Non-ergodic states induced by impurity levels in quantum spin chains

The semi-infinite XY spin chain with an impurity at the boundary has been chosen as a prototype of interacting many-body systems to test for non-ergodic behavior. The model is exactly solvable in analytic way in the thermodynamic limit, where energy eigenstates and the spectrum are obtained in closed form. In addition of a continuous band, localized states may split off from the continuum, for some values of the impurity parameters. In the next step, after the preparation of an arbitrary non-equilibrium state, we observe the time evolution of the site magnetization. Relaxation properties are described by the long-time behavior, which is estimated using the stationary phase method. Absence of localized states defines an ergodic region in parameter space, where the system relaxes to a homogeneous magnetization. Out of this region, impurity levels split from the band, and localization phenomena may lead to non-ergodicity.Comment: 10 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:1703.0344

Impedance-compensated grid synchronisation for extending the stability range of weak grids with voltage source converters

This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version

La desventaja de la media aritmética: cómo tratarla en clases

La enseñanza de la media aritmética dentro del programa de estadística se realiza desde la enseñanza primaria hasta la universitaria pero no es frecuente tratar, con ejemplos prácticos, la desventaja de este estadístico ante la presencia de datos extremos. El presente artículo expone una experiencia de clases donde es necesario tener en cuenta esta desventaja

La escritura gótica libraria: su trazado y técnica de ejecución

Ya el profesor J. Boussard advirtió en su momento la importancia del biselado de la pluma en la génesis y trazado de la escritura gótica; en la actualidad este condicionamiento sigue siendo uno de los temas más debatidos en el ámbito paleográfico. Por ende, la finalidad de este artículo es estudiar esta técnica, esencialmente desde una óptica caligráfica, con la intención de obtener unos resultados gráficos aclaratorios.Professor J. Boussard already pointed the importance of the oblique quill cutting concerning genesis and tracing of the gothic script; nowadays it continues to be one of the most debated palaeog-raphical subjects. So, the aim of this paper is studying this technique, essentially from a calligraphical point of view, in order to obtain some clarifying graphic results

Acciones didácticas para desarrollar la Resistencia Aeróbica en estudiantes de licenciatura en Ciencias Farmacéuticas

The article responds to the necessity of improving the Aerobic Endurance in the formation of the professional of students of the career Degree in Pharmaceutical Sciences by means of didactic actions that lead to the knowledge and importance from a physical-educational dynamics starting from implementing them driving to the understanding of a Contemporary Physical Education. It was worked with a deliberate sample of 22 students of first year of the career between 17 and 19 years old, 5 males and 17 females. Theoretical methods were applied as the analytic-synthetic, inductive-deductive; of empiric level the observation, the survey, the analysis of documents, the mensuration and statistical methods in the analysis of the developed tests.El artículo responde a la necesidad de mejorar la Resistencia Aeróbica en los estudiantes de la carrera Licenciatura en Ciencias Farmacéuticas mediante acciones didácticas que conduzcan al conocimiento e importancia desde una dinámica físico-educativa a partir de implementarlas conduciendo a la comprensión de una Educación Física Contemporánea. Se trabajó con una muestra intencionada de 22 estudiantes de primer año de la carrera entre 17 y 19 años de edad, 5 varones y 17 hembras. Se aplicaron métodos teóricos como el analítico-sintético, inductivo-deductivo; de nivel empírico la observación, la encuesta, el análisis de documentos, la medición y métodos estadísticos en el análisis de las pruebas realizadas