Metric characterization of cluster dynamics on the Sierpinski gasket

We develop and implement an algorithm for the quantitative characterization of cluster dynamics occurring on cellular automata defined on an arbitrary structure. As a prototype for such systems we focus on the Ising model on a finite Sierpsinski Gasket, which is known to possess a complex thermodynamic behavior. Our algorithm requires the projection of evolving configurations into an appropriate partition space, where an information-based metrics (Rohlin distance) can be naturally defined and worked out in order to detect the changing and the stable components of clusters. The analysis highlights the existence of different temperature regimes according to the size and the rate of change of clusters. Such regimes are, in turn, related to the correlation length and the emerging "critical" fluctuations, in agreement with previous thermodynamic analysis, hence providing a non-trivial geometric description of the peculiar critical-like behavior exhibited by the system. Moreover, at high temperatures, we highlight the existence of different time scales controlling the evolution towards chaos.Comment: 20 pages, 8 figure

Spin coupling around a carbon atom vacancy in graphene

We investigate the details of the electronic structure in the neighborhoods of a carbon atom vacancy in graphene by employing magnetization-constrained density-functional theory on periodic slabs, and spin-exact, multi-reference, second-order perturbation theory on a finite cluster. The picture that emerges is that of two local magnetic moments (one \pi-like and one \sigma-like) decoupled from the \pi- band and coupled to each other. We find that the ground state is a triplet with a planar equilibrium geometry where an apical C atom opposes a pentagonal ring. This state lies ~0.2 eV lower in energy than the open-shell singlet with one spin flipped, which is a bistable system with two equivalent equilibrium lattice configurations (for the apical C atom above or below the lattice plane) and a barrier ~0.1 eV high separating them. Accordingly, a bare carbon-atom vacancy is predicted to be a spin-one paramagnetic species, but spin-half paramagnetism can be accommodated if binding to foreign species, ripples, coupling to a substrate, or doping are taken into account

Metric Features of a Dipolar Model

The lattice spin model, with nearest neighbor ferromagnetic exchange and long range dipolar interaction, is studied by the method of time series for observables based on cluster configurations and associated partitions, such as Shannon entropy, Hamming and Rohlin distances. Previous results based on the two peaks shape of the specific heat, suggested the existence of two possible transitions. By the analysis of the Shannon entropy we are able to prove that the first one is a true phase transition corresponding to a particular melting process of oriented domains, where colored noise is present almost independently of true fractality. The second one is not a real transition and it may be ascribed to a smooth balancing between two geometrical effects: a progressive fragmentation of the big clusters (possibly creating fractals), and the slow onset of a small clusters chaotic phase. Comparison with the nearest neighbor Ising ferromagnetic system points out a substantial difference in the cluster geometrical properties of the two models and in their critical behavior.Comment: 20 pages, 15 figures, submitted to JPhys

A hungry need for knowledge on the black soldier fly digestive system

The interest towards the black soldier fly (BSF), Hermetia illucens, has grown impressively in the last few years, fostered by the legislative changes in the European landscape that have lifted the ban regarding the use of BSF larvae as feedstuff. In addition, bioconversion mediated by the larvae of the BSF is viewed as one of the most promising technologies for organic waste processing and valorisation. Finally, new, alternative applications to exploit various larval products such as lipids, chitin, antimicrobial peptides, and frass are being explored. However, this positive trend, confirmed by the increasing number of companies that deal with BSF mass rearing and processing, is in sharp contrast with the limited information on the biology of this insect, in particular on aspects related to its digestive features. This lack of knowledge needs to be carefully considered and filled in coming years, as a deep characterisation of the morphology, physiology, transcriptomics, and proteomics of the digestive system of the insect, as well a fine dissection of related aspects as gut microbiota and pathogens, is a prerequisite to improve the amazing bioconversion capabilities of this dipteron. So far, the larval stages received the most attention in research, but there might still be a lot to win by focusing more on the adult stage. Further expanding the basic knowledge on both the larval and the adult gut could lead to unexpected findings and open new perspectives to produce value-added bioproducts

