587 research outputs found
Wasps (Insecta: Vespida = Hymenoptera) from the Early Cretaceous of Spain
Wasps and their relatives from the Lower Cretaceous lithographic limestones of Spain have been studied. Thirty specimens representing 30 species (4 of them with undetermined placement), at least 21 genera and 11 families are recorded. We erect 1 new family - Andrenelidae-, 6 new genera and 11 new species: Meiaghilarella cretacica n.gen., n.sp. (Sepulcidae Ghilarellinae), Eosyntexis catalonicus n.sp., Cretosyntexis montsecensis n.gen., n.sp. (Anaxyelidae Syntexinae), Montsecephialtites zherikhini n.gen., n.sp. (Ephialtitidae Ephialtitinae), Karataus hispanicus n.sp. (Ephialtitidae Symphytopterinae), Manlaya ansorge i n.sp. (Gasteruptiidae Baissinae), Andrenelia pennata n.gen., n.sp. (Andrenelidae n. fam.), Cretoserphus gomezi n.gen., n.sp. (Mesoserphidae), Montsecosphex jarzembow skii n.gen., n.sp., Angarosphex penyalveri n.sp., Pompilopterus (?) noguerensis n.sp. (Sphecidae Angarosphecinae), Cretoscolia conquensis n.sp. (Scoliidae Archaeoscoliinae). The Mesozoic family Ephialtitidae is revisited based on the restudy of the type-species. We compare these Spanish Cretaceous assemblages with other ones from various parts of the world: Central and Eastern Asia, England, Australia, and Brazil. The number of genera and families identified in the Spanish fossil-sites is almost the same as in the English Purbeck and Wealden. The absence of some hymenopteran groups as Xyelidae, is consistent with the warm climate know to exist in Spain during the Early Cretaceous. We conclude that both La Cabrúa and La Pedrera assemblages - the two sites that have yielded the greatest number of species- correspond to the Lower Cretaceous “Baissin type” (sensu Rasnitsyn et al., 1998), but including some Jurassic “survivors”. La Pedrera assemblage fits equally well in the “angarosphecine subtype”, while La Cabrúa roughly corresponds to the “proctotrupid” one, although shows a comparative ly high proportion of angarosphecins. This fact may suggest: a) possibly asynchrony between these two fossilsites, b) environmental differences not reflected in the lithological record, c) different taphonomic processes and/or, d) insufficient sample size - to reflect the reality of the source populations-. La Pedrera assemblage is very similar to those from Weald Clay (England), Bon Tsagan (Mongolia) and Santana (Brazil). La Cabrúa approaches to a some extent, though not quite agrees with the Purbeck (UK), Koonwarra (Australia), and most Lower Cretaceous Asian assemblages
The Kagome Antiferromagnet with Defects: Satisfaction, Frustration, and Spin Folding in a Random Spin System
It is shown that site disorder induces noncoplanar states, competing with the
thermal selection of coplanar states, in the nearest neighbor, classical kagome
Heisenberg antiferromagnet (AFM). For weak disorder, it is found that the
ground state energy is the sum of energies of separately satisfied triangles of
spins. This implies that disorder does not induce conventional spin glass
behavior. A transformation is presented, mapping ground state spin
configurations onto a folded triangular sheet (a new kind of ``spin origami'')
which has conformations similar to those of tethered membranes.Comment: REVTEX, 11 pages + 3 pictures upon reques
Molecular phylogenetics of subfamily Urgineoideae (Hyacinthaceae): Toward a coherent generic circumscription informed by molecular, morphological, and distributional data
The taxonomy and systematics of Urgineoideae (Hyacinthaceae) have been controversial in recent decades, with contrasting taxonomic treatments proposed based on preliminary and partial studies that have focused on morphology and/or solely plastid DNA sequence data. Some authors have recognized only two genera, with a very broadly conceived Drimia, while others have accepted several genera that, although better defined morphologically, were doubtfully monophyletic. Here, we present phylogenetic analyses involving four plastid DNA regions (trnL intron, trnL-F spacer, matK, and the trnCGCA-ycf6 intergenic region), a nuclear region (Agt1), and a selection of 40 morphological characters. Our study covers 293 samples and ca. 160 species of Urgineoideae (ca. 80% of its global diversity). Bayesian inference, maximum likelihood, and maximum parsimony analyses were performed to derive the phylogenetic patterns. The combination of data yielded phylogenetic trees with 31 well-defined clades or lineages, most corresponding to previously described genera, although some have required description or revised circumscription. As with other monocot families, a considerable degree of homoplasy was observed in morphological characters, especially in those groups with unspecialized flowers; nonetheless, consistent syndromes of traditional and novel characters are shown to support clade recognition at genus rank. The forthcoming revised classification of Urgineoideae is outlined here
Machine learning for ecosystem services
