# Vienna Theory Lunch Seminar

by Sabine Ertl, Andreas Gabriel, Patrick Ludl and Nils-Ole Walliser

Tuesdays 12:15-13:30

held alternatively at:

TU Wien ( Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room E136 )

Uni Wien ( Boltzmanngasse 5, 5th floor, Schrödinger HS / large seminar room )

 Daniel Grumiller

 Idee:   Idea: Wie auf vielen Universitäten praktiziert wollen wir ein Lunch-Seminar etablieren, das aktuelle Themen der Theoretischen Physik, die von DiplomandInnen, DoktorandInnen und PostDocs behandelt werden, aufgreift. Das Niveau soll so gewählt werden, dass jeder Student und jede Studentin am Ende des Studiums dem Vortrag folgen kann. Die Vortragenden werden ermutigt keinen "perfekten" Vortrag zu halten, und sollen hauptsächlich Ihre Motivation, warum sie dieses Thema gewählt haben, wiedergeben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden. Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza, Sushi) gratis zur Verfügung gestellt. We want to establish a lunch seminar as practiced at other universities. The focus is on recent theoretical research done by Master students, PhDs and PostDocs. The seminar is designed for graduate students but should also be comprehensible to advanced undergraduate students. Pre-diploma students are particularly encouraged to attend so that they receive an overview of research activities conducted at both universities. Speakers are also encouraged to focus on their motivation for choosing their particular topic and to present open questions. In order to avoid any "loss of time" we provide a free lunch (pizza, sushi). Wie kann ich teilnehmen? How can I join? Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailinglist eintragen oder Mail an dburke at hep.itp.tuwien.ac.at oder an christoph.spengler at univie.ac.at Just attend! To receive informations via email go to the Mailinglist or drop an email to dburke at hep.itp.tuwien.ac.at or christoph.spengler at univie.ac.at

