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 CientíficaLeido 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 aquí 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 epistemologías 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

Violation of the strong Huygen's principle and timelike signals from the early Universe

We analyze the implications of the violations of the strong Huygens principle in the transmission of information from the early universe to the current era via massless fields. We show that much more information reaches us through timelike channels (not mediated by real photons) than it is carried by rays of light, which are usually regarded as the only carriers of information.Comment: 5 pages, 2 figures. RevTeX 4.1. V2: Updated to match published version. Previous title "A glimpse of the early universe without real light" modified to match Physical Review Letters published versio

Space and time error estimates for a first order, pressure stabilized finite element method for the incompressible Navier–Stokes equations

In this paper we analyze a pressure stabilized, finite element method for the unsteady, incompressible Navier–Stokes equations in primitive variables; for the time discretization we focus on a fully implicit, monolithic scheme. We provide some error estimates for the fully discrete solution which show that the velocity is first order accurate in the time step and attains optimal order accuracy in the mesh size for the given spatial interpolation, both in the spaces L2(Ω) and H10(Ω); the pressure solution is shown to be order 12 accurate in the time step and also optimal in the mesh size. These estimates are proved assuming only a weak compatibility condition on the approximating spaces of velocity and pressure, which is satisfied by equal order interpolations

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