
Vienna
Theory Lunch Seminar
by Iva Lovrekovic (VUT),
Max Riegler (VUT), Sebastian Frank (HEPHY), Tuesdays 12:1513:30 held alternatively
at: Vienna University of Technology
(VUT): Wiedner Hauptstr. 810, yellow area,
10th floor, seminar
room E136 University of Vienna (UV):
Boltzmanngasse 5, 5th floor, Schrödinger
Lecture Hall / large seminar
room We thank
our kind
sponsors:

Idee: Idea: 
Wie auf vielen
Universitäten praktiziert wollen wir ein LunchSeminar
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) 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.
Prediploma 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
Mailinglist eintragen oder Mail an
albert.georg.passegger at
univie.ac.at schicken 
Just
attend! To
receive informations
via email go
to the
Mailinglist or
drop an email to
albert.georg.passegger
at univie.ac.at 
Oct 8 2013 UV 
Leif Eric Hintzsche

Defect states at cSi/aSi_{3}N_{x}H_{y}
interfaces Abstract:
Amorphous
silicon nitrides are deposited on crystalline silicon as
antireflection and passivating coatings. Up to date detailed
knowledge about the interfaces is largely lacking. We have
investigated the electronic and structural properties of
cSi/aSi_{3}N_{x}H_{y} interfaces
obtained by large scale ab initio molecular dynamics
simulations. Over 500 independent samples have been
generated for each considered stoichiometry to perform a
reliable defect analysis. While the classes of dominant
defect states coincided with previous bulk calculations, we
found a considerably increased defect density at the
interface. By applying an energy and spatially resolved
defect analysis, we observed that most of the defect states
originate from the first silicon nitride layer at the
interface. Additionally, we examined passivation effects of
hydrogen at the interface which play an importantrole to
increase the efficiency of modern solar cells. 
Oct 15 2013 VUT 
Florian Libisch 
Tackling MoleculeSurface Interactions Using Embedding
Methods Abstract:
Quantum
mechanical embedding methods allow for the theoretical
description of problems that cannot be tackled with one
single abinitio method. One prominent example is the
interaction of a gas molecule with a metal surface. Density
functional theory is perfectly suited to describe extended
metal surfaces, but fails to correctly account for subtle
details of the moleculesurface interaction, e.g., charge
transfer processes. More expensive correlated wavefunction
methods, in turn, can very accurately describe the molecule
itself, but become prohibitively expensive even for
moderatelysized surface clusters. Embedding methods combine
the advantages of different methods: a system of interest is
partitioned into different parts (e.g. the approaching
molecule interacting with a small surface cluster, and the
remainder of the surface), which are each described by a
suitable method. Their interaction is mediated by a global
embedding potential. We present different approaches to
determine this embedding potential, and discuss two
applications: the dissociation of oxygen molecules by an
aluminum surface, and the hotelectron mediated dissociation
of hydrogen on a gold surface. For both cases we compare to
experimental results. 
Oct 22 2013 UV 
Felix Hummel 
Evaluating
the second order exchange contribution to the
polarizability by means of Feynman Diagrams Abstract:
Many
methods in correlated electronic structure calculation
estimate the dominant set of interaction processes among all
possible ones to improve precision while maintaining
computational efficiency. We approximate the second order
exchange (SOEX) contribution to the polarizability of
materials by the respective SOEX contribution in the uniform
electron gas. This can be computed much faster while we
still believe to capture the dominant part of this
contribution for many materials. Monte Carlo integration
with importance sampling and symmetrization considerations
is used to integrate the divergent kernels of the SOEX
contribution to the polarizability numerically as a function
of momentum and frequency. Correlated electronic structure
calculation methods such as the Random Phase Approximation
(RPA) can then be amended using this polarizability. 
Oct 29 2013 VUT 
Renate Pazourek 
TimeResolved Photoemission on the Attosecond Scale Abstract:
One
of the most fundamental processes in the interaction of
light with matter is the photoelectric effect, i.e. the
emission of an electron after the absorption of a photon.
