Events: Interesting happenings in the next 4 weeks.

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September 2018

Thu., 13.

Doctoral thesis presentation

Colloidal suspensions driven out of the equilibrium

Alberto SCACCHI, 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

For a system of Brownian particles interacting via a soft exponential potential we investigate the interaction between equilibrium crystallization and spatially varying shear flow. For thermodynamic state points within the liquid part of the phase diagram, but close to the crystallization phase boundary, we observe that imposing a Poiseuille fl ow can induce non-equilibrium crystalline ordering in regions of low shear gradient. The physical mechanism responsible for this phenomenon is shear induced particle migration, which causes particles to drift preferentially towards the center of the channel, thus increasing the local density in the channel center. The method employed is classical dynamical density functional theory.

Wed., 19.

Colloquium Physics

Laser driven phase transitions, topology and black holes

Prof. Takashi OKA, Max Planck Institute for the Physics of Complex Systems, Dresden

Time: 17:15 - 18:15h

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

Materials driven out of equilibrium by lasers and other dynamical fields provide an exciting research frontier in condensed mater physics. Realization of new quantum states including topological bands is possible with the help of Floquet engineering, i.e., control of quantum states by time periodical external fields [1]. Less understood is how strongly interacting systems such as correlated electronic materials behave in strong fields. In systems coupled to baths, heating and relaxation may balance and interesting nonequilibrium steady states can be realized. One approach to study this problem is to employ a quantum field theory that has a holographic dual. The D3/D7 model is a string theory derived system which describes three dimensional Dirac fermions interacting with a Coulomb force mediated by nonlinear bosons (SUSY QCD). This model shows many features similar to Dirac materials that can be examined in condensed matter experiments. When the system is driven by rotating electric fields, we can use a simple theoretical trick to map it to a static system [1,2] in the dual model. An effective black hole is formed that signals a transition from an insulator to a metal. It is intriguing to compare the results with those directly obtained from numerical approaches [4]. References 1. T. Oka, S. Kitamura, (Floquet Engineering of Quantum Materials), arXiv:1804.03212 2. K. Hashimoto, S. Kinoshita, K. Murata, T. Oka, (Holographic Floquet states I: a strongly coupled Weyl semimetal), JHEP 5, 127 (2017). 3. S. Kinoshita, K. Murata, T. Oka, (Floquet condensation of vector mesons in nonequilibrium phase diagram), JHEP 6, 96 (2018). 4. Y. Murakami, P. Werner, (Nonequilibrium steady states of electric field driven Mott insulators), Phys. Rev. B 98, 075102 (2018)

Fri., 21.

Seminar

Novel two-dimensional electron systems at the surface of functional oxides

Andres Santander-Syro, CSNSM, Universite Paris-Sud, Orsay, France

Time: 10:00 - 11:00h

Location: UNI-Perolles, Department of Physics, building 8, auditorium 0.58.5, Chemin du Musee 3, 1700 Fribourg

 Les oxydes de métaux de transition (TMO) présentent des propriétés remarquables, que l’on ne trouve pas dans les semi-conducteurs standard, tels que la supraconductivité à haute température, la capacité photocatalytique ou les transitions métal sur isolant. La réalisation de gaz d'électrons bidimensionnels (2DEG) dans les TMO est cruciale pour exploiter les fonctionnalités de ces matériaux pour des applications futures. De plus, ces 2DEG offrent la possibilité d’explorer de nouvelles physiques issues des effets combinés des corrélations d’électrons et du confinement à faible dimension.

Nous avons constaté que les 2DEG peuvent être simplement réalisés à la surface de divers TMO isolants, tels que le paraélectrique quantique SrTiO 3  [1], le fort couplage spin-orbite KTaO 3 [2] ou le photocatalyseur TiO 2 [3] . Je montrerai comment le choix de la terminaison de surface permet d'adapter la structure électronique et les symétries de ces 2DEG [4-5], ouvrant la voie à la recherche d'états topologiques dans les oxydes corrélés. En outre, je discuterai de notre observation du magnétisme dans le 2DEG à la surface déficiente en oxygène de SrTiO 3  [6]. Je décrirai ensuite notre récente mise au point d’une méthode simple pour fabriquer ces 2DEGs dans plusieurs autres oxydes, tels que le BaTiO 3 ferroélectrique. [7] ou le semi-conducteur ZnO, qui se révèle être un cas d'école d'un liquide de Fermi 2D couplé à une distribution de Debye de phonons [8]. Cette nouvelle technique de fabrication permet de réaliser des interfaces fonctionnelles entre les couches magnétiques 2DEG et, par exemple , accordables [9], et de mesurer leurs caractéristiques de transport [10] - et est donc prometteuse pour les applications.

Sat., 25.08.2018 - Fri., 21.09.2018

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