Luttinger liquid superlattices: realization of gapless insulating phases

We investigate Luttinger Liquid superlattices, a periodic structure composed of two kinds of one-dimensional systems of interacting electrons. We calculate several properties of the low-energy sector: the effective charge and spin velocities, the compressibility, various correlation functions, the Landauer conductance and the Drude weight. The low-energy properties are subsumed into effective parameters, much like homogeneous one-dimensional systems. A generic result is the weighted average nature of these parameters, in proportion to the spatial extent of the underlying subunits, pointing to the possibility of ``engineered'' structures. As a specific realization, we consider a one-dimensional Hubbard superlattice, which consists of a periodic arrangement of two long Hubbard chains with different coupling constants and different hopping amplitudes. This system exhibits a rich phase diagram with several phases, both metallic and insulating. We have found that gapless insulating phases are present over a wide range of parameters.Comment: 16 pages, 15 figures, RevTeX

Magnetic-field effects in defect-controlled ferromagnetic Ga_{1-x}Mn_xAs semiconductors

We have studied the magnetic-field and concentration dependences of the magnetizations of the hole and Mn subsystems in diluted ferromagnetic semiconductor Ga_{1-x}Mn_xAs. A mean-field approximation to the hole-mediated interaction is used, in which the hole concentration p(x) is parametrized in terms of a fitting (of the hole effective mass and hole/local moment coupling) to experimental data on the Tc critical temperature. The dependence of the magnetizations with x, for a given temperature, presents a sharply peaked structure, with maxima increasing with applied magnetic field, which indicates that application to diluted-magnetic-semiconductor devices would require quality-control of the Mn-doping composition. We also compare various experimental data for Tc(x) and p(x) on different Ga_{1-x}Mn_xAs samples and stress the need of further detailed experimental work to assure that the experimental measurements are reproducible.Comment: RevTeX 4, 3 two-column pages, 4 colour figures; to appear in J Appl Phy

Ferramenta de suporte para apresentação e gestão de dados recolhidos por sistemas autónomos

A medida que as tecnologias relacionadas com os sistemas autónomos progridem, a quantidade de informação obtida dos mesmos também progride. Os sensores que constituem estes sistemas estão a tornar-se progressivamente mais eficientes e a cooperação entre grupos de sensores torna-se cada vez mais complexa. Tudo isto justifica a necessidade da existência de um sistema de uso simples, o qual obtém dados de um número elevado de sensores ou grupo de sensores e os interpreta e visualiza de forma imediata. Uma solução a esta necessidade deve procurar adquirir informação providenciada por sensores associados a sistemas autónomos, seja esta informação obtida através de ficheiros de log ou através de comunicação direta através de sockets, seguida da escrita para bases de dados da informação interpretada. A ferramenta de manipulação de bases de dados, desenvolvida em Python, utiliza “MySQL” queries de forma a criar, alterar ou apagar tabelas dentro de uma base de dados, tabelas as quais serão preenchidas com os dados interpretados a partir de logs ou comunicação com sensores em tempo real, via sockets. Por outro lado, a ferramenta de visualização online, desenvolvida em HTML (Hypertext Markup Language), PHP (PHP: Hypertext Preprocessor) e JavaScript, utiliza PHP de forma a obter dados através de “MySQL” queries e utiliza gráficos e outras ferramentas visuais, disponibilizadas através de JavaScript, de forma a demonstrar os dados obtidos de forma simplificada.As the technology of autonomous systems evolves with the passage of time, so does the amount of information we can obtain from them. The sensors that are part of these systems are becoming progressively more efficient and cooperation between groups of sensors is becoming ever more complex. All of this creates a need for a simple-to-use system, that obtains sensor data from a plethora of different sensors or groups of sensors and parses and visualizes them in an immediate fashion. A solution to this need should aim to acquire information provided by sensors associated with autonomous systems, be that information obtained from previously existent log files or through direct communication utilizing sockets, followed by the writing to databases of the interpreted information. Once that information is acquired, interpreted and stored, it should then be easy to visualize and access on any platform. The database manipulation tool, developed in Python, uses “MySQL” queries in order to create, alter or delete tables inside a database, tables which will be filled with the interpreted data obtained from logs or real-time sensor communication. On the other hand, the online visualization tool, developed in HTML (Hypertext Markup Language), PHP (PHP: Hypertext Preprocessor) and JavaScript, uses PHP coding to obtain data through “MySQL” queries and uses graphs and other visual tools, available through JavaScript, to convey the acquired data in a simplified fashion

