Disordered two-dimensional superconductors: roles of temperature and interaction strength

We have considered the half-filled disordered attractive Hubbard model on a square lattice, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. Through Quantum Monte Carlo (QMC) simulations for several values of $f$ and $U$, and for system sizes ranging from $8\times 8$ to $16\times 16$, we have calculated the configurational averages of the equal-time pair structure factor $P_s$, and, for a more restricted set of variables, the helicity modulus, $\rho_s$, as functions of temperature. Two finite-size scaling {\it ansatze} for $P_s$ have been used, one for zero-temperature and the other for finite temperatures. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to U=4 (in units of the hopping energy). Also, the normalized zero-temperature gap as a function of $f$ shows a maximum near $f\sim 0.07$, for $2\lesssim U\lesssim 6$. Analyses of the helicity modulus and of the pair structure factor led to the determination of the critical temperature as a function of $f$, for $U=3,$ 4 and 6: they also show maxima near $f\sim 0.07$, with the highest $T_c$ increasing with $U$ in this range. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to "push" electrons towards attractive sites, the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe

Siscero - Sistema de clipping da Embrapa Rondônia.

O Siscero, ou Sistema de clipping da Embrapa Rondônia, é uma aplicação desenvolvida em plataforma web para o registro de matérias jornalísticas ou outras inserções na mídia que envolvam a Embrapa Rondônia. As informações são armazenadas em banco de dados relacional e o sistema permite o registro de arquivos digitais, como imagem, PDF, áudio ou vídeo. Uma interface de pesquisa oferece uma série de filtros para busca de registros e gráficos interativos permitem avaliar, comparar e monitorar o desempenho da Empresa na mídia. Este documento contextualiza o clipping no universo corporativo, faz uma retrospectiva das iniciativas passadas e da situação atual do clipping realizado pela Embrapa e apresenta o Siscero como uma ferramenta estratégica. Um manual para utilização do sistema e um guia de instalação também fazem parte do conteúdo.bitstream/item/35944/1/doc137-siscero.pd

Immunization and Aging: a Learning Process in the Immune Network

The immune system can be thought as a complex network of different interacting elements. A cellular automaton, defined in shape-space, was recently shown to exhibit self-regulation and complex behavior and is, therefore, a good candidate to model the immune system. Using this model to simulate a real immune system we find good agreement with recent experiments on mice. The model exhibits the experimentally observed refractory behavior of the immune system under multiple antigen presentations as well as loss of its plasticity caused by aging.Comment: 4 latex pages, 3 postscript figures attached. To be published in Physical Review Letters (Tentatively scheduled for 5th Oct. issue