Past Talks (2015-2019)

9. 12. 2014 {$\hspace{0.5cm}$} Kirill Boguslavski (Heidelberg)

Title: Non-thermal fixed points (II): From superfluid Bose-gases to heavy-ion collisions

Abstract: Strongly correlated quantum systems have been observed to approach non-thermal fixed-points of their evolution during the thermalization process. Such fixed-points form universality classes far from equilibrium, which may be shared by different many-body systems (one example in A. Orioli's previous talk). In this talk, we will present theoretical evidence using classical-statistical simulations that important aspects of longitudinally expanding non-abelian gauge theory in the ultrarelativistic limit admit a dual description in terms of a Bose condensed scalar field theory. Remarkably, these non-Abelian dynamics may be relevant at early stages of ultrarelativistic heavy-ion collisions. The scalar system also reveals a low momentum fixed-point, which shares universal properties with superfluid Bose gases.

25. 11. 2014 {$\hspace{0.5cm}$} Asier Pineiro Orioli (Heidelberg)

Title: Non-thermal fixed points (I): Universal self-similar dynamics of relativistic and non-relativistic scalar field theories

Abstract: The dynamics of quantum fields far from equilibrium play an important role in systems ranging from early universe cosmology and relativistic heavy-ion collisions to ultra cold quantum gases. Strikingly, universal features emerge during the respective thermalisation processes. This universality is based on the existence of non-thermal fixed points, which are attractor solutions characterised by turbulence and self-similar time evolution. In this talk we will show that the (massless) relativistic and the non-relativistic (Gross-Pitaevskii) scalar field theory belong to the same universality class in the infrared. We compute the scaling exponents and scaling functions in this non-perturbative regime in two ways: first by performing classical statistical lattice simulations and second by using the resummed 2PI $1/N$ expansion to NLO.

25. 11. 2014 {$\hspace{0.5cm}$} Maximilian Demmel (Uni Mainz)

Title: RG flow of W(F^2) Yang-Mills theory

Abstract: The non-perturbative renormalisation group flow of a W_k(F^2)-type Yang-Mills theory is presented. The flow equation for this type of truncation generalises to a partial differential equation. In this talk, different methods to construct solutions of this equation are presented. For the gauge coupling a non-Gaußian infrared fixed point has been found. Moreover, it is shown that the flow equation admits solutions with a nontrivial minimum allowing for gluon condensation.

18. 11. 2014 {$\hspace{0.5cm}$} Valentin Kasper (Heidelberg)

Title: Real-Time Dynamics of Fermions in Gauge Theories

Abstract: In this talk I will show how it is possible to study real-time dynamics of fermions in strong gauge fields. At the heart of this is the production of fermions, hence the interest is focused on the dynamical version of the Schwinger effect. However, the used method is not restricted to this problem, but is a generalization of the classical statistical method in the presence of fermions. As another example I will show how these ideas can be applied to study string breaking in lattice gauge theories realizable by cold atom experiments.

11. 11. 2014 {$\hspace{0.5cm}$}Stefan Lippoldt (Uni Jena)

Title: Fermions in gravity with local spin-base invariance

Abstract: We study a formulation of Dirac fermions in curved spacetime that respects general coordinate invariance as well as invariance under local spin-base transformations. The natural variables for this formulation are spacetime-dependent Dirac matrices subject to the Clifford-algebra constraint. In particular, a coframe, i.e. vierbein field is not required. This observation is of particular relevance for field theory approaches to quantum gravity, as it can serve for a purely metric-based quantization scheme for gravity even in the presence of fermions.

4. 11. 2014 {$\hspace{0.5cm}$}Rene Sondenheimer (Uni Jena)

Title: Higgs mass bounds from the functional renormalization group

Abstract: We investigate various Higgs-Yukawa models to mimic the Higgs sector of the standard model within the framework of the functional renormalization group. Futhermore, we re-analyze the conventional arguments that relate a lower bound for the Higgs mass with vacuum stability in the light of exact results for the regularized fermion determinant as well as in the framework of the FRG. In both cases, we find no indication for vacuum instability nor meta-stability induced by top-fluctuations if the cutoff is kept finite but arbitrary. For the class of standard bare potentials of \phi^4-type at a given ultraviolet cutoff, we show that a finite infrared Higgs mass range emerges naturally from the renormalization group flow itself. By contrast, more general bare potentials allow to diminish the lower bound considerably. If active also in the full standard model, Higgs masses smaller than the conventional infrared window do not necessarily require new physics at low scales or give rise to instability problems.

