Modelling exchange bias in core/shell nanoparticles

We present an atomistic model of a single nanoparticle with core/shell structure that takes into account its lattice strucutre and spherical geometry, and in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and interfacial regions. By means of Monte Carlo simulations of the hysteresis loops based on this model, we have determined the range of microscopic parameters for which loop shifts after field cooling can be observed. The study of the magnetic order of the interfacial spins for different particles sizes and values of the interfacial exchange coupling have allowed us to correlate the appearance of loop asymmetries and vertical displacements to the existence of a fraction of uncompensated spins at the shell interface that remain pinned during field cycling, offering new insight on the microscopic origin of the experimental phenomenology.Comment: 7 pages, 3 figures. Contribution presented at HMM 2007 held at Napoli 4-6 June 2007. To be published in J. Phys. Condens. Matte

Oxygen vacancies in strained SrTiO3_{3} thin films: formation enthalpy and manipulation

We report the enthalpy of oxygen vacancy formation in thin films of electron-doped SrTiO$_{3}$, under different degrees of epitaxial stress. We demonstrate that both compressive and tensile strain decrease this energy at a very similar rate, and promote the formation of stable doubly ionized oxygen vacancies. Moreover, we also show that unintentional cationic vacancies introduced under typical growth conditions, produce a characteristic rotation pattern of TiO$_6$ octahedra. The local concentration of oxygen vacancies can be modulated by an electric field with an AFM tip, changing not only the local electrical potential, but also producing a non-volatile mechanical response whose sign (up/down) can be reversed by the electric field.Comment: Physical Review B (accepted for publication

Multiple merging in the Abell cluster 1367

We present a dynamical analysis of the central ~1.3 square degrees of the cluster of galaxies Abell 1367, based on 273 redshift measurements (of which 119 are news). From the analysis of the 146 confirmed cluster members we derive a significantly non-Gaussian velocity distribution, with a mean location C_{BI} = 6484+/-81 km/s and a scale S_{BI} = 891+/-58 km/s. The cluster appears elongated from the North-West to the South-East with two main density peaks associated with two substructures. The North-West subcluster is probably in the early phase of merging into the South-East substructure (~ 0.2 Gyr before core crossing). A dynamical study of the two subclouds points out the existence of a group of star-forming galaxies infalling into the core of the South-East subcloud and suggests that two other groups are infalling into the NW and SE subclusters respectively. These three subgroups contain a higher fraction of star-forming galaxies than the cluster core, as expected during merging events. Abell 1367 appears as a young cluster currently forming at the intersection of two filaments.Comment: 15 pages, 13 figures, 7 tables. Accepted for publication on A&A. High resolution figures at http://goldmine.mib.infn.it/papers/a1367.htm

Model Calculation of Electron-Phonon Couplings in a Dimer with a Non-Degenerate Orbital

We evaluate all the electron-phonon couplings derived from the one-body electronic interactions, in both the adiabatic and extreme non-adiabatic limit, for a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape. We find largely different values of the coupling parameters in the two cases, as well as different expressions of the corresponding terms in the Hamiltonian.Comment: 5 postscript figure

Magnetic Field scaling of Relaxation curves in Small Particle Systems

We study the effects of the magnetic field on the relaxation of the magnetization of small monodomain non-interacting particles with random orientations and distribution of anisotropy constants. Starting from a master equation, we build up an expression for the time dependence of the magnetization which takes into account thermal activation only over barriers separating energy minima, which, in our model, can be computed exactly from analytical expressions. Numerical calculations of the relaxation curves for different distribution widths, and under different magnetic fields H and temperatures T, have been performed. We show how a \svar scaling of the curves, at different T and for a given H, can be carried out after proper normalization of the data to the equilibrium magnetization. The resulting master curves are shown to be closely related to what we call effective energy barrier distributions, which, in our model, can be computed exactly from analytical expressions. The concept of effective distribution serves us as a basis for finding a scaling variable to scale relaxation curves at different H and a given T, thus showing that the field dependence of energy barriers can be also extracted from relaxation measurements.Comment: 12 pages, 9 figures, submitted to Phys. Rev.

Participatory Democracy in Local Government: An Online Platform in the City of Madrid

In 2015, the local government of the city of Madrid (Spain) introduced an electronic participation system. This initiative stemmed mainly from the social movements that had occupied the squares of many Spanish cities since 2011. As a result of the local elections in 2015, many of those same activists gained institutional power, took citizens’ participation very seriously, and decided to use the possibilities offered by the internet for political and administrative participation. In this article, we seek to assess the impacts of the Madrid city government with the e-democracy experiment – based mainly on establishing an online platform to facilitate citizen participation in political and administrative decision processes. Drawing on qualitative and documental data, our research indicates that whereas the overall aim of the project was to give citizens a say in local policy and decision making, our case study shows that participation was very low since most of the population does not feel concerned by these processes. Indeed, one of our findings showed that citizens’ involvement offline surpassed in some cases their online participation. To identify who is politically active online and offline is a great challenge, to which the promoters of the project did not pay much attention. Although e-participation was meagre in relation to the electoral turnout, the case study also shows that many proposals from the public were incorporated into the local policies, indicating that from a qualitative point of view, e-participation influences decision-making processes. Perhaps local governments should use a more strategic and integrated approach towards the use of electronic technologies to foster and motivate citizens’ involvement in local politics and administration. This more integrated approach should be less dependent on ideological incentives, more institutionalized, and must incorporate citizens’ perceptions and inputs before the introduction of new technologies

El romance de Góngora en 'El príncipe constante', de Calderón

El caso del gongorino romance “Entre los sueltos caballos,” ampliamente citado en El príncipe constante, de Calderón, resulta excepcional por el tamaño de la cita, nunca igualado por apariciones de versos de otros poetas en las comedias calderonianas. Este trabajo estudia en detalle cómo Calderón adaptó el romance a las circunstancias de la comedia y además resuelve la cuestión de cuál de las versiones del romance gongorino fue la que conoció el dramaturgo madrileño. The case of Góngora’s romance “Entre los sueltos caballos,” extensively quoted in El príncipe constante of Calderón, is exceptional due to the magnitude of the quotation, without equal among the verses of other poets included in Calderonian comedies. This paper studies in detail the ways in which Calderón adapted this romance to the circumstances of his comedy, also resolving the question of which version of Góngora’s romance the Madrid playwright knew