Events: Interesting happenings in the next 4 weeks.

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October 2017

Wed., 25.

Colloquium Physics

Exactly solvable models for light-matter interaction

Prof. Daniel Braak, University of Augsburg

Time: 17:15 - 18:15h

Location: UNU-Perolles, Physics Department, building 8, auditorium 0.51, Chemin du Musee 3, 1700 Fribourg

I shall review developments in the theory of elementary models to describe the interaction of light (bosons) with matter (fermions), which have been motivated by the recent dramatic progress towards experimental realization of the so-called deep strong coupling region with possible applications to quantum computing. Relations with the classical theory of analytic functions and the role of discrete and continuous symmetries are especially emphasized.

Thu., 26.

Master thesis presentation

Finite size effects in time-domain THz ellipsometry

Vjollca ZENKO,Physics Departement of the University of Fribourg

Time: 16:15 - 17:15h

Location: Uni-Perolles, Physics Department, building 8, auditoire 0.51, Chemin du Musee 3, 1700 Fribourg Switzerland

My Master thesis reports scattering effects observed on small transparent samples of acrylic and LSAT using time-domain terahertz rotating analyzer ellipsometer (TD THz RAE). Studies in diffraction with small samples has been done for conducting materials, using conventional ellipsometry, that is with continuous wave in visible and infrared range. Also, theoretical results for conducting material were found regarding the diffraction effects in the spectrum of the ellipsometric angles psi and delta. I extend the subject to pulsed radiation in the THz range and dielectric materials with low and high index of refraction. The focus of my experimental work, using TD THz RAE, was to scan THz pulse profiles in time-domain and investigating about the origin of some unexpected pulses appearing in my scans. Eventually, I found out that these parasitic pulses were results due to the diffraction with the edge of my sample (diffracted pulses), or pulses which propagate inside the material from a side face to the other (side-face pulses). I supported my hypothesis of the side-face effect with a geometrical model which enabled me to calculated the time delay of a side-face pulse and my theoretical data matched well the experimental ones. For the diffracted pulse with the edge, I simply considered, in the theory of diffraction, the shadow zone which is illuminated by radiation which undergoes diffraction with edge. As conclusion, I ran a simulation which showed the interaction of a pulse with a rectangular sample, and the out coming confirmed my assumptions.

November 2017

Wed., 08.

Colloquium Physics

Thermal convection under our feet and beyond

Prof. Jun Zhang, Physics and Mathematics, Courant Institute, New York University

Time: 17:15 - 18:15h

Location: UNI-Perolles, Physics Department, building 8, auditorium 0.51, Chemin du Musee 3, 1700 Fribourg, Switzerland

This talk is about interdisciplinary fluid dynamics that is based on lab experiments as well as terrestrial and celestial observations. Thermal convection is ubiquitous in nature and can be found in everyday life. This subject has been studied by scientists and engineers for many decades, for its rich dynamics and vast applications. In this talk, I will first discuss an experiment as a free-moving floating boundary interacts with a fluid, which is heated from under and cooled from above. The top boundary is mobile and thermally opaque (poor conductor), causing the coupled system to oscillate. The underlying mechanism is similar to what has been powering the geophysical process of continental drift, as continents interact with the convective mantle of the earth. In the second experiment, a few seemingly impeding partitions or dividers are inserted into a convective fluid, but the heat-flux that passes through is found to be boosted by several times. Theses results are explained and some new directions and phenomena (include the recent total solar eclipse) that extend the classical picture of thermal convection are also discussed.

Thu., 09.

Doctoral thesis presentation

Super-cooled and glassy states of charged microgel systems in the weak screesing regime

Francesco NAZZANI, Physics Department of the University of Fribourg

Time: 10:15 - 11:15h

Location: UNI-Perolles, Physics Department, building 8, auditorium 0.51, Chemin du Musee 3, 1700 Fribourg

The fragility of a glass system describes how its dynamics evolves with temperature. It is a fundamental parameter for the technological application of molecular glass formers, and a critical property for the engineering properties of the material. For some time now colloidal systems has been shown to be important model systems for glass transition. However hard spheres, that interact just via excluded volume, show only critical-like increase of the relaxation time, a behaviour shared only by the so called "fragile" molecular glass formers. In this work we instead look with light scattering techniques at charged colloids, characterized by long-range interactions, and show how this particular type of interaction lead to an exponential increase of the relaxation time with concentration, akin to "strong" glass formers.

Thu., 09.

Doctoral thesis presentation

Laboratory based XAS measurements, Electron-induced double K-shell ionization of Sc, Cr and Cu and Chemical sensitivity of photoinduced Kbeta1,3 hypersatellite transitions in Cr compounds

Faisal ZEESHAN, Physics Department of the University of Fribourg

Time: 17:00 - 18:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 0.51, Chemin du Musee 3, 1700 Fribourg, Switzerland

The thesis contains three different projects belonging to the domain of X-ray spectroscopy. The first one concerns the development of a laboratory-based setup for in-house X-ray absorption spectroscopy (XAS) measurements. The setup is based on the von Hamos crystal spectrometer of Fribourg using X-ray tubes as radiation sources. The new setup was characterized by measuring the XAS spectra around the K and L edges of several 3d, 4d and 5d transition metals. The edges energies, spectral shapes and absorption coefficients were determined from the data analysis and compared to existing experimental and theoretical data. In the second project the double K-shell ionization of Sc, Cr and Cu induced by electron impact was investigated. The project was also carried out in-house with the von Hamos crystal spectrometer operated in the direct geometry, using the anodes of Sc, Cr and Cu X-ray tubes as targets. The energies and natural line widths of the hypersatellites were determined. The ratios PKK of the double-to-single K-shell ionization cross sections were deduced from the measured relative intensities of the hypersatellites and the results were compared to the PKK values obtained by other groups with electrons, photons and via the nuclear electron capture process. In the third project, the influence of the chemical environment on the Kbeta1,3 hypersatellite transitions was investigated for the first time. The measurements were performed at SSRL using metallic Cr and three Cr compounds corresponding to different oxidation states. For comparison, the diagram Kbeta1,3 and valence-to-core (VtC) Kbeta2,5 transitions were also measured. Relevant dependencies of the energies, energy shifts, relative spectral intensities and line widths on the oxidation state, metal-ligand bond length or spin state could be demonstrated. The observed trends were compared to the ones of the diagram and VtC transitions.

Fri., 17.

Doctoral thesis presentation

Impact of Interaction Topology in Complex Networks

Zhuoming REN, Physics Department of the University of Fribourg

Time: 16:00 - 17:00h

Location: UNI-Perolles, Physics Department, buiding 8, auditorium 0.51, Chemin du Musee 3, 1700 Fribourg, Switzerland

Interaction in complex networks occurs everywhere, especially in man-made systems and in human social systems. For example, online social networks are organized around participating users who create interaction with who they associate in which people have been integrated to make friends, communicate with friends, share interests, spread ideas and so on. Meanwhile, the economic interaction between two countries and extend to do the same for all countries in the world, which can outline the level of economic growth and the country's productive structure. The main goal of this presentation is to introduce an interesting topic in my thesis : Nestedness: A new framework of the economy complexity.

Mon., 23.10.2017 - Sun., 19.11.2017

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