First-principles study of epitaxial strain in perovskites

Using an extension of a first-principles method developed by King-Smith and Vanderbilt [Phys. Rev. B {\bf 49}, 5828 (1994)], we investigate the effects of in-plane epitaxial strain on the ground-state structure and polarization of eight perovskite oxides: BaTiO$_3$, SrTiO$_3$, CaTiO$_3$, KNbO$_3$, NaNbO$_3$, PbTiO$_3$, PbZrO$_3$, and BaZrO$_3$. In addition, we investigate the effects of a nonzero normal stress. The results are shown to be useful in predicting the structure and polarization of perovskite oxide thin films and superlattices.Comment: 10 page

Geiger-Mode Avalanche Photodiodes in Particle Detection

It is well known that avalanche photodiodes operated in the Geiger mode above the breakdown voltage offer a virtually infinite sensitivity and time accuracy in the picosecond range that can be used for single photon detection. However, their performance in particle detection remains still unexplored. In this contribution, we are going to expose the different steps that we have taken in order to prove the efficiency of Geiger mode avalanche photodiodes in the aforementioned field. In particular, we will present an array of pixels of 1mmx1mm fabricated with a standard CMOS technology for characterization in a test beam.Comment: 7 pages, 2 figures, Proceedings of LCWS1

Diferencias sexuales en los primeros comportamientos coitales

[Resumen] Objetivos: Conocer las primeras prácticas sexuales no coita1es de la juventud y las diferencias en función del sexo

Megalitismo y medio edafológico en el noroeste peninsular

[Resumen] Se estudia la relación existente entre la distribución de tumbas megalíticas y las caracteristicas de los suelos en la provincia de La Coruña. Las mayores concentraciones de túmulos y la presencia de los de mayor tamaño están relacionadas con los suelos más aptos para el cultivo. Por el contrario en zonas con suelos poco aptos para la agricultura los túmulos son más escasos y de menor tamaño.[Abstract] The relations between the distribution of megali thic graves and the soil characteristics in the province of La Coruña are studied. The highest concentrations of tombs and the biggest ones are related to the best soils for the agriculture. On contrary in zones with poor soi ls the graves are less abundant and of smallest siz

Ab initio study of the phase diagram of epitaxial BaTiO3

Using a combination of first-principles and effective-Hamiltonian approaches, we map out the structure of BaTiO3 under epitaxial constraints applicable to growth on perovskite substrates. We obtain a phase diagram in temperature and misfit strain that is qualitatively different from that reported by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)], who based their results on an empirical thermodynamic potential with parameters fitted at temperatures in the vicinity of the bulk phase transitions. In particular, we find a region of `r phase' at low temperature where Pertsev et al. have reported an `ac phase'. We expect our results to be relevant to thin epitaxial films of BaTiO3 at low temperatures and experimentally-achievable strains.Comment: 4 pages, with 4 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/od_epi/index.htm

The importance of meteorological scales to forecast air pollution scenarios on coastal complex terrain

Some of the meteorological approaches commonly considered in urban air pollution models do not take into account the importance of the smaller scales in the meteorology of complex-terrain coastal sites. The aim of this work is to estimate the impact of using the proper meteorological scales when simulating the behaviour of the pollutant concentrations emitted in the lower layers over coastal complex terrain areas. The availability of experimental measurements of a power plant plume near the Castellón conurbation (on the Spanish Mediterranean coast) has allowed us to use this plume as a tracer of opportunity of the lower atmosphere to check the results of a simulation exercise using the RAMS mesoscale model coupled to the HYPACT particle model. The results obtained show that in a complex-terrain coastal site, because of the strong effect of the meteorological interactions between the different scales on the integral advection and the turbulent dispersion of pollutants, using an inadequate scale to solve the meteorology can result in a very big gap in the simulation of lower-layer pollutant behaviour at urban scales

Comparación de métodos de estimación de ajuste de las funciones Weibull, SB de Johnson y beta a masas de Pinus pinaster, Pinus radiata y Pinus sylvestris en el noroeste de España