21. 10. 2014 {$\hspace{0.5cm}$}Walid Ahmed Mian (TU Darmstadt)

Title: Magnetische Katalyse in der QCD

Abstract: Ich werde die spontane chirale Symmetriebrechung in der Gegenwart eines konstanten externen magnetischen Feldes vorstellen. Für die Untersuchung wurde der Einfluss des Magnetfeldes auf dem Materiesektor und dem Eichsektor betrachtet.

30. 9. 2014 {$\hspace{0.5cm}$}Sergej Moroz (CU Boulder)

Title: Effective theory of p-wave interacting fermions in two dimensions

Abstract: Due to the current search of Majorana fermions, the physics of two-dimensional identical fermions with short-range p-wave interactions is of considerable interest. My talk will be about the effective theory of a chiral p+ip fermionic superfluid at zero temperature. This theory naturally incorporates the parity and time reversal violating effects such as the Hall viscosity and the edge current. I will present some applications of this theory such as the linear response to external electromagnetic and gravitational fields and the density profile of an isolated vortex. Finally, the dual gauge reformulation of this theory will be presented.

17.6. 2014 {$\hspace{0.5cm}$}Michael Scherer (Heidelberg)

Title: The Planck scale, the Higgs mass and scalar dark matter

Abstract: This study is inspired by a scenario, in which the Standard Model, enhanced by an additional dark matter scalar, could be extended up to the Planck scale, while accommodating the low measured value of the Higgs mass. To that end, we study a toy model for a gauge singlet dark matter scalar coupled to the Higgs-top-quark sector of the Standard Model. Using functional methods to derive Renormalization Group flow equations in that model, we examine several choices for the ultraviolet, bare potential in the Higgs-dark-matter sector. Our results indicate that the dark matter scalar can decrease the lower bound on the Higgs mass in the Standard Model. We then use the fact that higher-order couplings which are driven to tiny values by the Renormalization Group flow towards low energies can easily be of order one at the ultraviolet cutoff scale. Our study indicates that the inclusion of these couplings can significantly increase the ultraviolet cutoff scale and therefore the range of validity of the model while yielding a low value for the Higgs mass in the infrared. This is achieved within a setting where the dark matter scalar accounts for the complete dark matter relic density in our universe.

3. 6. 2014 {$\hspace{0.5cm}$}Michael Golor (Aachen)

Title: Quantum Monte Carlo for fermionic Many-Body-Systems

Abstract: In the field of strongly-correlated electrons we try to understand the effects of electron-electron interactions in solid-state systems. These can lead to such different phenomena as (anti-)ferromagnetism, high-temperature superconductitivity or even more exotic phases. Unfortunately, already the most simplified models that are used to describe these systems, usually refuse a solution by conventional means. In some cases however, valuable insight can be gained by Quantum Monte Carlo (QMC) methods, which can provide exact numerical solutions. I will present my method of choice, the "Determinantal QMC" and discuss some examples of its application but also its major shortcoming, the notorious "sign problem".

4. 2. 2014 {$\hspace{0.5cm}$}Fabian Rennecke (Heidelberg)

Title: The chiral phase transition of QCD

28. 1. 2014 {$\hspace{0.5cm}$}Christoph Rahmede (KIT)

Title: Asympotic safety, higher-derivative couplings and inflationary cosmology

Abstract: I will discuss recent results for the renormalisation group flow of higher-derivative couplings in gravity which show evidence for the existence of a suitable renormalisation group fixed point as required by Weinberg's asymptotic safety hypothesis. I will specify conditions under which the fixed point regime can be connected successfully with viable models of inflationary cosmology.

21. 1. 2014 {$\hspace{0.5cm}$}Luca Zambelli (TPI Jena)

Title: A Hamiltonian formulation of the functional renormalization group equations

Abstract: An exact renormalization group (RG) equation for Wilson's effective action is translated into a generalized Hamiltonian framework, as an equation for an effective Hamiltonian density that depends on configuration variables, on momenta fields with higher spins, but on no derivatives of these `phase space' coordinates. By neglecting the spacetime dependence of the couplings parameterizing such a Hamiltonian, one can boil down the RG equation to a partial differential equation for a function of infinitely-many fields. This allows for a momenta expansion of the Hamiltonian, that might be considered as an approximation scheme alternative to the usual derivative expansion. This construction is described in the simple case of a scalar field theory, whose critical behavior is addressed, in order to test and compare the new framework and approximations to known results.

14. 1. 2014 {$\hspace{0.5cm}$}Matin Mojaza (CP3-Origins, University of Southern Denmark)

Title: Conformal Windows, Mass Hierarchies and Perturbatively Natural Conformality

Abstract: After describing the progress in our understanding of the conformal window of gauge theories, I will outline its relation to theories also containing scalar fields. Here the Coleman-Weinberg analysis is an integral part of understanding the moduli space and it's quantum corrections. In the attempt to apply these ideas to electroweak symmetry breaking, the theory gets plagued with the hierarchy problem. I will present one recently proposed way out of this problem, dubbed Perturbative Natural Conformality, and present an intriguing prediction testable at the LHC.

7. 1. 2014 {$\hspace{0.5cm}$}Timo Reckling (Heidelberg)

Title: Quantum strategies in simple game theoretic situations

Abstract: After discussing some elementary classical game theory concepts we will investigate 3 simple examples of "quantum"(?) 2-player-games.

10. 12. 2013 {$\hspace{0.5cm}$}Felix Rennecke (MPI for Physics, Munich)

Title: O(d,d) Geometry in String Theory

3.12.2013 {$\hspace{0.5cm}$}Conrad Albrecht (Heidelberg)

Title: Numerics for Functional Renormalization

26.11.2013 {$\hspace{0.5cm}$}Benjamin Knorr (Heidelberg)

Title: Global Completeness of Quantum Gravity

19.11.2013 {$\hspace{0.5cm}$}Laura Classen (Heidelberg)

Title: Interplay between electron-phonon and Coulomb interactions in the honeycomb lattic

16. 7. 2013 {$\hspace{0.5cm}$}Ydalia Delgado Mercado (Graz)

Title: Dual Representation and the Complex Phase Problem

Abstract: On the lattice, the QCD action can be interpreted as a probability weight during the Monte Carlo simulation. Such interpretation is spoiled at finite quark density because the action turns complex. However, mapping the theory onto new degrees of freedom, so called dual variables, all the terms of the partition sum become real and positive. Thus the sign problem is solved and the simulations feasible. In this talk, I present the dual representation for a U(1) gauge model coupled with two Higgs fields. We study its dual formulation and explore its phase diagram at zero and finite density.

2. 7. 2013 {$\hspace{0.5cm}$}Christian Kohlfuerst (Graz)

Title: Electron-Positron Pair Production in Strong Electric Fields

25. 6. 2013 {$\hspace{0.5cm}$}Tobias Henz (Heidelberg)

Title: Dilaton Quantum Gravity

Abstract: We propose a simple fixed point scenario in the renormalization flow of a scalar dilaton coupled to gravity. This would render gravity non-perturbatively renormalizable and thus constitute a viable theory of quantum gravity. On the fixed point dilatation symmetry is exact and the quantum effective action takes a very simple form. Realistic gravity with a nonzero Planck mass is obtained through a nonzero expectation value for the scalar field, constituting a spontaneous scale symmetry breaking. Furthermore, relevant couplings for the flow away from the fixed point can be associated with a'dilatation anomaly' that is responsible for dynamical dark energy. For the proposed fixed point and flow away from it the cosmological'constant' vanishes for asymptotic time.

