117 research outputs found
Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation
The manipulation of the antiferromagnetic interlayer coupling in the
epitaxial Fe/Cr/Fe(001) trilayer system by moderate 5 keV He ion beam
irradiation has been investigated experimentally. It is shown that even for
irradiation with very low fluences (10^14 ions/cm^2) a drastic change in
strength of the coupling appears. For thin Cr-spacers (below 0.6 - 0.7 nm) the
coupling strength decreases with fluence, becoming ferromagnetic for fluences
above (2x10^14 ions/cm^2). The effect is connected with the creation of
magnetic bridges in the layered system due to atomic exchange events caused by
the bombardment. For thicker Cr spacers (0.8 - 1.2 nm) an enhancement of the
antiferromagnetic coupling strength is found. A possible explanation of the
enhancement effect is given.Comment: Submitted to PR
Separation of the first- and second-order contributions in magneto-optic Kerr effect magnetometry of epitaxial FeMn/NiFe bilayers
The influence of second-order magneto-optic effects on Kerr effect
magnetometry of epitaxial exchange coupled FeMn/NiFe-bilayers is investigated.
A procedure for separation of the first- and second-order contributions is
presented. The full angular dependence of both contributions during the
magnetization reversal is extracted from the experimental data and presented
using gray scaled magnetization reversal diagrams. The theoretical description
of the investigated system is based on an extended Stoner-Wohlfarth model,
which includes an induced unidirectional and fourfold anisotropy in the
ferromagnet, caused by the coupling to the antiferromagnet. The agreement
between the experimental data and the theoretical model for both the first- and
second-order contributions are good, although a coherent reversal of the
magnetization is assumed in the model.Comment: 6 pages, 7 figures, submitted to J. Appl. Phy
Two new species of shrew-rats (Rhynchomys: Muridae; Rodentia) from Luzon Island, Philippines
The murine genus Rhynchomys includes the large-bodied Philippine âshrew-rats,â highly specialized members of the vermivorous clade of Philippine murids. Four species are recognized, all of which are endemic to Luzon Island: R. soricoides from mountains within the Central Cordillera, R. isarogensis from Mt. Isarog on the Bicol Peninsula, R. banahao from Mt. Banahaw in south-central Luzon, and R. tapulao from Mt. Tapulao in the Zambales Mountains. Field surveys in 2006 and 2008 revealed two additional populations of Rhynchomys, one from Mt. Labo (1544 m), a dormant stratovolcano at the base of the Bicol Peninsula, the other from Mt. Mingan (1901 m), the highest peak in the central Sierra Madre of east-central Luzon. Assessment of external and craniodental features of available specimens from throughout Luzon support our description of the populations on Mt. Labo and Mt. Mingan as new species. All species of Rhynchomys are restricted to high-elevation, montane, and mossy forest habitats, separated by intervening lowlands. These discoveries highlight the importance of isolated highland areas in the historical diversification of Southeast Asian murines, and as current centers of endemism
Realizations of Causal Manifolds by Quantum Fields
Quantum mechanical operators and quantum fields are interpreted as
realizations of timespace manifolds. Such causal manifolds are parametrized by
the classes of the positive unitary operations in all complex operations, i.e.
by the homogenous spaces \D(n)=\GL(\C^n_\R)/\U(n) with for mechanics
and for relativistic fields. The rank gives the number of both the
discrete and continuous invariants used in the harmonic analysis, i.e. two
characteristic masses in the relativistic case. 'Canonical' field theories with
the familiar divergencies are inappropriate realizations of the real
4-dimensional causal manifold \D(2). Faithful timespace realizations do not
lead to divergencies. In general they are reducible, but nondecomposable - in
addition to representations with eigenvectors (states, particle) they
incorporate principal vectors without a particle (eigenvector) basis as
exemplified by the Coulomb field.Comment: 36 pages, latex, macros include
Bohrification of operator algebras and quantum logic
Following Birkhoff and von Neumann, quantum logic has traditionally been
based on the lattice of closed linear subspaces of some Hilbert space, or, more
generally, on the lattice of projections in a von Neumann algebra A.
Unfortunately, the logical interpretation of these lattices is impaired by
their nondistributivity and by various other problems. We show that a possible
resolution of these difficulties, suggested by the ideas of Bohr, emerges if
instead of single projections one considers elementary propositions to be
families of projections indexed by a partially ordered set C(A) of appropriate
commutative subalgebras of A. In fact, to achieve both maximal generality and
ease of use within topos theory, we assume that A is a so-called Rickart
C*-algebra and that C(A) consists of all unital commutative Rickart
C*-subalgebras of A. Such families of projections form a Heyting algebra in a
natural way, so that the associated propositional logic is intuitionistic:
distributivity is recovered at the expense of the law of the excluded middle.
