Room Auditorium 00:00 11:00 Coffee, Tea and more TBA
Quantitative measurements of modulus and viscoelastic properties on the nanoscale: Challenges and new techniquesDr. Dalia Yablon
Room Room 404 11:00
Atomically controlled growth of functional, technologically important semiconductor systemsProf. Ana G Silva
Room Room 404 11:00
special faculty seminarProf. Roee Ozeri
Room Auditorium 11:15 11:00 REFRESHMENTS In this seminar I’ll review the rationale behind, and the essence of, the redefinition of the standard international systems of units that happened on May 20th 2019. I’ll review some of the possible experiments with which standard units can be realized for empirical references. .
Imaging the human brain: ultra-high field MRI and new biomarkersRita Schmidt
Room Drory Auditorium 13:00 Times New Roman (Headings CS)
Excitons in Flatland: Exploring and Manipulating Many-body Effects on the Optical Excitations in Quasi-2D MaterialsDr. Diana Qiu
Room Room 404 11:00 Since the isolation of graphene in 2004, atomically-thin quasi-two-dimensional (quasi-2D) materials have proven to be an exciting platform for both applications in novel devices and exploring fundamental phenomena arising in low dimensions. This interesting low-dimensional behavior is a consequence of the combined effects of quantum confinement and stronger electron-electron correlations due to reduced screening. In this talk, I will discuss how the optical excitations (excitons) in quasi-2D materials, such as monolayer transition metal dichalcogenides and few-layer black phosphorus, differ from typical bulk materials. In particular, quasi-2D materials are host to a wide-variety of strongly-bound excitons with unusual excitation spectra and massless dispersion. The presence of these excitons can greatly enhance both linear and nonlinear response compared to bulk materials, making them ideal candidates for optoelectronics and energy applications. Moreover, due to enhanced correlations and environmental sensitivity, the electronic and optical properties of these materials can be easily tuned. I will discuss how substrate engineering, stacking of different layers, and the introduction or removal of defects can be used to tune the band gaps and optical selection rules in quasi-2D materials.
Period doubling as an early warning signal for desertificationOmer Tzuk
Room Room A 14:15 The predictions for a warmer and drier climate and for increased likelihood of climate extremes raise high concerns about the possible collapse of dryland ecosystems, and about the formation of new drylands where native species are less tolerant to water stress. Using a dryland-vegetation model for plant species that display different tradeoffs between fast growth and tolerance to droughts, we find that ecosystems subjected to strong seasonal variability, typical for drylands, exhibit a period-doubling route to chaos that results in early collapse to bare soil. We further find that fast-growing plants go through period doubling sooner and span wider chaotic ranges than stress-tolerant plants. We propose the detection of period-doubling signatures in power spectra as early indicators of ecosystem collapse that outperform existing indicators in their ability to warn against climate extremes and capture the heightened vulnerability of newly-formed drylands.
Growth dynamics and complexity of national economies in theA.L. Stella
Room Room A 14:15 Methods of statistical physics allow to explore the quantitative nexus among economic growth of a country, diversity of its productions, and evolution in time of its export basket(*). A stochastic model of evolution, calibrated on data for 1238 exports from 223 countries in 21 years, enables counterfactual analyses based on estimates of the part of growth due to resource transfers between different productions. Original use of the Boltzmann-Shannon entropy function leads to the construction of consistent measures of the efficiency of national economies and of the specialization of productions. Comparisons with dynamical and GDP pc data lead to clear distinctions among developed, developing, underdeveloped and risky countries. Perspective applications of the entropic measures in other fields (ecology, microbiology,..) where diversity has to be estimated from bipartite networks will be shortly outlined. (Work in collaboration with G. Teza, University of Padova, and M. Caraglio, Katholieke Universiteit Leuven.)
Emergence and stability of a Brownian motorAlex Feigel
Room Room A 14:15 A Brownian motor rectifies thermal noise and creates useful work. Here we address how this machine can emerge without predefined energy minimum in a system out of thermal equilibrium. Intuitively, Brownian motor as any artificial or biological machine should degrade with time. I will show that on contrary, a system with multiple degrees of freedom out of thermal equilibrium can be stable at a state that generates useful work. It is demonstrated with the help of ab initio analysis of a modified Feynman-Smoluchowski ratchet with two degrees of freedom. Out of thermal equilibrium, an environment imposes effective mechanical forces on nano-fabricated devices as well as on microscopic chemical or biological systems. Thus out of thermal equilibrium environment can enforce a specific steady state on the system by creating effective potentials in otherwise homogeneous configuration space. I present an ab initio path from the elastic scattering of a single gas particle by a mechanical system to the transition rate probability between the states of the system with multiple degrees of freedom, together with the corresponding Masters-Boltzmann equation and the average velocities of the system’s degrees of freedom as functions of the macroscopic parameters of the out-of-equilibrium environment. It results in Onsager relations that include the influence of the different degrees of freedom on each other. An interesting finding is that some of these forces persist even in a single temperature environment if the thermodynamic limit does not hold. In addition, the spatial asymmetry of the system’s stable state, together with the corresponding directed motion, may possess preferred chiral symmetry.
Geometry, defects and motion in active matterLuca Giomi
Room Drory Auditorium 13:00 The paradigm of “active matter” has had notable successes over the past decade in describing self-organization in a surprisingly broad class of biological and bio-inspired systems: from flocks of starlings to robots, down to bacterial colonies, motile colloids and the cell cytoskeleton. Active systems are generic non-equilibrium assemblies of anisotropic components that are able to convert stored or ambient energy into motion. In this talk, I will discuss some recent theoretical and experimental work on active nematic liquid crystals confined on two-dimensional curved interfaces and highlight how the geometrical and topological structure of the environment can substantially affect collective motion in active materials, leading to spectacular life-like functionalities.