1,472 research outputs found
Construction of G_2-instantons via twisted connected sums
We propose a method to construct G_2-instantons over a compact twisted
connected sum G_2-manifold, applying a gluing result of S\'a Earp and Walpuski
to instantons over a pair of 7-manifolds with a tubular end (see
arXiv:1310.7933). In our example, the moduli spaces of the ingredient
instantons are non-trivial, and their images in the moduli space over the
asymptotic cross-section K3 surface intersect transversely. Such a pair of
asymptotically stable holomorphic bundles is obtained using a twisted version
of the Hartshorne-Serre construction, which can be adapted to produce other
examples. Moreover, their deformation theory and asymptotic behaviour are
explicitly understood, results which may be of independent interest.Comment: 22 pages. Final version to appear in Mathematical Research Letter
Coordination of Foliar and Wood Anatomical Traits Contributes to Tropical Tree Distributions and Productivity along the Malay-Thai Peninsula
Drought is a critical factor in plant species distributions. Much research points to its relevance even in moist tropical regions. Recent studies have begun to elucidate mechanisms underlying the distributions of tropical tree species with respect to drought; however, how such desiccation tolerance mechanisms correspond with the coordination of hydraulic and photosynthetic traits in determining species distributions with respect to rainfall seasonality deserves attention. In the present study, we used a common garden approach to quantify inherent differences in wood anatomical and foliar physiological traits in 21 tropical tree species with either widespread (occupying both seasonal and aseasonal climates) or southern (restricted to aseasonal forests) distributions with respect to rainfall seasonality. Use of congeneric species pairs and phylogenetically independent contrast analyses allowed examination of this question in a phylogenetic framework. Widespread species opted for wood traits that provide biomechanical support and prevent xylem cavitation and showed associated reductions in canopy productivity and consequently growth rates compared with southern species. These data support the hypothesis that species having broader distributions with respect to climatic variability will be characterized by traits conducive to abiotic stress tolerance. This study highlights the importance of the well-established performance vs. stress tolerance trade-off as a contributor to species distributions at larger scales
Cross-Lingual Classification of Crisis Data
Many citizens nowadays flock to social media during crises to share or acquire the latest information about the event. Due to the sheer volume of data typically circulated during such events, it is necessary to be able to efficiently filter out irrelevant posts, thus focusing attention on the posts that are truly relevant to the crisis. Current methods for classifying the relevance of posts to a crisis or set of crises typically struggle to deal with posts in different languages, and it is not viable during rapidly evolving crisis situations to train new models for each language. In this paper we test statistical and semantic classification approaches on cross-lingual datasets from 30 crisis events, consisting of posts written mainly in English, Spanish, and Italian. We experiment with scenarios where the model is trained on one language and tested on another, and where the data is translated to a single language. We show that the addition of semantic features extracted from external knowledge bases improve accuracy over a purely statistical model
Triplectic Quantization of W2 gravity
The role of one loop order corrections in the triplectic quantization is
discussed in the case of W2 theory. This model illustrates the presence of
anomalies and Wess Zumino terms in this quantization scheme where extended BRST
invariance is represented in a completely anticanonical form.Comment: 10 pages, no figure
Photon counting for quantum key distribution with Peltier cooled InGaAs/InP APD's
The performance of three types of InGaAs/InP avalanche photodiodes is
investigated for photon counting at 1550 nm in the temperature range of
thermoelectric cooling. The best one yields a dark count probability of per gate (2.4 ns) at a detection efficiency of 10% and a
temperature of -60C. The afterpulse probability and the timing jitter are also
studied. The results obtained are compared with those of other papers and
applied to the simulation of a quantum key distribution system. An error rate
of 10% would be obtained after 54 kilometers.Comment: 8 pages, 10 figures, submitted to Journal of Modern Optic
Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors
We measure the homogeneous excitation linewidth of regioregular
poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by
means of two-dimensional coherent photoluminescence excitation spectroscopy. At
a temperature of 8\,K, we find a linewidth that is always \,meV
full-width-at-half-maximum, which is a significant fraction of the total
linewidth. It displays a spectral dependence and is minimum near the 0--0
origin peak. We interpret this spectral dependence of the homogeneous
excitation linewidth within the context of a weakly coupled aggregate model.Comment: 13 pages, 4 figures, Supplementary Materia
A Frequency-Controlled Magnetic Vortex Memory
Using the ultra low damping NiMnSb half-Heusler alloy patterned into
vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile
memory controlled by the frequency. A perpendicular bias magnetic field is used
to split the frequency of the vortex core gyrotropic rotation into two distinct
frequencies, depending on the sign of the vortex core polarity inside
the dot. A magnetic resonance force microscope and microwave pulses applied at
one of these two resonant frequencies allow for local and deterministic
addressing of binary information (core polarity)
Interaction between static holes in a quantum dimer model on the kagome lattice
A quantum dimer model (QDM) on the kagome lattice with an extensive
ground-state entropy was recently introduced [Phys. Rev. B 67, 214413 (2003)].
The ground-state energy of this QDM in presence of one and two static holes is
investigated by means of exact diagonalizations on lattices containing up to
144 kagome sites. The interaction energy between the holes (at distances up to
7 lattice spacings) is evaluated and the results show no indication of
confinement at large hole separations.Comment: 6 pages, 3 figures. IOP style files included. To appear in J. Phys.:
Condens. Matter, Proceedings of the HFM2003 conference, Grenobl
Soliton binding and low-lying singlets in frustrated odd-legged S=1/2 spin tubes
Motivated by the intriguing properties of the vanadium spin tube Na2V3O7, we
show that an effective spin-chirality model similar to that of standard
Heisenberg odd-legged S=1/2 spin tubes can be derived for frustrated inter-ring
couplings, but with a spin-chirality coupling constant alpha that can be
arbitrarily small. Using density matrix renormalization group and analytical
arguments, we show that, while spontaneous dimerization is always present,
solitons become bound into low-lying singlets as alpha is reduced. Experimental
implications for strongly frustrated tubes are discussed.Comment: 4 pages, 4 figure
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