Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. Application of a magnetic field perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. Below approximately 0.9 K, intermolecular interactions turn this equilibrium state into a ferromagnetically ordered phase. However, long range ferromagnetic correlations nearly disappear for fields larger 5.5 T, possibly suggesting the existence of a quantum critical point.Comment: 4 pages, 4 figure

Optimization of soliton ratchets in inhomogeneous sine-Gordon systems

Unidirectional motion of solitons can take place, although the applied force has zero average in time, when the spatial symmetry is broken by introducing a potential $V(x)$, which consists of periodically repeated cells with each cell containing an asymmetric array of strongly localized inhomogeneities at positions $x_{i}$. A collective coordinate approach shows that the positions, heights and widths of the inhomogeneities (in that order) are the crucial parameters so as to obtain an optimal effective potential $U_{opt}$ that yields a maximal average soliton velocity. $U_{opt}$ essentially exhibits two features: double peaks consisting of a positive and a negative peak, and long flat regions between the double peaks. Such a potential can be obtained by choosing inhomogeneities with opposite signs (e.g., microresistors and microshorts in the case of long Josephson junctions) that are positioned close to each other, while the distance between each peak pair is rather large. These results of the collective variables theory are confirmed by full simulations for the inhomogeneous sine-Gordon system

Una visión de la obra y el herbario de Rouy a través del género Carex L.

Se analiza la obra de Georges Rouy, célebre botánico francés, mediante el estudio de sus materiales así como del tratamiento del género Carex (Cyperaceae) en la Flore de France. Se valora favorablemente el conocimiento que el autor demuestra sobre este género, puesto de manifi esto al incluir subgéneros en la clasifi cación taxonómica. Asimismo la sinonimia que propone para cada nombre y las descripciones de los taxones son muy completas. Al contrario, se valora negativamente la excesiva fragmentación que estableció, pues incluyó demasiadas secciones y subsecciones. Finalmente, su rechazo a utilizar las normas nomenclaturales establecidas por el Código de Nomenclatura Botánica conduce a la confusión e impide que la mayoría de sus nombres sean válidos.The George Rouy’s work, a famous French botanist, is studied by means of his material and literature of the genus Carex (Cyperaceae). We assess positively the knowledge that he demonstrated of this genus; because of the subgenera differentiated in his treatment, besides the complete synonymy presented for each name and the taxa descriptions. But he splitted excessively the infrageneric classifi cation including too many sections and subsections. Finally, most of the names he proposed are invalid because of his rejection of the International Code of Botanical Nomenclature recommendations and rules

Impregnación de platino y dimensión fractal en dos tipos de sólidos microporosos

El presente trabajo muestra la modificación de una zeolita-Y y una arcilla pilarizada (tipo saponita) mediante impregnación con diferentes cargas de Pt (catalizadores a base de Pt). Los sólidos se caracterizaron en cuanto a su composición química y su textura. La caracterización textural revela cambios importantes en los sólidos, mostrando que se genera microporosidad en la serie de catalizadores zeolíticos y se bloquean las estructuras microporosas en la serie de catalizadores obtenidos a partir de la arcilla pilarizada. La determinación de la dimensión fractal (mediante el método de Avnir-Jaroniec) indica que la modificación con Pt afecta significativamente las características topográficas de la superficie de la arcilla pilarizada como consecuencia de la posible formación de agregados de Pt

Justifications-on-demand as a device to promote shifts of attention associated with relational thinking in elementary arithmetic

Student responses to arithmetical questions that can be solved by using arithmetical structure can serve to reveal the extent and nature of relational, as opposed to computational thinking. Here, student responses to probes which require them to justify-on-demand are analysed using a conceptual framework which highlights distinctions between different forms of attention. We analyse a number of actions observed in students in terms of forms of attention and shifts between them: in the short-term (in the moment), medium-term (over several tasks), and long-term (over a year). The main factors conditioning students´ attention and its movement are identified and some didactical consequences are proposed

Dynamics of vortices in weakly interacting Bose-Einstein condensates

We study the dynamics of vortices in ideal and weakly interacting Bose-Einstein condensates using a Ritz minimization method to solve the two-dimensional Gross-Pitaevskii equation. For different initial vortex configurations we calculate the trajectories of the vortices. We find conditions under which a vortex-antivortex pair annihilates and is created again. For the case of three vortices we show that at certain times two additional vortices may be created, which move through the condensate and annihilate each other again. For a noninteracting condensate this process is periodic, whereas for small interactions the essential features persist, but the periodicity is lost. The results are compared to exact numerical solutions of the Gross-Pitaevskii equation confirming our analytical findings.Comment: 8 pages, 7 figures, one reference updated, typos correcte

Pseudo-dynamic tests on two buildings retrofitted with damped braces

Seismic tests have been conducted on two 3-storey buildings protected with fluid-viscous spring damper devices. One of the buildings was a reinforced concrete frame with clay elements in the slabs, while the other one was a steel frame with steel/concrete composite slabs. The spring dampers were installed through K bracing in between the floors. The tests were performed by means of the pseudodynamic method on full-size specimens and by implementing a specific compensation strategy for the strain-rate effects at the devices. The tests results allowed to compare the behaviour of the unprotected buildings with several protected configurations showing the benefits of the application of the devices and the characteristics of their behaviour