With the advent of ultrashort and wellcontrolled laser
pulses photoemission can now be studied with unprecedented
temporal resolution. The challenge lies in interpreting the
timing information obtained by attosecond pumpprobe
experiments. One fundamental question is whether an electron
is emitted instantaneously upon absorption of a photon or
with a certain time shift. In this talk, we will discuss how
such a time delay can be defined quantum mechanically and
classically. For an accurate theoretical description we
numerically solve the timedependent Schrödinger equation
for one and twoelectron atoms and can show that, as long
as all measurementinduced distortions can be quantified,
the intrinsic (EisenbudWignerSmith) time delay of
photoemission becomes accessible by attosecond pumpprobe
setups. 
Nov 5 2013 UV 
Ulf R. Pedersen 
The interface pinning method Abstract:
An
important aspect of computational condensed matter physics
is the computation of phase diagrams. The thermodynamically
stable phase is the one with the lowest Gibbs free energy.
"Interface pinning" is a method where the Gibbs free energy
differences between phases is computed directly in a single
equilibrium simulation. This is done by applying an
artificial external field that biases the system towards
twophase configurations. The Gibbs free energy difference
is then determined from the average force that the field
exert on the system. In addition, the method gives
information about the interface between the phases. 
Nov 12 2013 VUT SEMINAR STARTS AT 12:00 
Jérémie
Joudioux 
Decay of ﬁelds in general relativity Abstract:
One
of the main problem studied these days in the field of
mathematical general relativity is a proof of the stability
of the slowly rotating black holes, known as Kerr black
holes. The understanding of this stability mostly relies on
establishing good decay/scattering properties of higher spin
fields (such as Maxwell fields) evolving on the Kerr
background. After a short presentation on the meaning of the
stability of the Einstein equation in the context of the
Cauchy problem, the decay properties of the most simple toy
model, the scalar wave equation (spin0 field) on the flat
background, will be described and the obstructions to the
generalization of these scattering properties of solutions
of the scalar wave on black holes will be explained.
Finally, a reduction process of higher spin fields to the
scalar wave equation, introduced by Penrose, will be
given. 
Nov 19 2013 UV 
Hamid Afshar 
Conformal
gravity in three dimensions Abstract:
Using
the ChernSimons formulation of gravity in three dimensions
we study the asymptotic analysis of 3D pure/conformal
gravity with Minkowskian (flat) and AdS boundary conditions.

Nov 26 2013 VUT 
Carina Karner 
Hard
Particles in Theory, Simulation and Experiment Abstract:
In the
1950's, when computational methods were established as new
way to study and understand physical phenomena, hard spheres
were among the first systems studied. As early as 1957 Alder
and Wainwright showed that hard spheres exhibit a
first order ordering phase transition. Now, 50 years later,
hard spheres and their experimental realizations, colloidal
particles with nearly hard interactions are still a very
attractive object of research. Nowadays, new realspace
imagining techniques enable the experimental study of
nucleation of hard colloidal spheres and, by that,
testing theoretical findings and numerical results. Besides
hard colloidal spheres, experimentalists can now, due
to the advent of new fabrication techniques, produce
colloidal particles with various shapes or interactions. 
Dec 3 2013 UV 
Christian Leitold 
Monte Carlo Simulations in Statistical Physics: Theory and
Applications Abstract: In statistical mechanics, many quantities of interest, like a system's energy as a function of temperature, are given as a high dimensional integral in the configuration space of the system which cannot be evaluated directly. However, it is possible to approximate this integral by a sequence of random samples, distributed according to the underlying probability distribution, e. g. the Boltzmann distribution in the case of a system with fixed temperature, volume and particle number. One way to generate such a sequence is the Metropolis algorithm. In my talk, I will present the theoretical background of this algorithm and give a few examples of its applications in the field of soft and condensed matter simulations. This applications range from comparatively easy systems like the simulation of a LennardJones fluid to more complex cases like the sampling of typical folding pathways in a model protein 
Dec 10 2013 VUT 
Ivana Mustac 
Light
vectorlike quarks and the Higgs boson Abstract:
Vectorlike quarks are currently studied in increasing
detail at the LHC, due to their importance in numerous
models of New Physics. The current experimental nullresults
for their onshell production have pushed the lower bounds
on their masses near a TeV, however valid only under certain
assumptions. Embedding vectorlike quarks into models
addressing the electroweak hierarchy problem by treating the
Higgs as a pseudogoldstone boson of a global symmetry
changes current conclusions from flavour and electroweak
precision tests and can have interesting implications for
direct searches. We have studied the flavour phenomenology
of such new states and calculated the implications for the
electroweak hierarchy problem in light of recent Higgs data. 
