Native defects in hybrid C/BN nanostructures

First-principles calculations of substitutional defects and vacancies are performed for zigzag-edged hybrid C/BN nanosheets and nanotubes which recently have been proposed to exhibit half-metallic properties. The formation energies show that defects form preferentially at the interfaces between graphene and BN domains rather than in the middle of these domains, and that substitutional defects dominate over vacancies. Chemical control can be used to favor localization of defects at C- B interfaces (nitrogen-rich environment) or C-N interfaces (nitrogen-poor environment). Although large defect concentrations have been considered here (106 cm-1), half-metallic properties can subsist when defects are localized at the C-B interface and for negatively charged defects localized at the C- N interface, hence the promising magnetic properties theoretically predicted for these zigzag-edged nanointerfaces might not be destroyed by point defects if these are conveniently engineered during synthesis.Comment: 6 pages, 5 figure

Origin of half-semimetallicity induced at interfaces of C-BN heterostructures

First-principles density functional calculations are performed in C-BN heterojunctions. It is shown that the magnetism of the edge states in zigzag shaped graphene strips and polarity effects in BN strips team up to give a spin asymmetric screening that induces half-semimetallicity at the interface, with a gap of at least a few tenths of eV for one spin orientation and a tiny gap of hundredths of eV for the other. The dependence with ribbon widths is discussed, showing that a range of ribbon widths is required to obtain half-semimetallicity. These results open new routes for tuning electronic properties at nanointerfaces and exploring new physical effects similar to those observed at oxide interfaces, in lower dimensions.Comment: 4 pages, 4 figure

Short range repulsive interatomic interactions in energetic processes in solids

The repulsive interaction between two atoms at short distances is studied in order to explore the range of validity of standard first-principles simulation techniques and improve the available short-range potentials for the description of energetic collision cascades in solids. Pseudopotentials represent the weakest approximation, given their lack of explicit Pauli repulsion in the core-core interactions. The energy (distance) scale realistically accessible is studied by comparison with all-electron reference calculations in some binary systems. Reference calculations are performed with no approximations related to either core (frozen core, augmentation spheres) or basis set. This is important since the validity of such approximations, even in all-electron calculations, rely on the small core perturbation usual in low-energy studies. The expected importance of semicore states is quantified. We propose a scheme for improving the electronic screening given by pseudopotentials for very short distances. The results of this study are applied to the assessment and improvement of existing repulsive empirical potentials.Comment: 10 pages, 7 figure

First-principles study of structural, elastic, and bonding properties of pyrochlores

Density Functional Theory calculations have been performed to obtain lattice parameters, elastic constants, and electronic properties of ideal pyrochlores with the composition A$_2$B$_2$O$_7$ (where A=La,Y and B=Ti,Sn,Hf, Zr). Some thermal properties are also inferred from the elastic properties. A decrease of the sound velocity (and thus, of the Debye temperature) with the atomic mass of the B ion is observed. Static and dynamical atomic charges are obtained to quantify the degree of covalency/ionicity. A large anomalous contribution to the dynamical charge is observed for Hf, Zr, and specially for Ti. It is attributed to the hybridization between occupied $2p$ states of oxygen and unoccupied d states of the B cation. The analysis based on Mulliken population and deformation charge integrated in the Voronoi polyhedra indicates that the ionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The charge deformation contour plots support this assignment.Comment: Modified contact details, and acknowledgment

Determinantes y evolución de la motivación de los trabajadores en un contexto de crisis económica. El caso de España

