Speaker
Joakim Bergli

Description Coulomb glasses are materials with electron states localized by the disorder under conditions of long-range interactions between their particles. One realization of a Coulomb glass is a doped semiconductor at low temperatures. Such systems are used as low-temperature sensors and detectors. Another example is granular metals. Coulomb glasses show complex dynamics typical of other complex systems: sluggish, non-exponential, relaxation of the conductance as well as aging and memory effects similar to those observed in structural glasses. We report dynamical Monte Carlo simulations of relaxation processes in a Coulomb glass. Both the relaxation to equilibrium following an initial temperature quench and the fluctuations around equilibrium was studied. We find that the autocorrelation function of the energy decays as a streched exponential function indicating constrained dynamics of the system at low temperature. We also see that out of equilibrium there is an effective electron temperature established on a short timescale which slowly relaxes to the bath temperature. We also study the response of the system to an external perturbation and observe how it relaxes after such a perturbation.

Speaker
Roger Guimerà

Description In biological systems, individual components interact with each other giving rise to complex networks, which are neither totally regular nor totally random. Because of the interplay between network topology and dynamics, it is crucial to characterize the structure of these complex biological networks. In my talk, I will discuss how we can use statistical models and computational tools to tackle two very prominent network problems: the problem of data reliability and network discovery, and the problem of extracting meaningful information from network data. I will illustrate the methods with examples from systems biology, including how to identify potential metabolic targets for antibacterial drugs, or how to make sense of the machinery responsible for establishing cell polarity in yeast.

Speaker
Jochen Guck

Description While most current biological research focuses on molecular aspects of cells, we are interested in their global physical – mechanical and optical – properties. The mechanical properties of cells are largely determined by the cytoskeleton, an internal hybrid polymer network. This cytoskeleton evolves during the normal differentiation of cells, is involved in many cellular functions, and is characteristically altered in many diseases, including cancer. We can exploit the deformability of the cytoskeleton as a link between molecular structure and biological function to distinguish between different cells using a microfluidic optical stretcher. We find that optical deformability is sensitive enough to monitor the subtle changes during the progression of cells from normal to cancerous and even metastatic state. We can also distinguish stem cells from more differentiated cells. In both cases, their mechanical properties are tuned to the specific requirements imposed on the cells by physical constraints. Another example for the importance of physics are the optical properties of cells, specifically in the retina. We have recently shown that there are glial cells in the retina that act as optical fibers and that photoreceptor cells even invert their usual nuclear chromatin arrangement to turn them into microlenses. In both cases cells utilize physics in order to improve the light transmission through the retina and help to mitigate the disadvantage of its inverted structure.

Speaker
Joshua Garland

Description Recent work in the field of dynamical systems provides evidence that computer systems are nonlinear deterministic dynamical systems. This implies the existence of a deterministic update rule for the state variables of a running computer, which, in turn, implies the existence of a deterministic forecast rule for those variables. Even a short-term prediction of (computer performance), if accurate, could be effective in tailoring system resources on-the-fly to the dynamics of a computing application. For example, a good prediction of processor load could allow a computer to increase its energy efficiency by scaling back processor voltage and/or frequency. To explore this, I use a custom measurement infrastructure, delay-coordinate embedding, topology-based signal separation, and nonlinear time-series analysis to forecast processor load and cache performance of a variety of C programs running on an Intel Core2 R Duo. Since the dynamics of a computer undergoes bifurcations induced by the code that it is running, these time series data represent a mix of transients and attractors. I model computers as iterated function systems (IFS) with multiple dynamical regimes, then use changes in the continuity of the IFS to detect dynamical shifts and segment the output signal. I will demonstrate all of these modeling and forecasting algorithms on both synthetic data and experimental computer performance data. This methodology has a wide range of potential uses, ranging from change-point detection in time-series data to design improvements in digital communications and computer systems.

Speaker
Salvador Pueyo

Description "Self-organized criticality'" (SOC) is a concept of complex systems theory that has been used to explain the power-law frequency distribution observed in many kinds of catastrophic events. However, SOC is not a necessary condition for power laws. I have been working with empirical data to test the presence of SOC in several biomes, and the nontrivial consequences that it might have for the impact of climate change on fire dynamics. TRANSLATION: "Criticitat autoorganitzada i l'impacte del canvi climàtic en els incendis forestals" "Criticitat autoorganitzada" (SOC) és un concepte de teoria de sistemes complexos que s'ha fet servir per explicar la llei de potència que s'observa en les distribucions de freqüències de molts tipus d'esdeveniments catastròfics. Tanmateix, la SOC no és una condició necessària per a les lleis de potència. He estat treballant amb dades empíriques per posar a prova la presència de SOC en diferents biomes, i les conseqüències no trivials que podria tenir pel que fa a l'impacte del canvi climàtic sobre la dinàmica dels incendis.