Energy Transport in an Ising Disordered Model

We introduce a new microcanonical dynamics for a large class of Ising systems isolated or maintained out of equilibrium by contact with thermostats at different temperatures. Such a dynamics is very general and can be used in a wide range of situations, including disordered and topologically inhomogenous systems. Focusing on the two-dimensional ferromagnetic case, we show that the equilibrium temperature is naturally defined, and it can be consistently extended as a local temperature when far from equilibrium. This holds for homogeneous as well as for disordered systems. In particular, we will consider a system characterized by ferromagnetic random couplings $J_{ij} \in [ 1 - \epsilon, 1 + \epsilon ]$. We show that the dynamics relaxes to steady states, and that heat transport can be described on the average by means of a Fourier equation. The presence of disorder reduces the conductivity, the effect being especially appreciable for low temperatures. We finally discuss a possible singular behaviour arising for small disorder, i.e. in the limit $\epsilon \to 0$.Comment: 14 pages, 8 figure

POINT DEFECTS AND ADATOMS AT SURFACES:CHALLENGES AND OPPORTUNITIES

In many occasions defects have been proved to be an opportunity more than a limit, as they may be used to tailor the properties of a given material. To this end, a practical route to introduce a controlled amount of defects as well as a deep knowledge of the defect nature is always desirable. As well, recombinative and dissociative processes involving gas molecules are well known to likely occur on metal surfaces, that may then be used in a number of industrial applications. In this thesis I report on both the isolated carbon atom vacancy, that is a common lattice defect in graphene, and the Eley-Rideal formation of H2 molecules in the limit of an single adsorbed atom on the Ag(111) surface. In the first part of this thesis I consider the details of the electronic structure in the neighbourhoods of a carbon atom vacancy in graphene by employing magnetisation-constrained density-functional theory on periodic slabs, and spin-exact, multi-reference, second-order perturbation theory on a finite cluster. The picture that emerges is that of two local magnetic moments (one sigma-like and one pi-like) decoupled from the pi-band and coupled to each other. The ground state is identified as a triplet with a planar equilibrium geometry resulting upon a Jahn-Teller distortion, in which an apical C atom opposes a pentagonal ring. This state lies 0.2 eV lower in energy than the open-shell singlet with one spin flipped, which is a bistable system with two equivalent equilibrium lattice configurations (for the apical C atom above or below the lattice plane) and a barrier 0.1 eV high separating them. Accordingly, a bare carbon-atom vacancy is predicted to be a spin-one paramagnetic species, but spin-half paramagnetism can be accommodated if binding to foreign species, ripples, coupling to a substrate, or doping are taken into account. In the second part, I study by DFT means the process of hydrogenation of the carbon vacancy, starting from the bare defect atom up to the case of six hydrogen atoms chemisorbed onto its nearest neighbours. I initially consider the formation of a mono-hydrogenated vacancy, finding a binding energy of 4.2 eV and no activation barrier to the adsorption. As well, I study a variety of possible mutual arrangements of the adsorbates at higher coverages discussing their reactivity and local magnetic moments. In this way the overall hydrogenation process turns out to be thermodynamically favoured and exothermic with respect to both atomic and molecular hydrogen gas sources at least up to four H atoms. This follows from the fact that the driving force in this process is the saturation of the (3sigma + 1pi) unpaired electrons at the vacancy. Moreover, these DFT energies are used to build a phase diagram in a broad range of temperatures and H2 partial pressures, thus finding that at room T and p conditions, the magnetic (M = 1muB) 3H-anti structure is the most stable in agreement with recent magnetic measurements. In addition, by considering the stable phase at TEM conditions, it seems reasonable to identify the recently detected three-fold and distorted vacancy with the 3H-anti and the 1H vacancy, respectively. In the end, in these calculations the 2H-geminal phase detected in muSR experiments, is found to be unfavoured both from a thermodynamic and a kinetic point of view with respect to other di-hydrogenated structures. In the third part I consider the formation of hydrogen molecules on the Ag(111) surface by abstraction of the adsorbed H atom according to the Eley-Rideal reaction mechanism. To follow the time evolution of the system, I rely on ab initio molecular dynamics and on the quasiclassical trajectory method based onto an external potential energy surface, originally built for quantum calculations on the same system within the flat and rigid surface approximation. In general the reaction is not activated, in fact it has a sizeable cross section even at collision energies in the order of few meV. In terms of cross sections, the differences between ab initio and quasiclassical results at collision energies below 0.5 eV are proved to depend on the surface corrugation and the energy exchange between hydrogen atoms and surface atoms, which are ignored in the quasiclassical study, following from the reference PES used. In this energy interval, the target vibration may be safely neglected but this is not the case for higher collision energies where it strongly affects the final outcome. Moreover, by considering the product molecules the reaction mechanism is identified as mainly based on a non-collinear scheme with the reactive encounter occurring upon a bounce of the incident atom on the surface. By means of all these dynamics calculations a large cross-section (compared to the typical value on transition metals) is found in quite good agreement with a recent experimental estimate at very low coverage. Anyway, in future in order to get closer to the experimental result, it seems to be necessary to account for the initial surface temperature, the surface precoverage and the incident angle of the incoming atoms

