46,568 research outputs found
AA-stacked bilayer square ice between graphene layers?
Water confined between two layers with separation of a few Angstrom forms
layered two- dimensional ice structure. Using large scale molecular dynamics
simulations with the adoptable ReaxFF interatomic potential we found that flat
monolayer ice with a rhombic-square structure nucleates between graphene layers
which is non-polar and non-ferroelectric. Two layers of water are found to
crystallize into a square lattice close to the experimental found AA-stacking
[G. Algara- Siller et al. Nature 519, 443445 (2015)]. Each layer has a net
dipole moment which are in opposite direction. Bilayer ice is also non-polar
and non-ferroelectric. For three layer ice we found that each layer has a
crystal structure similar to monolayer ice
A singularity-free space-time
We show that the solution published in Ref.1 is geodesically complete and
singularity-free. We also prove that the solution satisfies the stronger energy
and causality conditions, such as global hyperbolicity, causal symmetry and
causal stability. A detailed discussion about which assumptions in the
singularity theorems are not fulfilled is performed, and we show explicitly
that the solution is in accordance with those theorems. A brief discussion of
the results is given.Comment: Latex 2.09, 14 page
The emergence of classical behavior in magnetic adatoms
A wide class of nanomagnets shows striking quantum behavior, known as quantum
spin tunneling (QST): instead of two degenerate ground states with opposite
magnetizations, a bonding-antibonding pair forms, resulting in a splitting of
the ground state doublet with wave functions linear combination of two
classically opposite magnetic states, leading to the quenching of their
magnetic moment. Here we study how QST is destroyed and classical behavior
emerges in the case of magnetic adatoms, as the strength of their coupling,
either to the substrate or to each other, is increased. Both spin-substrate and
spin-spin coupling renormalize the QST splitting to zero allowing the
environmental decoherence to eliminate superpositions between classical states,
leading to the emergence of spontaneous magnetization.Comment: 5 pages, 4 figure
A multi-layered Bayesian network model for structured document retrieval
New standards in document representation, like for example SGML, XML, and MPEG-7, compel Information Retrieval to design and implement models and tools to index, retrieve and present documents according to the given document structure. The paper presents the design of an Information Retrieval system for multimedia structured documents, like for example journal articles, e-books, and MPEG-7 videos. The system is based on Bayesian Networks, since this class of mathematical models enable to represent and quantify the relations between the structural components of the document. Some preliminary results on the system implementation are also presented
Plant clonal morphologies and spatial patterns as self-organized responses to resource-limited environments
We propose here to interpret and model peculiar plant morphologies (cushions,
tussocks) observed in the Andean altiplano as localized structures. Such
structures resulting in a patchy, aperiodic aspect of the vegetation cover are
hypothesized to self-organize thanks to the interplay between facilitation and
competition processes occurring at the scale of basic plant components
biologically referred to as 'ramets'. (Ramets are often of clonal origin.) To
verify this interpretation, we applied a simple, fairly generic model (one
integro-differential equation) emphasizing via Gaussian kernels non-local
facilitative and competitive feedbacks of the vegetation biomass density on its
own dynamics. We show that under realistic assumptions and parameter values
relating to ramet scale, the model can reproduce some macroscopic features of
the observed systems of patches and predict values for the inter-patch distance
that match the distances encountered in the reference area (Sajama National
Park in Bolivia). Prediction of the model can be confronted in the future to
data on vegetation patterns along environmental gradients as to anticipate the
possible effect of global change on those vegetation systems experiencing
constraining environmental conditions.Comment: 14 pages, 6figure
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