Microscopic origin of exchange bias in core/shell nanoparticles

We report the results of Monte Carlo simulations with the aim to clarify the microscopic origin of exchange bias in the magnetization hysteresis loops of a model of individual core/shell nanoparticles. Increase of the exchange coupling across the core/shell interface leads to an enhancement of exchange bias and to an increasing asymmetry between the two branches of the loops which is due to different reversal mechanisms. A detailed study of the magnetic order of the interfacial spins shows compelling evidence that the existence of a net magnetization due to uncompensated spins at the shell interface is responsible for both phenomena and allows to quantify the loop shifts directly in terms of microscopic parameters with striking agreement with the macroscopic observed values.

Collective dynamic properties in simple crystals: Influence of the structural disorder

Collective dynamic properties in Lennard-Jones crystals are investigated by molecular dynamics simulation. The study is focused on properties such as the dynamic structure factors, the longitudinal and transverse currents and the density of states. The influence on these properties of the structural disorder is analyzed by comparing the results for one-component crystals with those for liquids and supercooled liquids at analogous conditions. The effects of species-disorder on the collective properties of binary crystals are also discussed.

Neutral atoms in ionic lattices: Excited states of KCl:Ag(0)

The optical-absorption spectrum of a cationic Ag0 atom in a KCl crystal has been studied theoretically by means of a series of cluster models of increasing size. Excitation energies have been determined by means of a multiconfigurational self-consistent field procedure followed by a second-order perturbation correlation treatment. Moreover results obtained within the density-functional framework are also reported. The calculations confirm the assignment of bands I and IV to transitions of the Ag-5s electron into delocalized states with mainly K-4s,4p character. Bands II and III have been assigned to internal transitions on the Ag atom, which correspond to the atomic Ag-4d to Ag-5s transition. We also determine the lowest charge transfer (CT) excitation energy and confirm the assignment of band VI to such a transition. The study of the variation of the CT excitation energy with the Ag-Cl distance R gives additional support to a large displacement of the Cl ions due to the presence of the Ag0 impurity. Moreover, from the present results, it is predicted that on passing to NaCl:Ag0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of involved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is explained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R dependence of the s2-sp transitions due to s2 ions like Tl+.

Ground- and excited-state properties of M-center oxygen vacancy aggregates in the bulk and the surface of MgO

Aggregates of oxygen vacancies (F centers) represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and M+ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in M+ centers the highest level is singly occupied. For the ground-state properties we used a gradient corrected density functional theory approach. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and M+ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the M+ center. On the surface the transitions occur at 1.6 eV (M+) and 2 eV (M). The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

Theoretical modeling of photon and electron-stimulated Na and K desorption from SiO2

The mechanism of generation of atomic Na and K from SiO2 samples has been studied using explicitly correlated wave function and density functional theory cluster calculations. Possible pathways for the photon and electron stimulated desorption of Na and K atoms from silicates are proposed, thus providing new insight on the generation of the tenuous Na and K atmosphere of the Moon.

Química de COU : bases para una coordinación.

1. Unificar criterios en el desarrollo del programa de estudios. 2. Actualizar la terminología. 3. Proponer actividades de autoevaluación que permitan una valoración del grado de consecución de los objetivos propuestos. La programación de Química en COU, con el fin de sugerir la posibilidad del inicio de una coordinación entre los profesores, rompiendo con la dinámica de actuaciones individuales. Se abordan los temas que los autores consideran básicos de Química además de un tema que intenta conectar con el BUP, una introducción a la Química del Carbono y una referencia a lo que podría ser el tratamiento de la Química Inorgánica descriptiva. Los temas a tratar son: 1. Termodinámica, 2. Cinética de reacciones, 3. Equilibrio químico, 4. Ácidos y bases, 5. Reacciones de transferencia de electrones, 6. Reacciones de precipitación. 7. Estructura atómica y sistema periódico y 8. Enlace químico. En cada uno de los temas que se abordan dentro de la Química de COU, se discute el enfoque y comentan los aspectos más destacados de la programación con especial incidencia en la actualización terminológica. Se exponen los objetivos del tema, el programa, los comentarios y se proponen ejercicios múltiples de autoevaluación: frases incompletas, afirmaciones de las que el alumno debe comentar su veracidad o falsedad, ejercicios numéricos, formulación, etc., que, según los autores, permitirán una valoración del grado de consecución de los objetivos propuestos. Se proponen también una serie de cuestiones y ejercicios para un inicio de las clases de Química de COU, para detectar los conocimientos que los alumnos poseen sobre cuestiones básicas. Se pone de manifiesto la necesidad de una coordinación entre los profesores de Química de COU y de los distintos niveles educativos. Viendo, además, la necesidad de abarcar otros aspectos como la actualización científica, la actualización pedagógica, orientación, investigación didáctica, etc.

Cicle de Stirling (applet) [versió comprimida]

Simulació del cicle d'Stirling

Cicles termodinàmics (applet) [versió comprimida]

Simulació de cicles termodinàmics

Dreta i esquerra: les molècules quirals

Molècules de dretes i d'esquerres? Una història científica amb contribució de la UdG: investigadors de l'Institut de Química Computacional de la UdG van participar en una recerca que amb posterioritat va ser premiada amb el Nobel de química del 2001

Cicle de Carnot (applet) [versió comprimida]

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