Bem vindo ao site do Grupo de Sistemas Complexos do Instituto de Física da Universidade Federal Fluminense. O grupo de Sistemas Complexos do Instituto de Física da UFF já tem tradição como um dos mais importantes em simulações de sistemas estatísticos, no Brasil e no exterior, tendo já formado vários pesquisadores que ocupam posições em várias instituições de pesquisa no país. O caráter interdisciplinar do grupo é um fator de atração de estudantes, tanto do Brasil quanto do exterior, em especial da América do Sul. Já há vários anos que o grupo se reúne semanalmente para apresentação e discussão do andamento de seus vários projetos de pesquisa.


Novo Doutor

O estudante de doutorado Angelo Mondaini Calvão defendeu sua tese intitulada “Estudos de Sistemas Dinâmicos não lineares: Pêndulo Duplo, Batimentos Cardíacos e Coletivos de Animais”. A defesa ocorreu no prédio do Instituto de Física, Campus da Praia Vermelha, no último dia 19/02/2014. O orientador foi o Professor Thadeu Penna. Parabéns Angelo!

2014/02/24 10:00 · Nuno Crokidakis · 7 Comentários · 0 Linkbacks


Dipolar interactions and critical behavior in two dimensional ferromagnets: an avoided phase transition scenario

  • Data: 03/07/17 às 16:00 h
  • Local: Sl A5-01
  • Apresentador: Sergio A. Cannas - Universidad Nacional de Córdoba, Argentina

Resumo: The paramagnetic-to-ferromagnetic phase transition is classified as a critical phenomenon due to the power-law behavior shown by thermodynamic observables when the Curie point is approached. In this work we report the observation of such behavior over many decades of suitable scaling variables in ultrathin Fe films, for certain ranges of temperature T and applied field B. Moreover, both scaling functions and critical exponents agree with those corresponding to the two dimensional Ising model (Onsager). This despite the fact that the underlying critical point is completely unreachable because protected by a phase with a modulated domain structure, induced by dipole-dipole interactions. Monte Carlo simulations on a two dimensional Ising model with both ferromagnetic exchange interactions and dipolar interactions support the experimental results, showing the same scaling behavior associated with a temperature Tc that converges to the Onsager value in the limit of dipolar interactions going to zero. In both cases (experiments and simulations) the modulated phase is realized in a portion of the (T,B) plane that extends above the putative critical temperature, where the themodynamic quantities show no singularity. We show that scaling of the thermodynamic observables with the dipolar coupling allows to intrepret such avoided criticality scenario as a crossover phenomenon between a short range (i.e., exchange) dominated regime to a dipolar (i.e., with modulated domains) dominated one. Such scaling hipotesis is supported by Mean Field, Real Space Renormalization Group and Monte Carlo calculations.

A networked voting rule for democratic representation

  • Data: 19/06/17 às 16:00 h
  • Local: Sl 201
  • Apresentador: Alexis Hernández - UFRJ

Resumo: We introduce a general framework for exploring the problem of selecting a committee of representatives with the aim of studying a networked voting rule based on a decentralized large scale platform, which can assure a strong accountability of the elected. The results of our simulations suggest that this algorithm-based approach is able to obtain a high representativeness for relatively small committees, performing even better than a classical voting rule based on a closed list of candidates. We show that a general relation between committee size and representatives exists in the form of an inverse square root law and that the normalized committee size approximately scales with the inverse of the community size, allowing the scalability to very large populations. These findings are not strongly influenced by the different networks used to describe the individuals interactions, except for the presence of few individuals with very high connectivity which can have a marginal negative effect in the committee selection process.




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