TBA

Abstract: TBA

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 8 Mar 2011 TU Wien Florian Preis (TU Wien) Inverse magnetic catalysis in dense holographic matter Abstract: We study the chiral phase transition in a magnetic field at finite temperature and chemical potential within the Sakai-Sugimoto model, a holographic top-down approach to QCD with a large number of colors. We consider the limit of a small separation of the flavor D8-branes, which corresponds to a dual field theory comparable to a Nambu-Jona Lasinio (NJL) model. Mapping out the surface of the chiral phase transition in the parameter space of magnetic field strength, quark chemical potential, and temperature, we find that for small temperatures the addition of a magnetic field decreases the critical chemical potential for chiral symmetry restoration -- in contrast to the case of vanishing chemical potential where, in accordance with the familiar phenomenon of magnetic catalysis, the magnetic field favors the chirally broken phase. This "inverse magnetic catalysis" (IMC) appears to be associated with a previously found magnetic phase transition within the chirally symmetric phase that shows an intriguing similarity to a transition into the lowest Landau level. We estimate IMC to persist up to $10^{19} G$ at low temperatures. 15 Mar 2011 Uni Wien Pascal Anastasopoulos (TU Wien) Living in a world of strings Abstract: We will review the basic ideas on how the Standard Model can be realized as an Open String vacuum and we will analyze some phenomenological implications of such realization. 22 Mar 2011 TU Wien Paul Erker (Uni Wien) Genuine Multipartite Entanglement in Dicke States Abstract: Genuine multipartite entanglement has witnessed serious attention within the scientific community recently. It was found to play a role in various fields, from solid state phase transitions to biological systems, and to be an important resource for e.g. quantum computation or quantum secret sharing. Dicke states represent an important class of multipartite quantum states, which e.g. appear as ground states of Hamiltoneans in various systems. Criteria to detect genuine multipartite entanglement in such states were recently developed and will be presented. 29 Mar 2011 Uni Wien Thomas Zojer (TU Wien) Three dimensional gravity and logarithmic conformal field theories Abstract: Three-dimensional massive gravity has been extensively studied in the last years as a toy model for quantum gravity. It is simple enough to find 'solutions', e.g. propagating degrees of freedom, gravitons, or other interesting ingredients like black holes. An important tool is the gauge gravity duality, or AdS/CFT correspondence, which in the context of three-dimensional quantum gravity is easily tractable since a lot is known about two dimensional CFTs. I am going to address special deformations of gravity theories and discuss the respective changes on the CFT side that lead to the conjecture of AdS/LCFT, i.e. some gravity theories are dual to so-called logarithmic conformal field theories. 5 Apr 2011 TU Wien Ivan Coluzza (Uni Wien) A coarse-grained approach to protein design: learning from design to understand folding Abstract: Computational studies have given a great contribution in building our current understanding of the complex behavior of protein molecules; nevertheless, a complete characterization of their free energy landscape still represents a major challenge. Here, we introduce a new coarse-grained approach that allows for an extensive sampling of the conformational space of a large number of sequences. We explicitly discuss its application in protein design, and by studying four representative proteins, we show that  the method generates sequences with a relatively smooth free energy surface directed towards the target structures. 12 Apr 2011 Uni Wien Harald Oberhofer (TU München) Predicting electron transfer rates (with computer simulations) Abstract: We show the feasibility of using computer simulations to calculate electron exchange reaction rates in extended systems from scratch. We achieve this by determining the dependence of all parameters occurring in the Marcus theory rate expression on the distance $r$ of Donor and Acceptor. Using both, classical and density functional theory (DFT) simulation techniques we calculate the reorganisation energy $\lambda(r)$ and the electronic transition matrix element $H_\text{ab}(r)$. Together with the potential of mean force $G(r)$ as a function $r$ this allows us to calculate the overall reaction rate. We also take into account quantum corrections due to the classical nature of the vibrational modes in our model. We illustrate this approach with our results for the Ru$^{2+}$ - Ru$^{3+}$ electron self-exchange reaction in water, which are in good agreement with experiment. Also we show preliminary results for a large modified-fullerene cluster, a system of high relevance to modern organic solar cells. 3 Mai 2011 TU Wien Denise Lorenz (Uni Wien) LPVs in NGC147 and NGC185 - Probing a new tool to measure extragalctic distances Abstract: Most of the variable stars change their brightness periodically and can therefore be classified according to their periods and amplitudes. Of special interest are variables that increase their mean luminosity according to the length of their period, as has been found for Cepheid stars. They can be used to establish a period-luminosity-relation (PLR) which enables us to determine the distance just by measuring the period of the star. Huge surveys of variable stars in the Magellanic Clouds - the neighbourhood of the Milky Way - revealed sequences of Long Period Variables (LPVs) in a period-luminosity-diagram. These findings encouraged the search of a PLR valid for LPVs and the investigation of its universality. LPVs are intrinsically brighter than Cepheid variables, hence, at greater distances LPVs are still observable. Therefore, a universal LPV-PLR would allow to measure distances of all stellar systems in which we are able to detect LPVs. Meanwhile, technical progress allows to resolve single stars in galaxies not only in the surroundings of the Milky Way but also at a distance of the Andromeda Galaxy and beyond. Our work describes the search of LPVs in the two neighbouring dwarf galaxies of Andormeda, NGC147 and NGC185, and their use in order to establish a universal PLR. 10 Mai 2010 Uni Wien Florian Hebenstreit (Uni Graz) Quantum kinetics of non-perturbative electron-positron production - The influence of temporal and spatial inhomogeneities Abstract: Non-perturbative electron-positron pair creation in