Recent developments in machine learning have expanded data-driven modelling (DDM) capabilities, allowing artificial intelligence to infer the behaviour of a system by computing and exploiting correlations between observed variables within it. Machine learning algorithms may enable the use of increasingly available ‘big data’ and assist applying ecosystem service models across scales, analysing and predicting the flows of these services to disaggregated beneficiaries. We use the Weka and ARIES software to produce two examples of DDM: firewood use in South Africa and biodiversity value in Sicily, respectively. Our South African example demonstrates that DDM (64–91% accuracy) can identify the areas where firewood use is within the top quartile with comparable accuracy as conventional modelling techniques (54–77% accuracy). The Sicilian example highlights how DDM can be made more accessible to decision makers, who show both capacity and willingness to engage with uncertainty information. Uncertainty estimates, produced as part of the DDM process, allow decision makers to determine what level of uncertainty is acceptable to them and to use their own expertise for potentially contentious decisions. We conclude that DDM has a clear role to play when modelling ecosystem services, helping produce interdisciplinary models and holistic solutions to complex socio-ecological issues
c-Axis tunneling in YBa2Cu3O7-\delta/PrBa2Cu3O7-\delta superlattices
In this work we report c-axis conductance measurements done on a superlattice
based on a stack of 2 layers YBa2Cu3O{7-\delta} and 7 layers
PrBa2Cu3O{7-\delta} (2:7). We find that these quasi-2D structures show no clear
superconducting coupling along the c-axis. Instead, we observe tunneling with a
gap of \Delta_c=5.0\pm 0.5 meV for the direction perpendicular to the
superconducting planes. The conductance spectrum show well defined
quasi-periodic structures which are attributed to the superlattice structure.
From this data we deduce a low temperature c-axis coherence length of
\xi_c=0.24\pm 0.03 nm.Comment: 15 pages, 5 figures. To appear in Phys.Rev.
Spectral functions and pseudogap in the t-J model
We calculate spectral functions within the t-J model as relevant to cuprates
in the regime from low to optimum doping. On the basis of equations of motion
for projected operators an effective spin-fermion coupling is derived. The self
energy due to short-wavelength transverse spin fluctuations is shown to lead to
a modified selfconsistent Born approximation, which can explain strong
asymmetry between hole and electron quasiparticles. The coupling to
long-wavelength longitudinal spin fluctuations governs the low-frequency
behavior and results in a pseudogap behavior, which at low doping effectively
truncates the Fermi surface.Comment: Minor corrections; to appear in Phys. Rev. B (RC
Comparative pattern of Octopus vulgaris life cycle with environmental parameters in the Northern Alboran Sea (Western Mediterranean Sea)
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First order transition and phase separation in pyrochlores with colossal-magnetoresistance
TlMnO pyrochlores present colossal magnetoresistance (CMR)
around the long range ferromagnetic ordering temperature (T). The
character of this magnetic phase transition has been determined to be first
order, by purely magnetic methods, in contrast to the second order character
previously reported by Zhao et al. (Phys. Rev. Lett. 83, 219 (1999)). The
highest CMR effect, as in TlCdMnO, corresponds to a
stronger first order character. This character implies a second type of
magnetic interaction, besides the direct superexchange between the Mn
ions, as well as a phase coexistence. A model is proposed, with a complete
Hamiltonian (including superexchange and an indirect interaction), which
reproduce the observed phenomenology.Comment: 6 pages. Figures include
Discrete Nonholonomic Lagrangian Systems on Lie Groupoids
This paper studies the construction of geometric integrators for nonholonomic
systems. We derive the nonholonomic discrete Euler-Lagrange equations in a
setting which permits to deduce geometric integrators for continuous
nonholonomic systems (reduced or not). The formalism is given in terms of Lie
groupoids, specifying a discrete Lagrangian and a constraint submanifold on it.
Additionally, it is necessary to fix a vector subbundle of the Lie algebroid
associated to the Lie groupoid. We also discuss the existence of nonholonomic
evolution operators in terms of the discrete nonholonomic Legendre
transformations and in terms of adequate decompositions of the prolongation of
the Lie groupoid. The characterization of the reversibility of the evolution
operator and the discrete nonholonomic momentum equation are also considered.
Finally, we illustrate with several classical examples the wide range of
application of the theory (the discrete nonholonomic constrained particle, the
Suslov system, the Chaplygin sleigh, the Veselova system, the rolling ball on a
rotating table and the two wheeled planar mobile robot).Comment: 45 page
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