96 research outputs found
Direct observation of nm-scale Mg- and B-oxide phases at grain boundaries in MgB2
Here we describe the results of an atomic resolution study of the structure
and composition of both the interior of the grains, and the grain boundaries in
polycrystalline MgB2. We find that there is no oxygen within the bulk of the
grains but significant oxygen enrichment at the grain boundaries. The majority
of grain boundaries contain BOx phases smaller than the coherence length, while
others contain larger areas of MgO sandwiched between BOx layers. Such results
naturally explain the differences in connectivity between the grains observed
by other techniques
Observation of coherent oxide precipitates in polycrystalline MgB2
Here we describe the results of an atomic resolution study of oxygen
incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are
formed by ordered substitution of oxygen atoms onto boron lattice sites, while
the basic bulk MgB2 crystal structure and orientation is preserved. The
periodicity of the oxygen ordering is dictated by the oxygen concentration in
the precipitates and primarily occurs in the (010) plane. The presence of these
precipitates correlates well with an improved critical current density and
superconducting transition behavior, implying that they act as pinning centers.Comment: Submitted to Applied Physics Letters, 6 pages, 3 figure
Ab Initio Structural Energetics of Beta-Si3N4 Surfaces
Motivated by recent electron microscopy studies on the Si3N4/rare-earth oxide
interfaces, the atomic and electronic structures of bare beta-Si3N4 surfaces
are investigated from first principles. The equilibrium shape of a Si3N4
crystal is found to have a hexagonal cross section and a faceted dome-like base
in agreement with experimental observations. The large atomic relaxations on
the prismatic planes are driven by the tendency of Si to saturate its dangling
bonds, which gives rise to resonant-bond configurations or planar sp^2-type
bonding. We predict three bare surfaces with lower energies than the open-ring
(10-10) surface observed at the interface, which indicate that
non-stoichiometry and the presence of the rare-earth oxide play crucial roles
in determining the termination of the Si3N4 matrix grains.Comment: 4 Pages, 4 Figures, 1 tabl
Atomically sharp domain walls in an antiferromagnet
The interest in understanding scaling limits of magnetic textures such as
domain walls spans the entire field of magnetism from its relativistic quantum
fundamentals to applications in information technologies. The traditional focus
of the field on ferromagnets has recently started to shift towards
antiferromagnets which offer a rich materials landscape and utility in
ultra-fast and neuromorphic devices insensitive to magnetic field
perturbations. Here we report the observation that domain walls in an epitaxial
crystal of antiferromagnetic CuMnAs can be atomically sharp. We reveal this
ultimate domain wall scaling limit using differential phase contrast imaging
within aberrationcorrected scanning transmission electron microscopy, which we
complement by X-ray magnetic dichroism microscopy and ab initio calculations.
We highlight that the atomically sharp domain walls are outside the remits of
established spin-Hamiltonian theories and can offer device functionalities
unparalleled in ferromagnets.Comment: 8 pages, 4 figures, Supplementary informatio
The role of Indigenous peoples and local communities in effective and equitable conservation
Debate about what proportion of the Earth to protect often overshadows the question of how nature should be conserved and by whom. We present a systematic review and narrative synthesis of 169 publications investigating how different forms of governance influence conservation outcomes, paying particular attention to the role played by Indigenous peoples and local communities. We find a stark contrast between the outcomes produced by externally controlled conservation, and those produced by locally controlled efforts. Crucially, most studies presenting positive outcomes for both well-being and conservation come from cases where Indigenous peoples and local communities play a central role, such as when they have substantial influence over decision making or when local institutions regulating tenure form a recognized part of governance. In contrast, when interventions are controlled by external organizations and involve strategies to change local practices and supersede customary institutions, they tend to result in relatively ineffective conservation at the same time as producing negative social outcomes. Our findings suggest that equitable conservation, which empowers and supports the environmental stewardship of Indigenous peoples and local communities represents the primary pathway to effective long-term conservation of biodiversity, particularly when upheld in wider law and policy. Whether for protected areas in biodiversity hotspots or restoration of highly modified ecosystems, whether involving highly traditional or diverse and dynamic local communities, conservation can become more effective through an increased focus on governance type and quality, and fostering solutions that reinforce the role, capacity, and rights of Indigenous peoples and local communities. We detail how to enact progressive governance transitions through recommendations for conservation policy, with immediate relevance for how to achieve the next decade’s conservation targets under the UN Convention on Biological Diversity
Behavioral Differences in Violence: The Case of Intra-Group Differences of Paramilitaries and Guerrillas in Colombia
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