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 · 0 Comments · 0 Linkbacks


A full mean field description of the nematic to smectic phase transitions

  • Data: 01/12/14 às 11:00 h
  • Local: Sl A5-01
  • Apresentador: Dora Izzo - Instituto de Física, UFRJ

Resumo: Recent experiments on thermotropic liquid crystals have shown the occurrence of an additional phase transition: the biaxial nematic to smectic-C phase transition. This possibility has not been predicted by theory yet. We consider the traditional model, where the ordering in the liquid crystal is described by a tensorial order parameter associated to the molecule quadrupole moment. The phase behavior of the system is obtained using a mean field approach: we use Landau-de Gennes theory. The free energy is an expansion in terms of the invariants associated to tensorial order parameter. Our study resembles that of Galerne and Marcerou [1] for the nematic phases in a lyotropic liquid crystal. The equations obtained from the mean field theory were minimized analitically leading to closed expressions for the invariants. Nevertheless, to compare the free energies of the different phases, we had to rely on numerical computations. A rich phase diagram was obtained. We observe the occurrence of all phases observed traditionally: isotropic, uniaxial nematic, biaxial nematic, smectic phases of two types, but the novelty is the possibility of the biaxial nematic to smectic transitions.

[1] Y.Galerne and J.P.Marcerou, J. Physique 46,589-594 (1985).

2014/11/18 13:56 · Nuno Crokidakis · 0 Comments

On quasi-stationary states of the conservative dynamics of long-range interacting systems

  • Data: 22/09/14 às 16:00
  • Local: Sl A5-01
  • Apresentador: Ana Carolina Ribeiro Teixeira - UFRGS

Resumo: Long-range interacting (LRI) systems are ubiquitous in nature. They range from the astronomical scale (e.g. self-gravitating systems), to the macroscopic (e.g. non-neutral plasmas, wave-plasma interacting systems, two-dimensional geophysical vortex systems), down to the atomic scale (e.g. classical and quantum cold atoms interacting via quasi-resonant lasers). Despite their importance, much of the behaviour of these systems still remains poorly understood. It is known that LRI systems can exhibit ergodicity-breaking, anomalous relaxation and diffusion, quasi-stationary states (qSS), vanishing Lyapunov exponents, ensemble inequivalence, negative specific heat, temperature discontinuities. Thermodynamic anomalies result from the non-additivity of energy, while dynamical ones arise from the complexity of collisionless relaxation driven by the wave-particle interactions.

In the thermodynamic limit, dynamics is governed by the Vlasov equation, which has an infinity of stationary solutions. Approaching the thermodynamic limit, LRI systems do not relax to equilibrium, but become trapped in qSS, whose life-time diverges with the particle number, whence the importance of studying Vlasov stationary states.

In this talk we will explore the relaxation to qSS of two long-range potential models, namely, the Hamiltonian Mean-Field (HMF) model and gravitation in three dimensions, for some classes of initial conditions (ICs) of interest. In the case of the HMF we show that if the initial distribution satisfies the virial condition, thereby reducing mean-field changes, the final distribution in the qSS can be predicted very accurately using a reduced exactly integrable model. For gravitation in 3d, we have tested spherically symmetric initial conditions satisfying the virial condition and isotropy in velocity. Again, the integrable approximation fits well qSS radial and velocity marginal distributions obtained through molecular dynamics. The distribution functions obtained using this approach are found to be more accurate than the ones predicted by the ergodic Lynden-Bell theory.

For ICs not obeying the virial condition, ergodicity-breaking has been shown to happen through parametric resonances between certain particles and the strong mean-field oscillations felt by particles. This mechanism is analogous to the nonlinear Landau damping seen in particle-wave interactions in plasmas. For these ICs Lynden-Bell distributions do not coincide with qSS distributions obtained through molecular dynamics. The present work goes a step beyond, showing that the ergodicity hypothesis is neither applicable to ICs obeying the virial condition, since an exactly integrable model provides a better approximation to the aforementioned distributions.

* Ana C. Ribeiro-Teixeira, Fernanda P. C. Benetti, Renato Pakter, Yan Levin, Phys. Rev. E 89 (2014) 022130

* Fernanda P. C. Benetti, Ana C. Ribeiro-Teixeira, Renato Pakter, Yan Levin, Phys. Rev. Lett. 113 (2014) 100602.

2014/09/19 13:45 · Nuno Crokidakis · 0 Comments




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