Microscopic energy flows in disordered Ising spin systems

An efficient microcanonical dynamics has been recently introduced for Ising spin models embedded in a generic connected graph even in the presence of disorder i.e. with the spin couplings chosen from a random distribution. Such a dynamics allows a coherent definition of local temperatures also when open boundaries are coupled to thermostats, imposing an energy flow. Within this framework, here we introduce a consistent definition for local energy currents and we study their dependence on the disorder. In the linear response regime, when the global gradient between thermostats is small, we also define local conductivities following a Fourier dicretized picture. Then, we work out a linearized "mean-field approximation", where local conductivities are supposed to depend on local couplings and temperatures only. We compare the approximated currents with the exact results of the nonlinear system, showing the reliability range of the mean-field approach, which proves very good at high temperatures and not so efficient in the critical region. In the numerical studies we focus on the disordered cylinder but our results could be extended to an arbitrary, disordered spin model on a generic discrete structures.Comment: 12 pages, 6 figure

Digital technologies, teacher training and teaching practices

This article presents the research results involving teacher training in the era of cyberculture and its interrelation with knowledge, skills and attitudes by teachers in an educational context influenced by the use of Digital Technologies (DTs). The discussion advanced here results from a qualitative investigation, consisting of a case study, with data collected from semi-structured interviews with teachers of courses with a hybrid approach, in the setting of a non-profit university in the south of Brazil. To analyze the data, we used a methodology associated to discourse textual analysis. This investigation provided an understanding of the competencies evidenced by teachers who engage in good teaching practices with DTs, resulting in elements that allow us to reflect on planning educational actions in the scope of the academic/professional development of working professors. Four major competencies were identified as the object of reflection: digital fluency, teaching practice, planning and pedagogical intervention. In this article, we discuss the developments of digital fluency, for understanding its contribution to teachers that wish to use/create teaching practices with DTs, based on their experiences and those of their peers. The subjects interviewed in this study demonstrated a heightened level of familiarity regarding the use of technologies, but had limited training with respect to their didactic-methodological aspects. The results provided us with indicators to reflect on the organization of strategically-prepared training spaces so that the faculty can experiment, test, discuss and exchange experiences regarding teaching activities to include in their practices with the use of DTs.Este artigo apresenta os resultados de pesquisa envolvendo a formação docente em tempos de cibercultura e sua inter-relação com conhecimentos, habilidades e atitudes para atuação de professores no cenário educacional influenciado pelo uso de Tecnologias Digitais (TDs). A discussão aqui proposta é resultante de uma pesquisa qualitativa, apoiada por um estudo de caso, com dados coletados por meio de entrevistas semiestruturadas, no cenário de uma universidade comunitária no sul do Brasil, tendo como sujeitos os docentes que ministram disciplinas com abordagem semipresencial. Utilizou-se, para realizar a análise dos dados, uma metodologia associada à análise textual discursiva. A investigação possibilitou compreender as competências evidenciadas pelos docentes que empreendem boas práticas pedagógicas com TDs, resultando em elementos que nos permitem refletir acerca do planejamento de ações formativas no âmbito do desenvolvimento acadêmico/ profissional de professores em serviço. Foram identificadas quatro grandes competências a serem objeto de reflexão: fluência digital, prática pedagógica, planejamento e mediação pedagógica. Neste artigo, discutimos os desdobramentos da fluência digital, por entender sua contribuição para docentes que desejam utilizar/criar práticas pedagógicas com TDs, a partir de suas experiências e das de seus pares. Os sujeitos entrevistados no estudo demostraram um nível de familiaridade diferenciado quanto ao uso de tecnologias, mas evidenciaram restrições na sua formação no que tange a aspectos didático-metodológicos. Os resultados forneceram-nos indicadores para refletirmos a respeito da organização de espaços de formação estrategicamente pensados para que o corpo docente experimente, teste, discuta e troque experiências acerca das possibilidades didáticas para compor suas práticas com uso de TDs