talks #research groups

Co-existing mesoscale patterns in bipartite networks: modularity, nestedness, in-block nestedness

talk picture

Aula 3.20. Departament Fisica de la Materia Condensada, Facultat de Fisica UB 2017-11-29 12:00:00

Speaker: Javier Borge-Holthofer

Abstract: The identification of mesoscale connectivity patterns in complex networks has been central to the development of the field. Besides an interest in the methodological challenges, these patterns matter to the community inasmuch they result from a complex structure-dynamics interactions. It is in this context –network architecture as emergent phenomena– that nestedness and modularity arise as prominent macrostructural signatures to study. Furthermore, their prevalence in many natural and socio-technical systems has spurred research on the (possible) co-existence of both features. Here we will focus on particular socio-technical settings in which modularity and nestedness are observed together, and discuss some possible explanations and methodological problems. Then, we will present a brand new formulation of the problem where nestedness and modularity can coexist in the form of nested blocks within the network. Finally, we will discuss possible directions from here.

2d active dumbbell model: Diffusive behavior and aggregation phenomena

Aula seminari 3.20 2017-11-24 12:00:00

Pascuale DiGregorio
Department of Condensed Matter Physics, University of Barcelona.

Active matter is constituted by self-propelled units that extract
energy from internal sources or its surroundings, and are also in
contact with an environment which allows for both dissipation and
thermal fluctuations. The locally gained energy is partially converted
into work and partially dissipated into the bath. Due to this energy
consumption detailed balance is broken in active matter, pushing these
systems out of thermodynamic equilibrium.
Our active matter model is a 2d Active Brownian Particle model in
which the constituents are dumbbells, simplified model for diatomic
molecules, interacting with each other by means of a short-range
purely repulsive WCA potential.
We have studied the mechanism for melting of our system in the passive
limit and we found a non-trivial KTHN-like scenario with a first-order
phase transition, marked by a region of co-existence between
disordered liquid/gas regions and regions with hexatic order.
Moreover, we have extended the analysis to the active case, and we
have found co-existence over a finite interval of packing fractions
for each value of activity. We didn’t find no discontinuous behavior
upon increasing activity from the passive limit.

Active spinner materials

Room Pere Pascual, Departament Física de la Matèria Condensada (Planta 5, Facultat de Física, Universitat de Barcelona, c/ Martí i Franqués 1) 2017-06-29 10:30:00

Strongly interacting colloids driven out-of-equilibrium by an external periodic forcing often develop nontrivial collective dynamics. Active magnetic colloids proved to be excellent model experimental systems to explore emergent behavior and active (out-of-equilibrium) self-assembly phenomena. While colloidal systems are relatively simple, understanding their collective response, especially in out of equilibrium conditions, remains elusive. 
Ferromagnetic micro-particles immersed in water and sediment on the bottom surface of the flat cell are energized by a single-axis homogeneous alternating magnetic field applied perpendicular to the surface supporting the particles. Upon application of the alternating magnetic field the magnetic torque on each particle is transferred to the mechanical torque giving rise to a rolling motion of the particle in a certain range of excitation parameters. 
Experiments reveal a rich collective dynamics of magnetic rollers. Flocking and spontaneous formation of steady vortex motion have been observed. The effects are fine-tuned and controlled by the parameters of the driving magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms leading to the emergence of largescale collective motion: spontaneous symmetry breaking of the clock/counterclockwise particle rotation, 
collisional alignment of particle velocities, and random particle re-orientations due to shape imperfections. 
Ferromagnetic micro-particles, suspended at a liquid interface and energized 
by a rotational homogeneous alternating magnetic field applied along the supporting interface, spontaneously form ensembles of synchronized self-assembled spinners with well-defined characteristic length. The size and the torque of an individual self-assembled spinner are controlled by the frequency of the driving magnetic field. Experiments reveal nontrivial collective dynamics in large ensembles of synchronized magnetic spinners that can spontaneously form dynamic spinner lattices at the interface in a certain range of the excitation parameters. Unusual dynamics inside of the formed spinner lattices is observed. Transport of passive cargo particles in a gas of spinners and structure of the underlying self-induced surface flows is analyzed. Active turbulent behavior of induced flows is reported.

Self-organization and criticality in martensite

Facultat de Física, Aula Pere Pascual, planta 6 2017-06-08 12:00:00

A martensitic phase-transformation is a first-order diffusionless transition occurring in elastic crystals and characterized by an abrupt change of shape of the underlying crystal lattice. It is the basic activation mechanism for the Shape-Memory effect. In this talk we present a probabilistic model for the description of martensitic microstructure as an avalanche process. Our approach to the analysis of the model is based on an associated general branching random walk process. Comparisons are reported for numerical and analytical solutions and experimental observations.

Deliberate exotic magnetism via frustration and topology

Aula Eduard Fontseré - Facultat de Física UB 2017-06-01 14:00:00

So called "Artificial Spin Ices" are two dimensional arrays of magnetic, interacting nanostructures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice, rare heart pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain understanding of degenerate ensembles. With the evolution of nano fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials. We will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe directions, which includes the design of desired topologically protected states and their implications to kinetics.