4,031 research outputs found
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
Miguel de Unamuno: «Teresa»
Producción CientÃficaLeido como colecci6n de poemas, Teresa es un anacronismo en
1923, y una anomalia en la escritura unamuniana. Leido, sin embargo, como novela (tal y com aquà se propone) el texto de Teresa no solo conecta plenamente con el experimentalismo narrativo de la epoca, sino que nos revela una gran densidad de significado
Interacción en el grupo y el encuentro con la exhibición : un estudio de aprendizaje en el museo de historia natural mediante el análisis de las epistemologÃas prácticas
En este estudio se presenta una forma de analizar el aprendizaje a un nivel discursivo en un museo de ciencias naturales. El material utilizado procede de la grabación en video de una actividad educativa en el Museo de Historia Natural en Estocolmo, Suecia. En dicha actividad los estudiantes, en pequeños grupos y con los textos de la exposición ocultos, realizan una interpretación de dioramas que representan escenas naturales. Mediante el uso de un análisis de las epistemológias practicas se describe como los estudiantes construyen el significado y desarrollan el conocimiento a partir de la interaccion en el grupo y el encuentro con el diorama en la exhibición. Los resultados del presente estudio demuestran que el uso de este tipo de análisis puede ser muy útil para describir el proceso de aprendizaje de las ciencias en museos y otros ambientes pedagógicos extramurales
The Quantum Echo of the Early Universe
We show that the fluctuations of quantum fields as seen by late comoving
observers are significantly influenced by the history of the early Universe,
and therefore they transmit information about the nature of spacetime in
timescales when quantum gravitational effects were non-negligible. We discuss
how this may be observable even nowadays, and thus used to build falsifiability
tests of quantum gravity theories.Comment: 3 pages. 2 Figures. Proceedings Theory Canada 9. Published in
Canadian Journal of Physics.
(http://www.nrcresearchpress.com/doi/abs/10.1139/cjp-2014-0567
X-ray diffraction analysis to clarify the unusual origin of an intraocular foreign body
[Purpose]: To clarify the origin of a metallic intraocular foreign body (IOFB) due to a firecracker injury and to show the utility of X-ray diffraction analysis in the identification process. [Methods]: Interventional case report using X-ray diffraction technique. [Results]: We report a mechanism of firecracker ocular injury not previously described in the published literature. It involved penetration into the eye by a ground particle of calamine (originated from the area of detonation) and not, as it could be suspected, by a firecracker fragment. [Conclusion]: X-ray diffraction analysis showed environmental debris in the area of detonation can be injected as foreign material into the eye.Peer Reviewe
Hipomotilidad gastrointestinal en conejos : 7 casos clÃnicos
Se describen siete casos clÃnicos de hipomotilidad gastrointestinal (HGI) en conejos mantenidos como mascotas. En todos los casos se intenta establecer la causa primaria que inició el proceso de HGI. Finalmente se discuten las ventajas e inconvenientes de cada una de las medidas terapéuticas aplicadas.
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