Plurality Voting: the statistical laws of democracy in Brazil

We explore the statistical laws behind the plurality voting system by investigating the election results for mayor held in Brazil in 2004. Our analysis indicate that the vote partition among mayor candidates of the same city tends to be "polarized" between two candidates, a phenomenon that can be closely described by means of a simple fragmentation model. Complex concepts like "government continuity" and "useful vote" can be identified and even statistically quantified through our approach.Comment: 4 pages, 4 figure

The biogeographic basis of Ebola-virus disease outbreaks: A model for other zoonotic diseases?

We first determined the differential role of favorability of environmental conditions and mammalian chorotypes in explaining the presence of the Ebola virus in Africa. We then combined environmental factors and chorotypes using fuzzy logic, which better explained the distribution of Ebola virus. The core area for the virus was associated with human infections of known animal origin, with infections of unknown source detected in areas that are biogeographically more peripheral. Variation in the environmental favorability for disease outbreaks may be monitored using indices of macroclimatic oscillations. This may provide the basis for an early warning system based on the variation in macroclimatic indices and the locations where human contact with multiple animal species tend to occur. We propose to study the biogeography of zoonoses by: 1) determining the potential spatial distribution of these diseases, according to environmental factors and the biogeographic structure of animals linked to the zoonosis cycle; 2) search for relationships between disease outbreaks and global atmospheric oscillations to forecast periods of higher risk of emergence of the infectious diseases.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Hole concentration in a diluted ferromagnetic semiconductor

We consider a mean-field approach to the hole-mediated ferromagnetism in III-V Mn-based semiconductor compounds to discuss the dependence of the hole density on that of Mn sites in Ga_{1-x}Mn_xAs. The hole concentration, p, as a function of the fraction of Mn sites, x, is parametrized in terms of the product m*J_{pd}^2 (where m* is the hole effective mass and J_{pd} is the Kondo-like hole/local-moment coupling), and the critical temperature Tc. By using experimental data for these quantities, we have established the dependence of the hole concentration with x, which can be associated with the occurrence of a reentrant metal-insulator transition taking place in the hole gas. We also calculate the dependence of the Mn magnetization with x, for different temperatures (T), and found that as T increases, the width of the composition-dependent magnetization decreases drammatically, and that the magnetization maxima also decreases, indicating the need for quality-control of Mn-doping composition in diluted magnetic semiconductor devices.Comment: 4 pages, 3 figures, RevTeX 3; Fig. 1 changed, new references adde

Report Card on Physical Activity for Children and Youth

Background: This article describes the procedures and development of the rst Portuguese Report Card on Physical Activity in Children and Adolescents. Methods: Comprehensive searches for data related to indicators of physical activity (PA) were completed by a committee of physical activity and sports specialists. Grades were assigned to each indicator consistent with the process and methodology outlined by the Active Healthy Kids Canada Report Card model. Results: Nine indicators of PA were graded. The following grades were assigned: Overall Physical Activity Levels, D; Organized Sport Participation, B; Active Play, D; Active Transportation, C; Sedentary Behaviors, D; Family and Peers, C; Schools, B; Community and the Built Environment, D; and Government, C. Conclusions: Portuguese children and adolescents do not reach suf cient physical activity levels and spend larger amounts of time spent in sedentary behaviors compared with recommendations. Effective policies of PA promotion and implementation are needed in different domains of young people’s daily lives

Modulation of charge-density waves by superlattice structures

We discuss the interplay between electronic correlations and an underlying superlattice structure in determining the period of charge density waves (CDW's), by considering a one-dimensional Hubbard model with a repeated (non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix renormalization group diagonalization of finite systems (up to 120 sites) is used to calculate the charge-density correlation function and structure factor in the ground state. The modulation period can still be predicted through effective Fermi wavevectors, k_F*, and densities, and we have found that it is much more sensitive to electron (or hole) doping, both because of the narrow range of densities needed to go from q*=0 to \pi, but also due to sharp 2k_F*-4k_F* transitions; these features render CDW's more versatile for actual applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev