Vienna Theory Lunch Seminar

by Elke Aeikens (UV), Maximillian Löschner (UV), Alexander Soloviev (VUT)
and David Toneian (VUT)

Tuesdays 12:15-13:30

held alternately at:

Vienna University of Technology (VUT): Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room E136

University of Vienna (UV): Boltzmanngasse 5, 5th floor, Schrödinger Lecture Hall

We thank our kind sponsors:

Dean of physics, TU

Faculty of Physics, UV

Daniel Grumiller, TU



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 Beginn des Masterstudiums dem Vortrag folgen kann. BachelorstudentInnen können besonders von dem Seminar profitieren, da es ihnen ermöglicht einen Eindruck in die Forschungsarbeit beider Universitäten zu erhalten. Die Vortragenden werden dabei auch ermutigt darüber zu sprechen, warum sie ein gewisses Forschungsgebiet gewählt haben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden und es ist nicht notwendig einen Vortrag über eine "perfekte", abgeschlossene Arbeit zu halten.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza) 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. Undergraduate 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).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailingliste eintragen.

Just attend! To receive informations via email register for the Mailinglist.

Mar 8 2016


Lisa Weiß
UV Computational Physics

Simulations of a Polyoxometalate in aqueous solution

Colloids represent an important part in our every day lifes ranging from food science to colours and coatings. Therefore, experimentalists developed multiple parameters and models to characterise the properties of colloids. In this work molecular dynamics simulations are used to explore the validity of these concepts for a small colloid. The influence of charge and salt content are evaluated to obtain a detailed picture how water molecules and ions arrange themselves in the surroundings of a colloid in equilibrium and non equilibrium conditions.

Mar 15 2016


Marissa Giustina
UV Quantum Information and Foundations of Physics

Significant-Loophole-Free Test of Bell’s Theorem with Entangled Photons

Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell’s theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell’s inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here, we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance.

Mar 22-29 2016

No Lunch Seminar

Osterferien - Easter Break

Apr 5 2016


Flaminia Giacomini
UV Quantum Foundations and Quantum Information Theory

Quantum mechanics with indefinite causal order

In all our well-established theories, it is assumed that events are embedded in a global causal structure such that, for every pair of events, the causal order between them is always fixed. However, the possible interplay between quantum mechanics and general relativity may require a revision of such a paradigm, for instance when the metric tensor, and therefore the causal structure, is subject to quantum fluctuations. The process matrix framework is an operational approach to this question, and uses techniques typical of quantum information to address a problem which has been typically considered in quantum gravity. The framework retains the validity of quantum physics locally but doesn’t assume the existence of a global causal order. For example, it allows to describe causal structures corresponding to a quantum superposition of ‘A is before B’ and ‘B is before A’. The framework has been developed for both finite and infinite-dimensional Hilbert spaces. The long term goal is formulating quantum fields on indefinite causal structure, as required by the high-energy regimes at which the interplay between quantum mechanics and general relativity is expected.

Apr 12 2016


Luis Cortés Barbado
UV Quantenoptik, Quantennanophysik und Quanteninformation

Hawking versus Unruh effects: What do you see when you fall into a black hole?

Arguably, the most important milestone of Quantum Field Theory in curved spacetime is the discovery by Stephen Hawking that black holes should evaporate by emitting a Planckian spectrum of particles, the so-called Hawking radiation. With a similar derivation, Bill Unruh postulated that accelerated observers in empty space should perceive a thermal bath of particles with temperature proportional to their acceleration, the so-called Unruh effect. It seems clear that, for an observer following an arbitrary trajectory outside a black hole, these two effect must be present together. But, how do they combine to give the observer's net particle perception? In this talk we will address this question, within a restricted but conceptually clear framework, by using the so-called effective-temperature function. Far from just getting a set of concrete quantitative results for different trajectories of the observer, we will obtain general results which are clearly interpretable in terms of well-known physical phenomena. Furthermore, these results will let us to address some interesting questions: Which part of the radiation perceived can be assigned to Hawking radiation and which to the Unruh effect? Can these two effects interfere destructively? Does always the Unruh temperature scale with the proper acceleration of the observer? Is it strictly necessary to form a horizon in order to have Hawking radiation emitted? Can Hawking radiation make a test particle to float nearby a black hole due to radiation pressure?

