315 research outputs found
Spin dependent electron transport through a magnetic resonant tunneling diode
Electron transport properties in nanostructures can be modeled, for example,
by using the semiclassical Wigner formalism or the quantum mechanical Green's
functions formalism. We compare the performance and the results of these
methods in the case of magnetic resonant-tunneling diodes. We have implemented
the two methods within the self-consistent spin-density-functional theory. Our
numerical implementation of the Wigner formalism is based on the
finite-difference scheme whereas for the Green's function formalism the
finite-element method is used. As a specific application, we consider the
device studied by Slobodskyy et all. [Phys. Rev. Lett. 90, 246601 (2003)] and
analyze their experimental results. The Wigner and Green's functions formalisms
give similar electron densities and potentials but, surprisingly, the former
method requires much more computer resources in order to obtain numerically
accurate results for currents. Both of the formalisms can successfully be used
to model magnetic resonant tunneling diode structures.Comment: 13 pages and 12 figure
Twin Paradox and the logical foundation of relativity theory
We study the foundation of space-time theory in the framework of first-order
logic (FOL). Since the foundation of mathematics has been successfully carried
through (via set theory) in FOL, it is not entirely impossible to do the same
for space-time theory (or relativity). First we recall a simple and streamlined
FOL-axiomatization SpecRel of special relativity from the literature. SpecRel
is complete with respect to questions about inertial motion. Then we ask
ourselves whether we can prove usual relativistic properties of accelerated
motion (e.g., clocks in acceleration) in SpecRel. As it turns out, this is
practically equivalent to asking whether SpecRel is strong enough to "handle"
(or treat) accelerated observers. We show that there is a mathematical
principle called induction (IND) coming from real analysis which needs to be
added to SpecRel in order to handle situations involving relativistic
acceleration. We present an extended version AccRel of SpecRel which is strong
enough to handle accelerated motion, in particular, accelerated observers.
Among others, we show that the Twin Paradox becomes provable in AccRel, but it
is not provable without IND.Comment: 24 pages, 6 figure
Polyteam Semantics
Team semantics is the mathematical framework of modern logics of dependence and independence in which formulae are interpreted by sets of assignments (teams) instead of single assignments as in first-order logic. In order to deepen the fruitful interplay between team semantics and database dependency theory, we define "Polyteam Semantics" in which formulae are evaluated over a family of teams. We begin by defining a novel polyteam variant of dependence atoms and give a finite axiomatisation for the associated implication problem. We also characterise the expressive power of poly-dependence logic by properties of polyteams that are downward closed and definable in existential second-order logic (ESO). The analogous result is shown to hold for poly-independence logic and all ESO-definable properties.Peer reviewe
A Geometrical Characterization of the Twin Paradox and its Variants
The aim of this paper is to provide a logic-based conceptual analysis of the
twin paradox (TwP) theorem within a first-order logic framework. A geometrical
characterization of TwP and its variants is given. It is shown that TwP is not
logically equivalent to the assumption of the slowing down of moving clocks,
and the lack of TwP is not logically equivalent to the Newtonian assumption of
absolute time. The logical connection between TwP and a symmetry axiom of
special relativity is also studied.Comment: 22 pages, 3 figure
Model-Driven Chatbot Development
Esta versiĂłn del artĂculo ha sido aceptada para su publicaciĂłn, despuĂŠs de la revisiĂłn por pares (cuando corresponda) y estĂĄ sujeta a los tĂŠrminos de uso de AM de Springer Nature, pero no es la VersiĂłn de Registro y no refleja mejoras posteriores a la aceptaciĂłn, ni ninguna correcciĂłn. La versiĂłn del registro estĂĄ disponible en lĂnea en: https://doi.org/10.1007/978-3-030-62522-1_15Chatbots are software services accessed via conversation in natural language. They are increasingly used to help in all kinds of procedures like booking flights, querying visa information or assigning tasks to developers. They can be embedded in webs and social networks, and be used from mobile devices without installing dedicated apps. While many frameworks and platforms have emerged for their development, identifying the most appropriate one for building a particular chatbot requires a high investment of time. Moreover, some of them are closed â resulting in customer lock-in â or require deep technical knowledge. To tackle these issues, we propose a model-driven engineering approach to chatbot development. It comprises a neutral meta-model and a domainspecific language (DSL) for chatbot description; code generators and parsers for several chatbot platforms; and a platform recommender. Our approach supports forward and reverse engineering, and model-based analysis. We demonstrate its feasibility presenting a prototype tool and an evaluation based on migrating third party Dialogflow bots to RasaWork funded by the Spanish Ministry of Science (RTI2018095255-B-I00) and the R&D programme of Madrid (P2018/TCS-4314
Axiomatizing relativistic dynamics without conservation postulates
A part of relativistic dynamics (or mechanics) is axiomatized by simple and
purely geometrical axioms formulated within first-order logic. A geometrical
proof of the formula connecting relativistic and rest masses of bodies is
presented, leading up to a geometric explanation of Einstein's famous .
