79 research outputs found
A Semantic Characterization for ASP Base Revision
International audienceThe paper deals with base revision for Answer Set Programming (ASP). Base revision in classical logic is done by the removal of formulas. Exploiting the non-monotonicity of ASP allows one to propose other revision strategies, namely addition strategy or removal and/or addition strategy. These strategies allow one to define families of rule-based revision operators. The paper presents a semantic characterization of these families of revision operators in terms of answer sets. This semantic characterization allows for equivalently considering the evolution of syntactic logic programs and the evolution of their semantic content. It then studies the logical properties of the proposed operators and gives complexity results
Excitons and stacking order in h-BN
The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN)
makes this material one of the most promising candidate for light emitting
devices in the far ultraviolet (UV). However, single excitons occur only in
perfect monocrystals that are extremely hard to synthesize, while regular h-BN
samples present a complex emission spectrum with several additional peaks. The
microscopic origin of these additional emissions has not yet been understood.
In this work we address this problem using an experimental and theoretical
approach that combines nanometric resolved cathodoluminescence, high resolution
transmission electron microscopy and state of the art theoretical spectroscopy
methods. We demonstrate that emission spectra are strongly inhomogeneus within
individual flakes and that additional excitons occur at structural
deformations, such as faceted plane folds, that lead to local changes of the
h-BN stacking order
Revisiting graphene oxide chemistry via spatially-resolved electron energy loss spectroscopy
The type and distribution of oxygen functional groups in graphene oxide and
reduced graphene oxide remain still a subject of great debate. Local analytic techniques
are required to access the chemistry of these materials at a nanometric scale. Electron
energy loss spectroscopy in a scanning transmission electron microscope can provide the
suitable resolution, but GO and RGO are extremely sensitive to electron irradiation.
In this work we employ a dedicated experimental set-up to reduce electron illumina-
tion below damage limit. GO oxygen maps obtained at a few nanometres scale show
separated domains with diferent oxidation levels. The C/O ratio varies from about 4:1 to 1:1, the latter corresponding to a complete functionalization of the graphene flakes.
In RGO the residual oxygen concentrates mostly in regions few tens nanometres wide. Specific energy-loss near-edge structures are observed for diferent oxidation levels. By combining these findings with first principles simulations we propose a model for the highly oxidized domains where graphene is fully functionalized by hydroxyl groups forming a 2D-sp3 carbon network analogous to that of graphane.AT, AZ and OS acknowledge support from the Agence Nationale de la Recherche (ANR), program of future investment TEMPOS-CHROMATEM (No. ANR-10-EQPX-50). The work has also received funding from the European Union in Seventh Framework Programme (No. FP7/2007 -2013) under Grant Agreement No. n312483 (ESTEEM2). AMB and WKM are grateful for Financial support from the Spanish Ministry MINECO and the European Regional development Fund (project ENE2013-48816-C5-5-R) and from the Regional Government of Aragon and the European Social Fund (DGA-ESF-T66 Grupo Consolidado).
The authors are grateful to P. Launois, S. Rouziere and C.P. Ewels for useful discussion.Peer reviewe
Computing Query Answering With Non-Monotonic Rules: A Case Study of Archaeology Qualitative Spatial Reasoning
International audienceThis paper deals with querying ontology-based knowledge bases equipped with non-monotonic rules through a case study within the framework of Cultural Heritage. It focuses on 3D underwater surveys on the Xlendi wreck which is represented by an OWL2 knowledge base with a large dataset. The paper aims at improving the interactions between the archaeologists and the knowledge base providing new queries that involve non-monotonic rules in order to perform qualitative spatial reasoning. To this end, the knowledge base initially represented in OWL2-QL is translated into an equivalent Answer Set Programming (ASP) program and is enriched with a set of non-monotonic ASP rules suitable to express default and exceptions. An ASP query answering approach is proposed and implemented. Furthermore due to the increased expressiveness of non-monotonic rules it provides spatial reasoning and spatial relations between artifacts query answering which is not possible with query answering languages such as SPARQL and SQWRL
Time calibration studies for the Timepix3 hybrid pixel detector in electron microscopy
Direct electron detection is currently revolutionizing many fields of
electron microscopy due to its lower noise, its reduced point-spread function,
and its increased quantum efficiency. More specifically to this work, Timepix3
is a hybrid-pixel direct electron detector capable of outputting temporal
information of individual hits in its pixel array. Its architecture results in
a data-driven detector, also called event-based, in which individual hits
trigger the data off the chip for readout as fast as possible. The presence of
a pixel threshold value results in an almost readout-noise-free detector while
also defining the hit time of arrival and the time the signal stays over the
pixel threshold. In this work, we have performed various experiments to
calibrate and correct the Timepix3 temporal information, specifically in the
context of electron microscopy. These include the energy calibration, and the
time-walk and pixel delay corrections, reaching an average temporal resolution
throughout the entire pixel matrix of ns. Additionally, we have
also studied cosmic rays tracks to characterize the charge dynamics along the
volume of the sensor layer, allowing us to estimate the limits of the
detector's temporal response depending on different bias voltages, sensor
thickness, and the electron beam ionization volume. We have estimated the
uncertainty due to the ionization volume ranging from about 0.8 ns for 60 keV
electrons to 8.8 ns for 300 keV electrons
Revisiting Graphene Oxide Chemistry via Spatially-Resolved Electron Energy Loss Spectroscopy
Bismuth iron garnet: ab initio study of electronic properties
Bismuth iron garnet (BIG), i.e. Bi3Fe5O12, is a strong ferrimagnet that also
possess outstanding magneto-optical properties such as the largest known
Faraday rotation. These properties are related with the distribution of
magnetic moments on octahedral and tetrahedral sites, the presence of spin gaps
in the density of state and a strong spin-orbit coupling. In this work,
first-principles ab initio calculations are performed to study the structural,
electronic and magnetic properties of BIG using Density Functional Theory with
Hubbard+U (DFT+U) correction including spin-orbit coupling and HSE06 hybrid
functional. We found that the presence of spin gaps in the electronic structure
results from the interplay between exchange and correlation effects and the
crystal field strengths for tetrahedral and octahedral iron sublattices. The
DFT+U treatment tends to close the spin-gaps for larger U due to
over-localization effects, notably in the octahedral site. On the other hand,
the hybrid functional confirms the occurrences of three spin gaps in the iron
states of the conduction band as expected from optical measurements. A strong
exchange splitting at the top of the valence bands associated with a lone-pair
type mixture of O p and Bi s,p states is also obtained. Similar exchange
splitting was not previously observed for other iron based garnets, such as for
yttrium iron garnet. It follows that hole doping, as obtained by Ca
substitution at Bi sites, results in a full spin polarized density at the Fermi
energy. This work helps to shed more light on the theoretical comprehension of
the properties of BIG and opens the route towards the use of advanced Many Body
calculations to predict the magneto-optical coupling effects in BIG in a direct
comparison with the experimental measurements
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