Subsequently, generalizing an earlier computation for n-by-n matrices, we
prove that the Heyting algebra thus associated to A arises as a basis for the
internal Gelfand spectrum (in the sense of Banaschewski-Mulvey) of the
"Bohrification" of A, which is a commutative Rickart C*-algebra in the topos of
functors from C(A) to the category of sets. We explain the relationship of this
construction to partial Boolean algebras and Bruns-Lakser completions. Finally,
we establish a connection between probability measure on the lattice of
projections on a Hilbert space H and probability valuations on the internal
Gelfand spectrum of A for A = B(H).Comment: 31 page
Ensembles and experiments in classical and quantum physics
A philosophically consistent axiomatic approach to classical and quantum
mechanics is given. The approach realizes a strong formal implementation of
Bohr's correspondence principle. In all instances, classical and quantum
concepts are fully parallel: the same general theory has a classical
realization and a quantum realization.
Extending the `probability via expectation' approach of Whittle to
noncommuting quantities, this paper defines quantities, ensembles, and
experiments as mathematical concepts and shows how to model complementarity,
uncertainty, probability, nonlocality and dynamics in these terms. The approach
carries no connotation of unlimited repeatability; hence it can be applied to
unique systems such as the universe.
Consistent experiments provide an elegant solution to the reality problem,
confirming the insistence of the orthodox Copenhagen interpretation on that
there is nothing but ensembles, while avoiding its elusive reality picture. The
weak law of large numbers explains the emergence of classical properties for
macroscopic systems.Comment: 56 page
Do Stacked Species Distribution Models Reflect Altitudinal Diversity Patterns?
The objective of this study was to evaluate the performance of stacked species distribution models in predicting the alpha and gamma species diversity patterns of two important plant clades along elevation in the Andes. We modelled the distribution of the species in the Anthurium genus (53 species) and the Bromeliaceae family (89 species) using six modelling techniques. We combined all of the predictions for the same species in ensemble models based on two different criteria: the average of the rescaled predictions by all techniques and the average of the best techniques. The rescaled predictions were then reclassified into binary predictions (presence/absence). By stacking either the original predictions or binary predictions for both ensemble procedures, we obtained four different species richness models per taxa. The gamma and alpha diversity per elevation band (500 m) was also computed. To evaluate the prediction abilities for the four predictions of species richness and gamma diversity, the models were compared with the real data along an elevation gradient that was independently compiled by specialists. Finally, we also tested whether our richness models performed better than a null model of altitudinal changes of diversity based on the literature. Stacking of the ensemble prediction of the individual species models generated richness models that proved to be well correlated with the observed alpha diversity richness patterns along elevation and with the gamma diversity derived from the literature. Overall, these models tend to overpredict species richness. The use of the ensemble predictions from the species models built with different techniques seems very promising for modelling of species assemblages. Stacking of the binary models reduced the over-prediction, although more research is needed. The randomisation test proved to be a promising method for testing the performance of the stacked models, but other implementations may still be developed
Lineage diversification and historical demography of a montane bird Garrulax elliotii - implications for the Pleistocene evolutionary history of the eastern Himalayas
<p>Abstract</p> <p>Background</p> <p>Pleistocene climate fluctuations have shaped the patterns of genetic diversity observed in many extant species. In montane habitats, species' ranges may have expanded and contracted along an altitudinal gradient in response to environmental fluctuations leading to alternating periods of genetic isolation and connectivity. Because species' responses to climate change are influenced by interactions between species-specific characteristics and local topography, diversification pattern differs between species and locations. The eastern Himalayas is one of the world's most prominent mountain ranges. Its complex topography and environmental heterogeneity present an ideal system in which to study how climatic changes during Pleistocene have influenced species distributions, genetic diversification, and demography. The Elliot's laughing thrush (<it>Garrulax elliotii</it>) is largely restricted to high-elevation shrublands in eastern Himalayas. We used mitochondrial DNA and microsatellites to investigate how genetic diversity in this species was affected by Pleistocene glaciations.</p> <p>Results</p> <p>Mitochondrial data detected two partially sympatric north-eastern and southern lineages. Microsatellite data, however, identified three distinct lineages congruent with the geographically separated southern, northern and eastern eco-subregions of the eastern Himalayas. Geographic breaks occur in steep mountains and deep valleys of the Kangding-Muli-Baoxin Divide. Divergence time estimates and coalescent simulations indicate that lineage diversification occurred on two different geographic and temporal scales; recent divergence, associated with geographic isolation into individual subregions, and historical divergence, associated with displacement into multiple refugia. Despite long-term isolation, genetic admixture among these subregional populations was observed, indicating historic periods of connectivity. The demographic history of <it>Garrulax elliotii </it>shows continuous population growth since late Pleistocene (about 0.125 mya).</p> <p>Conclusion</p> <p>While altitude-associated isolation is typical of many species in other montane regions, our results suggest that eco-subregions in the eastern Himalayas exhibiting island-like characteristics appear to have determined the diversification of <it>Garrulax elliotii</it>. During the Pleistocene, these populations became isolated on subregions during interglacial periods but were connected when these expanded to low altitude during cooler periods. The resultant genetic admixture of lineages might obscure pattern of genetic variation. Our results provide new insights into sky island diversification in a previously unstudied region, and further demonstrate that Pleistocene climatic changes can have profound effects on lineage diversification and demography in montane species.</p
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