Apr 19 2016


Miquel Triana
Universidad Barcelona

Shock wave collisions in a family of non-conformal theories

A compelling holographic model for the heavy ion collisions and the subsequent evolution of the resulting quark-gluon plasma are the shock wave collisions in AdS space. In this project we present for the first time a fully fledged shock wave collision evolution in a one parameter family of non-conformal field theories. The models studied show a non-trivial RG flow between two fixed points, and their thermodynamics resemble qualitatively (and for some magnitudes) the one of QCD. The preliminar results show how the relaxation times of the expanding plasma increase with the non-conformality, and differ very significantly from the ones computed at a linear level through the quasinormal modes spectra (wich saturate).

Apr 26 2016


Lukas Schneiderbauer
UV Mathematical Physics

Measuring finite Quantum Geometries via Quasi-Coherent States

A systematic approach to determine and measure numerically the geometry of generic quantum or “fuzzy” geometries realized by a set of finite-dimensional hermitian matrices is developed. The method is designed to recover the semi- classical limit of quantized symplectic spaces embedded in R^d including the well-known examples of fuzzy spaces, but it applies much more generally. The central tool is provided by quasi-coherent states, which are defined as ground states of Laplace- or Dirac operators corresponding to localized point branes in target space. The displacement energy of these quasi-coherent states is used to extract the local dimension and tangent space of the semi-classical geometry, and provides a measure for the quality and self-consistency of the semi-classical approximation.

May 3 2016


Alexander Komech
UV Mathematical Physics

On Bohr's transitions to quantum stationary states

The up-to-date mathematical structure of Quantum Theory is a mixture of dynamical (Schrödinger, Klein - Gordon, Dirac, Yang - Mills, Maxwell, ...) PDEs with Quantum Postulates (Bohr's transitions between stationary orbits, de Broglie's wave-particle duality, Born's probabilistic interpretation, ...). Our ultimate goal is to discover these postulates as inherent properties of dynamical equations. This is a two-fold problem: first, we should find the corresponding mathematical formulation of a postulate, and afterwords, justify it in a suitable framework. We suggest to treat i) the Bohr's transitions as a global long-time attraction to solitary waves, and ii) the wave-particle duality as a global long-time attraction to solitons.
We will describe the results on the global attraction to solitary waves and solitons obtained in the last two decades for wave, Klein - Gordon, Dirac and Maxwell equations coupled to a particle or nonlinear oscillators. In particular, we have rigorously justified for the first time the radiation damping in Classical Electrodynamics.
This global attraction is caused by energy radiation to infinity in dispersive Hamilton PDEs. These results were obtained starting from 1995 in collaboration with H. Spohn (München Technical University) and other colleagues. Our approach relies on subtle methods of Harmonic Analysis for analysis of the energy radiation: Wiener Tauberian theorem, Titchmarsh Convolution theorem, the theory of quasimeasures, and other techniques.
This programm is inspired by the absense of rigorous `ab initio' description of fundamental quantum phenomena: black body radiation, laser coherent radiation, thermal electron emission, photo-electric emission, electric conductivity and heat conductivity of metals (Ohm and Fourier laws), the Hall and Meissner effects, superconductivity, superfluidity, ...
A rigorous description of these phenomena is indispensable for numerical design and computer control of high frequency electron devices: magnetron, klystron, traveling wave tube, laser, synchrotron, electron microsope, etc."

May 10 2016


Moritz Preisser
UV Particle Physics Group

The top quark mass: Calibrating PYTHIA's top quark mass parameter using massive event shapes

Abstract: The top quark is the heaviest particle in the Standard Model (SM) of particle physics. For a variety of reasons it is very important to know its mass with high precision. In this talk I am going to review a class of precise top quark mass determinations and their relation with General Purpose Monte Carlo event generators (GPMCs) such as PYTHIA. After that I will present a method for extracting a theoretically well defined top quark mass value from such determinations using Effective Field Theory (EFT) methods. In the end I am going to show recent results and an outlook on future work.