El grado de exactitud o fiabilidad de la información cuantitativa y cualitativa, derivada de la elaboración propia por parte del autor, es de la exclusiva responsabilidad de éste.La motivación de los trabajadores es un factor de gran relevancia, tanto en su vertiente personal como en la empresarial. El objetivo principal de este estudio es analizar los determinantes de la motivación de los trabajadores en los últimos años, en el contexto de crisis en España. Este trabajo contribuye, además, a dar a conocer el nivel de motivación de la población ocupada. Partiendo de los datos de las oleadas de 2008 a 2010 de la Encuesta de Calidad de Vida en el Trabajo1, se emplean modelos logit ordenados para estimar los efectos marginales sobre los niveles de motivación de la población ocupada acerca de diversos factores sociodemográficos y laborales. Los resultados obtenidos indican que el nivel global medio de motivación no ha experimentado cambios en los últimos años, al contrario de lo que se observa para grupos de trabajadores específicos, como es el caso de quienes tienen una jornada semanal inferior a veinte horas o los extranjeros. Respecto a los determinantes de la motivación, no solo existen diferencias temporales, sino que, además, se observan cambios en función del nivel de estudios, de la categoría profesional o entre asalariados y autónomos, aunque es el salario el que arroja los efectos más significativos. Como conclusiones, se resalta el valor que este trabajo tiene por ser el primero de estas características realizado para España, además de aportar evidencia sobre la motivación en un contexto de decrecimiento económico y de apoyar la tesis que defiende el papel clave del salario como elemento motivador.Job motivation is a key factor from both a personal and an organizational perspective. The main objective of this study is to analyze the determinants of workers' motivation in recent years in Spain, and hence in a context of crisis. This work also contributes to the knowledge of the motivational situation of the Spanish workforce. Based on data drawn from the 2008-2010 waves of the Quality of Working Life Survey, ordered logit models are used to estimate the marginal effects of sociodemographic, occupational and organizational variables. The results show that the overall level of motivation has not changed in recent years, contrary to what is found for specific groups of workers, such as those who have a working week of less than twenty hours or foreigners. Regarding the determinants of motivation, not only are there differences along time, but changes can also be observed in terms of education, job category or between waged workers and self-employed individuals. Nevertheless, monthly net income yields the most significant effect. In conclusion, the value of this research is threefold: it is the first of its kind to be conducted for Spain, it provides new evidence on motivation in a context of economic crisis, and its results support the thesis that economic incentives are one of the most powerful motivators

Anàlisi comparativa de l’eficiència en el tractament quirúrgic de la placa plantar segons el tipus de sutura aplicada

Treball Final de Grau de Podologia, Facultat de Medicina i Ciències de la Salut, Universitat de Barcelona, curs: 2016-2017, Tutor: Elena de Planell i MasLa placa plantar és una estructura anatòmica situada sota l’articulació metatarsofalàngica (AMTF) del peu i es considerada la principal estructura estabilitzadora de l’AMTF. L’objectiu del treball és la realització d’una comparativa de l’eficiència del tractament quirúrgic de la ruptura de la placa plantar, a partir d’un abordatge dorsal o plantar, segons el tipus de sutura aplicada i analitzar les contraindicacions del tractament. La recerca bibliogràfica dels articles es va realitzar a partir de dues bases de dades, PubMed i Scopus, per valorar l’eficiència del tractament quirúrgic de la placa plantar segons el tipus de sutura, publicats entre el 2010 i l’actualitat. Sis articles han estat inclosos en l’estudi final, amb un total de 294 plaques plantars estudiades en el treball. L’anàlisi dels estudis mostra una evolució satisfactòria del tractament en un percentatge elevat dels estudis. Els tipus de tècniques quirúrgiques són eficaces per la reparació de la placa plantar, tot i que la falta d’evidència científica en l’abordatge plantar i la fixació de l’AMTF amb cargols canulats no permet treure resultats representatius

ab inito local vibrational modes of light impurities in silicon

We have developed a formulation of density functional perturbation theory for the calculation of vibrational frequencies in molecules and solids, which uses numerical atomic orbitals as a basis set for the electronic states. The (harmonic) dynamical matrix is extracted directly from the first order change in the density matrix with respect to infinitesimal atomic displacements from the equilibrium configuration. We have applied this method to study the vibrational properties of a number of hydrogen-related complexes and light impurities in silicon. The diagonalization of the dynamical matrix provides the vibrational modes and frequencies, including the local vibrational modes (LVMs) associated with the defects. In addition to tests on simple molecules, results for interstitial hydrogen, hydrogen dimers, vacancy-hydrogen and self-interstitial-hydrogen complexes, the boron-hydrogen pair, substitutional C, and several O-related defects in c-Si are presented. The average error relative to experiment for the aprox.60 predicted LVMs is about 2% with most highly harmonic modes being extremely close and the more anharmonic ones within 5-6% of the measured values.Comment: 18 pages, 1 figur