Dec 17 2013 UV 
Thomas Schäfer 
Beware of ... ”Dragons”: Divergent Precursors of the
MottHubbard metalinsulator Transition Abstract:
Bulk
electronic correlated systems are often well described by
dynam ical mean field theory (DMFT)[1]. Amongst several
successes, DMFT is able to properly describe the
MottHubbard metaltoinsulator transition (MIT), which is
an intrinsic nonperturbative phenomenon. In this talk,
after a short introduction to DMFT and the MIT at the
oneparticlelevel, the notion of twoparticle vertex
functions and their necessity for exten sions of DMFT are
discussed[2]. Furthermore, within this twoparticle level
framework, hallmarks of the MIT are already identified well
inside the metallic phase in terms of divergences of the
(frequency resolved) ir reducible vertex [2,3].
Specifically, the strong enhancements and the sign changes
of the irreducible vertex functions, which mark this
precursor of the MIT, stem from enhanced local scattering
processes and can be traced in the high temperature regime
up to the atomic limit.
[1]
A. Georges et al., Rev. Mod. Phys. 68, 13125 (1996) 
Jan 7 2014 VUT 
Andrej Ficnar 
Finite
endpoint momentum strings and applications to energy loss
Abstract: I will argue that classical strings, both bosonic and supersymmetric, can have finite energy and momentum at their endpoints. I will also show that, in a general curved background, the string endpoints must propagate along null geodesics as long as their energy remains finite. Finite endpoint momentum allows strings with a fixed energy to travel a greater distance in an AdS5Schwarzschild background than has been possible for classical solutions considered previously. I will also review the relevance to heavy ion phenomenology of the dependence of this distance on energy and propose a scheme for determining the instantaneous rate of energy loss. 
Jan 14 2014 UV 
Christine Gruber 
Quantum
phenomena in the realm of cosmology and astrophysics Abstract:
Modern physical theories provide explanations for processes
in both the small dimensions of the quantum world and on
cosmologically large scales, but unfortunately a unifying
theory valid on all scales does still not exist. In this
seminar, we will  in the framework of existing theories 
introduce scenarios where quantum effects can however still
have consequences on astrophysical or cosmological scales. 
Jan 21 2014 VUT 
Iva HungerBrezinova 
Solitons, nonlinear evolution equations and inverse
scattering
Abstract:
Nonlinear
evolution equations play a pivotal role in many areas of
physics ranging from fluid dynamics, plasma physics,
nonlinear optics, quantum field theory, to approximative
descriptions of the manybody quantum problem such as the
GrossPitaevskii equation, the timedependent Hartree Fock
equations, or timedependent density functional theory. In
some cases nonlinear evolution equations are exactly
solvable via the inverse scattering transform  a fact
closely related to the existence of solitons. In this talk a
short introduction to the inverse scattering transform will
be given. As an example we will discuss the Kortewegde
Vries equation which describes propagation of water waves. 
Jan 28 2014 UV 
Frederic Brünner 
Glueballs in Holographic QCD Abstract:
Glueballs, bound states of gluonic degrees of freedom, are a prediction
of quantum chromodynamics. Due to lack of theoretical input, they have
not yet been established experimentally. We discuss how this problem
can be solved within the framework of the AdS/CFT correspondence, a
holographic duality between a strongly coupled gauge theory in four
dimensions and a string theory in a fivedimensional Antide Sitter
geometry. We explain how in a particular realization of holographic
QCD, the SakaiSugimoto model, one can describe glueball interactions
and explicitly calculate decay rates relevant for theexperimental search. 
Program Winter Semester
2012/2013
Program Winter Semester
2011/2012