The purpose of this study was to compare the accuracy of the Weibull, Johnson’s SB and beta distributions, fitted with some of the most usual methods and with different fixed values for the location parameters, for describing diameter distributions in even-aged stands of Pinus pinaster, Pinus radiata and Pinus sylvestris in northwest Spain. A total of 155 permanent plots in Pinus sylvestris stands throughout Galicia, 183 plots in Pinus pinaster stands throughout Galicia and Asturias and 325 plots in Pinus radiata stands in both regions were measured to describe the diameter distributions. Parameters of the Weibull function were estimated by Moments and Maximum Likelihood approaches, those of Johnson’s SB function by Conditional Maximum Likelihood and by Knoebel and Burhart’s method, and those of the beta function with the method based on the moments of the distribution. The beta and the Johnson’s SB functions were slightly superior to Weibull function for Pinus pinaster stands; the Johnson’s SB and beta functions were more accurate in the best fits for Pinus radiata stands, and the best results of the Weibull and the Johnson’s SB functions were slightly superior to beta function for Pinus sylvestris stands. However, the three functions are suitable for this stands with an appropriate value of the location parameter and estimation of parameters method.El objetivo de este estudio fue comparar la precisión de las distribuciones Weibull, SB de Johnson y beta, ajustadas por alguno de los métodos más habituales y fijando diferentes valores para los parámetros de localización, para describir distribuciones diamétricas en masas regulares de Pinus pinaster, Pinus radiata y Pinus sylvestris en el noroeste de España. Se midieron un total de 155 parcelas permanentes en masas de Pinus sylvestris en Galicia, 183 parcelas de Pinus pinaster en Galicia y en Asturias y 325 parcelas de Pinus radiata en ambas regiones para describir sus distribuciones diamétricas. Los parámetros de la función Weibull fueron estimados por las aproximaciones de los Momentos y Máxima Verosimilitud, los de la función SB de Johnson por los estimadores condicionados de Máxima Verosimilitud y por el método de Knoebel y Burkhart, y los de la función beta por el método basado en los Momentos de la distribución. Las funciones beta y SB de Johnson fueron ligeramente superiores a la función Weibull en las masas de Pinus pinaster; las funciones SB de Johnson y beta fueron más precisas en los mejores ajustes en las masas de Pinus radiata, y los mejores resultados de las funciones Weibull y SB de Johnson fueron ligeramente superiores a los de la función beta en las masas de Pinus sylvestris. No obstante, las tres funciones son apropiadas para estas masas siempre que se elija un valor de localización y método de estimación de los parámetros apropiado

High dynamic range diamond detector acquisition system for beam wire scanner applications

The CERN Beam Instrumentation group has been working during the last years on the beam wire scanners upgrade to cope up with the increasing requirements of CERN experiments. These devices are used to measure the beam profile by crossing a thin wire through a circulating beam, the resulting secondary particles produced from beam/wire interaction are detected and correlated with the wire position to reconstruct the beam profile. The upgraded secondary particles acquisition electronics will use polycrystalline chemical vapour deposition (pCVD) diamond detectors for particle shower measurements, with low noise acquisitions performed on the tunnel, near the detector. The digital data is transmitted to the surface through an optical link with the GBT protocol. Two integrator ASICs (ICECAL and QIE10) are being characterized and compared for detector readout with the complete acquisition chain prototype. This contribution presents the project status, the QIE10 front-end performance and the first measurements with the complete acquisition system prototype. In addition, diamond detector signals from particle showers generated by an operational beam wire scanner are analysed and compared with an operational system

First-principles modeling of strain in perovskite ferroelectric thin films

We review the role that first-principles calculations have played in understanding the effects of substrate-imposed misfit strain on epitaxially grown perovskite ferroelectric films. We do so by analyzing the case of BaTiO$_3$, complementing our previous publications on this subject with unpublished data to help explain in detail how these calculations are done. We also review similar studies in the literature for other perovskite ferroelectric-film materials.Comment: 14 pages, submitted to Phase Transition