electric fields (Schwinger effect) has been a long-standing but still unobserved prediction of QED. Due to the advent of a new generation of high-intensity laser systems such as the European XFEL or the Extreme Light Infrastructure (ELI) it might, however, become possible to observe this effect within the next decades. Previous investigations led to a good understanding of the general mechanisms behind the pair creation process, however, realistic electric fields as they might be present in upcoming experiments have not been fully considered yet. In this talk I focus on various aspects of the Schwinger effect in the presence of inhomogeneous electric fields: First, I consider the pair creation process in the presence of a spatially homogeneous, time-dependent electric field. Most notably, the momentum distribution of created particles in the presence of a  pulsed electric field with sub-cycle structure, which serves as a simple model of the time-dependence of a realistic laser pulse, is presented. Moreover, I introduce a formalism by means of which the Schwinger effect in the presence of space- and time-dependent electric fields can be treated properly. Finally, I present the time evolution of various observable quantities (charge distribution, momentum spectrum, number of created particles) in the presence of a simple space- and time-dependent electric field which have been calculated for the first time. 17 Mai 2011 TU Wien Michal Michalčík (TU Wien) Stringy effects in the AdS/CFT correspondence Abstract: Even though the AdS/CFT correspondence is originally formulated via string theory, most of its aspects were investigated in the string theory low energy limit - the Supergravity. It is therefore of interrest to look for stringy effects. There are two essential ingredients to describe a conformal field theory : the spectrum of conformal weights and coefficients of the three-point functions of primary operators. The latter one causes more problems in the string context, since the computational methods based on the AdS/CFT correspondence are (mostly) restricted to the operators dual to supergravity fields. I will present a method to calculate correlation functions of a class of operators with large R-charges which are dual to classical string states at strong coupling. 24 Mai 2011 Uni Wien Jan Kurzidim (TU Wien) Glass formation of colloids in porous media Abstract: We have studied a fluid of hard spheres that evolves in the presence of randomly-placed hard-sphere obstacles. In particular, we are interested in the prolongation of the fluid's structural relaxation, i.e. in the formation of a glass phase. Using molecular dynamics simulations, we found two arrest mechanisms: the localization of fluid particles is dominant at high obstacle densities, $\phi_m$, whereas mutual blocking among the fluid particles prevails at large fluid densities, $\phi_f$. Similar effects have been shown to play vital roles in real systems like the movement of proteins in the cytoplasm of living cells. Upon varying $\phi_m$ and $\phi_f$, we unveiled very different scenarios of dynamic arrest, anomalous diffusion, and the presence of multiple non-trivial time scales in the system. To elucidate these phenomena, we geometrically distinguished the pores formed by the obstacles, which enabled us to identify particles located (a) in the percolating backbone of accessible space and (b) in pores of finite volume. We separately evaluated various characteristics for particles in (a) and (b) and demonstrated that they exhibit significant differences. 31 Mai 2011 TU Wien Jan Schlemmer (Uni Wien) Local thermal equilibrium in relativistic quantum field theory Abstract: In the description of spatially extended physical systems one often assigns to them thermal quantities (temperature, pressure, ...) varying in space and time. While there is a well established theory (statistical mechanics) for linking the thermodynamic description of systems with constant temperature ("global equilibrium") to microscopic models, for the case of varying thermal parameters the situation is much less clear cut and in fact there are many proposed frameworks to link the microscopic and macroscopic level. In my talk I will present a framework especially suited for relativistic quantum field theory, talk about its generalization to quantum fields on curved spacetime and try to summarize some results obtained for specific systems, both on flat and curved spacetime. 7 Jun 2011 Uni Wien Renate Pazourek (TU Wien) Attosecond physics - Ultrafast electron dynamics in atoms Abstract: With the ability to control and shape infrared laser fields and the generation of isolated attosecond light pulses by high harmonic generation a new research field ? attoscience - was born about ten years ago. Since then it has become possible to study electron dynamics in atoms, molecules and solids on it's natural time scale. After a general introduction to attosecond physics I will focus on ultrafast electron dynamics in atoms, in particular correlated electron dynamics in helium for which the time-dependent Schr¿dinger equation can still be solved numerically. I will present simulations for two-photon double ionization of helium and compare some of the calculated results with experiments. In addition, I will give an introduction to attosecond streaking, a pump-probe technique using an ultrashort extreme ultaviolet pulse and a synchronized IR laser field, which allows to extract time shifts between different photoionization events with attosecond precision. 21 Jun 2011 TU Wien Andrea Puhm (CEA-Saclay Paris) Black Hole Microstates Abstract: A century after the first black hole solution to Einstein's equation was found black holes remain an intense and puzzling area of research. While general relativity is a very good description far away from the black hole horizon quantum mechanical effects like Hawking radiation are present close to the horizon leading to apparent inconsistencies like information loss. A theory of quantum gravity is needed in order to describe black holes in full glory and to resolve these puzzles. In this talk I will introduce how black holes can be described in string theory and how ''black hole microstates'' may cure these paradoxes. Constructing such microstates is a difficult task and so far they have been found only for extremal (supersymmetric and non-supersymmetric) black holes. I will present a new approach that enabled us to find microstates corresponding to non-extremal (non-supersymmetric) black holes, thus taking a step forward to a stringy description of realistic black holes.