May 17 2016

No Lunch Club

Pfingsten - Pentecost

May 24 2016


Silvano Ferrari
TU Soft Matter Theory

Self-assembly of inverse-patchy colloids into Manner Schnitten

Inverse-patchy colloids (IPC) are heterogeneously charged; two small sites (the so-called patches) carry charge of the opposite sign of the rest of the surface. The interplay between the regions of like-unlike charge gives rise to quite a few unusual lattices; among them, we studied a very peculiar structure where liquid regions are trapped between solid planar layers, like the melted chocolate is trapped between wafer layers in a Manner Schnitten in a hot summer day. Using molecular dynamics, we determined the stability of this structure with respect to the model parameters (charge and patch amplitude).

May 31 2016



Jun 7 2016


Dr. Miguel Verdugo
UV Department for Astrophysics

On the origin and life of cluster galaxies

Clusters of galaxies are the most massive objects in the Universe. As such they sit at the crossroads between cosmology and astrophysics, allowing the investigation of a variety of phenomena. Clusters of galaxies also feature a different galaxy population in comparison to their surroundings, with galaxies in the field being mostly star-forming whereas galaxies in clusters are mostly passive. This dichotomy has baffled astronomers for years and no model currently predicts all the observed features. In this talk I will concentrate in the possible origins of this population and the efforts being done in Vienna to make progress in some long standing questions. I will also introduce aspects of structure formation and galaxy evolution which are relevant for the topic of this talk.

Jun 14 2016


Dr. Johannes Lachner
UV Isotope Research and Nuclear Physics

Accelerator Mass Spectrometry at VERA

Accelerator Mass Spectrometry (AMS) is a powerful experimental technique to determine trace amounts of nuclides in environmental or artificial samples. This finds many applications, e.g. in nuclear astrophysics, geology, or oceanography. The setup in Vienna (VERA: Vienna Environmental Research Accelerator) is now in use since 20 years and can be used to measure many long-lived nuclides such as 10Be, 14C, 36Cl, 129I or U and Pu isotopes. The key parameters for a successful AMS measurement are the efficient counting of the ions of interest and the effective suppression of potential background arising from molecular and atomic isobars or from neighbouring abundant isotopes. I will present recent technical developments towards our aim to suppress isobaric background in an interaction of the beam particles with a LASER and show some exemplary applications of radionuclides in other sciences.

Jun 21 2016


Alexander Haber
TU Fundamental Interactions

Flux Tubes Interacting with Superfluids

In compact stars, nucleons form an interacting multi-fluid system of a neutron superfluid and a proton superconductor. The rotation of the star and the strong background magnetic fields generate superfluid vortices and superconducting flux tubes in the core. After a phenomenological introduction to superconductivity, I will review how these phenomena can be described in the context of quantum field theory. Using a microscopic field-theoretical model of two coupled bosonic fields with entrainment and density coupling, the phase structure of the multi-fluid system is examined. In particular, the superconducting type-I/type-II transition and multi-flux quantum phases under the influence of the neutron condensate are investigated, including an effective interaction between the flux tubes.

Jun 28 2016


Tin Ribic
TU Computational Materials Science

The Falicov-Kimball model - Dynamical Mean Field Theory and beyond

A lot of todays most intriguing effects in material science are observed in systems with strongly correlated electrons. Unfortunately, interacting quantum systems are computationally very expensive to treat. The Falicov-Kimball model is a rare example of a non-trivial, interacting quantum system, for which some analytical results can be obtained. Results on local corellators of mobile electrons in the Falicov-Kimball model within the framework of dynamical mean field theory will be discussed. These are then used as building blocks for reincluding non-local correlations into the model via expansion schemes. Since analytical expressions for the local correlators are available, many calculations which would usually need to be treated computationally can be performed by hand. This allows investigation of a simple, interacting quantum system on one hand, but simultanously provides a benchmark for the behaviour of methods developed for treatment of more general interacting systems.

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