8,447 research outputs found
Pequeñas historias de humor y de horror : usando narrativas y casos en la formación del profesorado de ciencias
En este trabajo se discute el uso de “pequeñas historias” en la formación inicial y continuada del profesorado de ciencias para todos los niveles educativos. Se presenta un proyecto de investigación e innovación didácticas dirigido a fomentar, en profesores y profesoras de ciencias, algunos procesos cognitivolingüísticos epitómicos (paradigmáticos) de las ciencias. Para tal proyecto se diseñaron dos relatos cortos basados en materiales de historia de la ciencia. Los relatos asumen un formato narrativo y se utilizan como casos de estudio; en ellos se recurre al humor y al horror como herramientas de estructuración del hilo argumental. El contenido y el formato de los relatos tiene su base en investigaciones previas acerca de la naturaleza de la ciencia; estas “pequeñas historias” pretendían construir una imagen más robusta de la ciencia como actividad profundamente humana
Generalised verification of the observer property in discrete event systems
The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system
Generalised verification of the observer property in discrete event systems
The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system
Efficient low-power terahertz generation via on-chip triply-resonant nonlinear frequency mixing
Achieving efficient terahertz (THz) generation using compact turn-key sources
operating at room temperature and modest power levels represents one of the
critical challeges that must be overcome to realize truly practical
applications based on THz. Up to now, the most efficient approaches to THz
generation at room temperature -- relying mainly on optical rectification
schemes -- require intricate phase-matching set-ups and powerful lasers. Here
we show how the unique light-confining properties of triply-resonant photonic
resonators can be tailored to enable dramatic enhancements of the conversion
efficiency of THz generation via nonlinear frequency down-conversion processes.
We predict that this approach can be used to reduce up to three orders of
magnitude the pump powers required to reach quantum-limited conversion
efficiency of THz generation in nonlinear optical material systems.
Furthermore, we propose a realistic design readily accesible experimentally,
both for fabrication and demonstration of optimal THz conversion efficiency at
sub-W power levels
Verification of the observer property in discrete event systems
The observer property is an important condition to be satisfied by abstractions of Discrete Event System (DES) models. This technical note presents a new algorithm that tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure, called OP-Verifier, can be applied to (potentially nondeterministic) automata, with no restriction on the existence of cycles of 'non-relevant' events. This procedure has quadratic complexity in the number of states. The performance of the algorithm is illustrated by a set of experiments
Rapid surfactant-free synthesis of Mg(OH)2 nanoplates and pseudomorphic dehydration to MgO
Magnesium hydroxide nanoplates ca. 50 nm in thickness can be prepared over minute timescales via hydrothermal synthesis in a multimode cavity (MMC) microwave reactor. This approach allows ca. 1 g of single-phase Mg(OH)2 to be synthesised in under 3 minutes without the requirement of surfactants or non-aqueous solvents. The hydroxide nanomaterial dehydrates at temperatures >200 K below that of the equivalent bulk material and can be utilised as a precursor for the pseudomorphic synthesis of nanoplates of MgO as investigated by TG-DTA-MS, XRD and SEM measurements. Equally, the pseudomorphic synthesis can be performed by irradiating the Mg(OH)2 nanomaterial with microwaves for 6 minutes to produce single phase MgO
Photonic crystal optical waveguides for on-chip Bose-Einstein condensates
We propose an on-chip optical waveguide for Bose-Einstein condensates based
on the evanescent light fields created by surface states of a photonic crystal.
It is shown that the modal properties of these surface states can be tailored
to confine the condensate at distances from the chip surface significantly
longer that those that can be reached by using conventional index-contrast
guidance. We numerically demonstrate that by index-guiding the surface states
through two parallel waveguides, the atomic cloud can be confined in a
two-dimensional trap at about 1m above the structure using a power of
0.1mW.Comment: 5 pages, 4 figure
Composition of the Innermost Core Collapse Supernova Ejecta
With presently known input physics and computer simulations in 1D, a
self-consistent treatment of core collapse supernovae does not yet lead to
successful explosions, while 2D models show some promise. Thus, there are
strong indications that the delayed neutrino mechanism works combined with a
multi-D convection treatment for unstable layers. On the other hand there is a
need to provide correct nucleosynthesis abundances for the progressing field of
galactic evolution and observations of low metallicity stars. The innermost
ejecta is directly affected by the explosion mechanism, i.e. most strongly the
yields of Fe-group nuclei for which an induced piston or thermal bomb treatment
will not provide the correct yields because the effect of neutrino interactions
is not included. We apply parameterized variations to the neutrino scattering
cross sections and alternatively, parameterized variations are applied to the
neutrino absorption cross sections on nucleons in the ``gain region''. We find
that both measures lead to similar results, causing explosions and a Ye>0.5 in
the innermost ejected layers, due to the combined effect of a short weak
interaction time scale and a negligible electron degeneracy, unveiling the
proton-neutron mass difference. We include all weak interactions (electron and
positron capture, beta-decay, neutrino and antineutrino capture on nuclei, and
neutrino and antineutrino capture on nucleons) and present first
nucleosynthesis results for these innermost ejected layers to discuss how they
improve predictions for Fe-group nuclei. The proton-rich environment results in
enhanced abundances of 45Sc, 49Ti, and 64Zn as requested by chemical evolution
studies and observations of low metallicity stars as well as appreciable
production of nuclei in the mass range up to A=80.Comment: 13 pages, 8 figures. Final versio
Gravitational settling of 22Ne and white dwarf evolution
We study the effects of the sedimentation of the trace element 22Ne in the
cooling of white dwarfs. In contrast with previous studies, which adopted a
simplified treatment of the effects of 22Ne sedimentation, this is done
self-consistently for the first time, using an up-to-date stellar evolutionary
code in which the diffusion equation is coupled with the full set of equations
of stellar evolution. Due the large neutron excess of 22Ne, this isotope
rapidly sediments in the interior of the white dwarf. Although we explore a
wide range of parameters, we find that using the most reasonable assumptions
concerning the diffusion coefficient and the physical state of the white dwarf
interior the delay introduced by the ensuing chemical differentation is minor
for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd
about 1.0 Msun, the delay turns out to be considerably larger. These results
are in qualitatively good accord with those obtained in previous studies, but
we find that the magnitude of the delay introduced by 22Ne sedimentation was
underestimated by a factor of about 2. We also perform a preliminary study of
the impact of 22Ne sedimentation on the white dwarf luminosity function.
Finally, we hypothesize as well on the possibility of detecting the
sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective
temperature range with accurately determined rates of change of the observed
periods.Comment: To apper in The Astrophysical Journa
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