielding detailed insights into the mechanism of action of these enzymes. Using a combination of structure and electron paramagnetic resonance spectroscopy, we reveal the means by which LPMOs interact with saccharide substrates. We further uncover electronic and structural features of the enzyme active site, showing how LPMOs orchestrate the reaction of oxygen with polysaccharide chains.We thank K. Rasmussen and R.M. Borup for experimental assistance, and MAXLAB, Sweden and the European Synchrotron Radiation Facility (ESRF), France, for synchrotron beam time and assistance. This work was supported by the UK Biotechnology and Biological Sciences Research Council (grant numbers BB/L000423 to P.D., G.J.D. and P.H.W., and BB/L021633/1 to G.J.D. and P.H.W.), Agence Française de l'Environnement et de la Maîtrise de l'Energie (grant number 1201C102 to B.H.), the Danish Council for Strategic Research (grant numbers 12-134923 to L.L.L. and 12-134922 to K.S.J.). Travel to synchrotrons was supported by the Danish Ministry of Higher Education and Science through the Instrument Center DANSCATT and the European Community's Seventh Framework Programme (FP7/2007-2013) under BioStruct-X (grant agreement 283570). L.M., S.F., S.C. and H.D. were supported by Institut de Chimie Moléculaire de Grenoble FR 2607, LabEx ARCANE (ANR-11-LABX-0003-01), the PolyNat Carnot Institute and the French Agence Nationale de la Recherche (PNRB2005-11).This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nchembio.202
Genetic association analysis identifies variants associated with disease progression in primary sclerosing cholangitis
Objective Primary sclerosing cholangitis (PSC) is a genetically complex, inflammatory bile duct disease of largely unknown aetiology often leading to liver transplantation or death. Little is known about the genetic contribution to the severity and progression of PSC. The aim of this study is to identify genetic variants associated with PSC disease progression and development of complications. Design We collected standardised PSC subphenotypes in a large cohort of 3402 patients with PSC. After quality control, we combined 130 422 single nucleotide polymorphisms of all patients-obtained using the Illumina immunochip-with their disease subphenotypes. Using logistic regression and Cox proportional hazards models, we identified genetic variants associated with binary and time-to-event PSC subphenotypes. Results We identified genetic variant rs853974 to be associated with liver transplant-free survival (p=6.07x10(-9)). Kaplan-Meier survival analysis showed a 50.9% (95% CI 41.5% to 59.5%) transplant-free survival for homozygous AA allele carriers of rs853974 compared with 72.8% (95% CI 69.6% to 75.7%) for GG carriers at 10 years after PSC diagnosis. For the candidate gene in the region, RSPO3, we demonstrated expression in key liver-resident effector cells, such as human and murine cholangiocytes and human hepatic stellate cells. Conclusion We present a large international PSC cohort, and report genetic loci associated with PSC disease progression. For liver transplant-free survival, we identified a genome-wide significant signal and demonstrated expression of the candidate gene RSPO3 in key liver-resident effector cells. This warrants further assessments of the role of this potential key PSC modifier gene.Peer reviewe
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
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