The connection of our geometrical axioms and the usual axioms on the
conservation of mass, momentum and four-momentum is also investigated.Comment: 21 pages, 7 figure
The neutrino signal at HALO: learning about the primary supernova neutrino fluxes and neutrino properties
Core-collapse supernova neutrinos undergo a variety of phenomena when they
travel from the high neutrino density region and large matter densities to the
Earth. We perform analytical calculations of the supernova neutrino fluxes
including collective effects due to the neutrino-neutrino interactions, the
Mikheev-Smirnov-Wolfenstein (MSW) effect due to the neutrino interactions with
the background matter and decoherence of the wave packets as they propagate in
space. We predict the numbers of one- and two-neutron charged and
neutral-current electron-neutrino scattering on lead events. We show that, due
to the energy thresholds, the ratios of one- to two-neutron events are
sensitive to the pinching parameters of neutrino fluxes at the neutrinosphere,
almost independently of the presently unknown neutrino properties. Besides,
such events have an interesting sensitivity to the spectral split features that
depend upon the presence/absence of energy equipartition among neutrino
flavors. Our calculations show that a lead-based observatory like the Helium
And Lead Observatory (HALO) has the potential to pin down important
characteristics of the neutrino fluxes at the neutrinosphere, and provide us
with information on the neutrino transport in the supernova core.Comment: 30 pages, 12 figures, 6 tables, minor correction
Changing a semantics: opportunism or courage?
The generalized models for higher-order logics introduced by Leon Henkin, and
their multiple offspring over the years, have become a standard tool in many
areas of logic. Even so, discussion has persisted about their technical status,
and perhaps even their conceptual legitimacy. This paper gives a systematic
view of generalized model techniques, discusses what they mean in mathematical
and philosophical terms, and presents a few technical themes and results about
their role in algebraic representation, calibrating provability, lowering
complexity, understanding fixed-point logics, and achieving set-theoretic
absoluteness. We also show how thinking about Henkin's approach to semantics of
logical systems in this generality can yield new results, dispelling the
impression of adhocness. This paper is dedicated to Leon Henkin, a deep
logician who has changed the way we all work, while also being an always open,
modest, and encouraging colleague and friend.Comment: 27 pages. To appear in: The life and work of Leon Henkin: Essays on
his contributions (Studies in Universal Logic) eds: Manzano, M., Sain, I. and
Alonso, E., 201
Investigation of bone resorption within a cortical basic multicellular unit using a lattice-based computational model
In this paper we develop a lattice-based computational model focused on bone
resorption by osteoclasts in a single cortical basic multicellular unit (BMU).
Our model takes into account the interaction of osteoclasts with the bone
matrix, the interaction of osteoclasts with each other, the generation of
osteoclasts from a growing blood vessel, and the renewal of osteoclast nuclei
by cell fusion. All these features are shown to strongly influence the
geometrical properties of the developing resorption cavity including its size,
shape and progression rate, and are also shown to influence the distribution,
resorption pattern and trajectories of individual osteoclasts within the BMU.
We demonstrate that for certain parameter combinations, resorption cavity
shapes can be recovered from the computational model that closely resemble
resorption cavity shapes observed from microCT imaging of human cortical bone.Comment: 17 pages, 11 figures, 1 table. Revised version: paper entirely
rewritten for a more biology-oriented readership. Technical points of model
description now in Appendix. Addition of two new figures (Fig. 5 and Fig. 9)
and removal of former Fig.
Features in air ions measured by an air ion spectrometer (AIS) at Dome C
An air ion spectrometer (AIS) was deployed for the first time at the Concordia station at Dome C (75 degrees 06'S, 123 degrees 23'E; 3220 ma.s.l.), Antarctica during the period 22 December 2010-16 November 2011 for measuring the number size distribution of air ions. In this work, we present results obtained from this air ion data set together with aerosol particle and meteorological data. The main processes that modify the number size distribution of air ions during the measurement period at this high-altitude site included new particle formation (NPF, observed on 85 days), wind-induced ion formation (observed on 36 days), and ion production and loss associated with cloud/fog formation (observed on 2 days). For the subset of days when none of these processes seemed to operate, the concentrations of cluster ions (0.9-1.9 nm) exhibited a clear seasonality, with high concentrations in the warm months and low concentrations in the cold. Compared to event-free days, days with NPF were observed with higher cluster ion concentrations. A number of NPF events were observed with restricted growth below 10 nm, which were termed as suppressed NPF. There was another distinct feature, namely a simultaneous presence of two or three separate NPF and subsequent growth events, which were named as multi-mode NPF events. Growth rates (GRs) were determined using two methods: the appearance time method and the mode fitting method. The former method seemed to have advantages in characterizing NPF events with a fast GR, whereas the latter method is more suitable when the GR was slow. The formation rate of 2 nm positive ions (J(2)(+)) was calculated for all the NPF events for which a GR in the 2-3 nm size range could be determined. On average, J(2)(+) was about 0.014 cm(-3) s(-1). The ion production in relation to cloud/fog formation in the size range of 8-42 nm seemed to be a unique feature at Dome C, which has not been reported elsewhere. These ions may, however, either be multiply charged particles but detected as singly charged in the AIS, or be produced inside the instrument, due to the breakage of cloud condensation nuclei (CCN), possibly related to the instrumental behaviour under the extremely cold condition. For the wind-induced ion formation, our observations suggest that the ions originated more likely from atmospheric nucleation of vapours released from the snow than from mechanical charging of shattered snow flakes and ice crystals.Peer reviewe
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