18. 6. 2013 {$\hspace{0.5cm}$}David Mesterhazy (Heidelberg)

Title: New Dynamic Critical Behavior for Model C

11. 6. 2013 {$\hspace{0.5cm}$}Michael Buchhold (Innsbruck)

Title: Spin and Photon Glasses in Open Quantum Systems

Abstract: In the context of ultracold atoms in multimode optical cavities, the appearance of a quantum-critical glass phase of atomic spins has been predicted recently. Due to the long-range nature of the cavity-mediated interactions, but also the presence of a driving laser and dissipative processes such as cavity photon loss, the quantum optical realization of glassy physics has no analog in condensed matter, and could evolve into a "cavity glass microscope" for frustrated quantum systems out-of-equilibrium. Here we develop the non-equilibrium theory of the multimode Dicke model with quenched disorder and Markovian dissipation. Using a unified Keldysh path integral approach, we show that the defining features of a low temperature glass, representing a critical phase of matter with algebraically decaying temporal correlation functions, are seen to be robust against the presence of dissipation due to cavity loss. The universality class however is modified due to the Markovian bath. The presence of strong disorder leads to an enhanced equilibration of atomic and photonic degrees of freedom, including the emergence of a common low-frequency effective temperature. The imprint of the atomic spin glass physics onto a "photon glass" makes it possible to detect the glass state by standard experimental techniques of quantum optics.

11. 6. 2013 {$\hspace{0.5cm}$}Lukas Sieberer (Innsbruck)

Title: Dynamical Critical Phenomena in Driven-Dissipative Systems

Abstract: Based on our recent investigation of the Bose condensation transition in three-dimensional driven open quantum systems (Phys. Rev. Lett. 110, 195301 (2013)), we will follow two lines: The first part of my talk will be concerned with the question how dynamical critical behavior is modified in two-dimensional systems such as the experimentally most relevant case of exciton-polariton condensates. In the second part I will discuss thermalization in these systems and its relation to emergent symmetries close to criticality.

28. 5. 2013 {$\hspace{0.5cm}$}Discussion of 4D cutoffs cont.

4. 6. 2013 {$\hspace{0.5cm}$}Discussion of 4D cutoffs

7. 5. 2013 {$\hspace{0.5cm}$}Jan Borchmann (Heidelberg)

Title: Introduction to F-Theory

23. 4. 2013 {$\hspace{0.5cm}$}Joerg Jaeckel (Heidelberg)

Title: Probing the Hidden Sector @ Low Energies

8. 1. 2013 {$\hspace{0.5cm}$}Walter Heupel (Giessen)

Title: Tetraquarks in a covariant Dyson-Schwinger/Bethe-Salpeter approach

18. 12. 2012 {$\hspace{0.5cm}$}'''Dachzimmer christmas party

11. 12 2012 {$\hspace{0.5cm}$}Dorival Goncalves-Netto (Heidelberg)

Title: Measuring Higgs Quantum Numbers

Abstract: Recently, the ATLAS and CMS experiments have reported the discovery of a Higgs like resonance at the LHC. The next analysis step will include the determination of its spin and CP quantum numbers or the form of its interaction Lagrangian channel-by-channel. We show how weak-boson-fusion Higgs production and associated ZH production can be used to separate different spin and CP states.

20. 11. 2012 {$\hspace{0.5cm}$}Vladimir Skokov (Brookhaven National Laboratory)

Title: Azimuthal anisotropy of photon production in heavy-ion collisions

Abstract: Measurements by the PHENIX collaboration revealed that the anisotropy of produced photons is very close to that of hadrons at sqrt(s) =200 GeV . Very recent data from the ALICE collaboration at the LHC affirms this at significantly higher collision energies. These results contradict the current theory of photon production: It has been expected that the elliptic flow of direct photons would be much smaller than that of hadrons because a significant fraction of photons has to be produced at early times, when the temperature is the highest.
In this talk, I will discuss the novel photon production mechanism that is able to resolve the above problem. It stems from the conformal anomaly of QCD+QED and the existence of strong (electro)magnetic field in heavy-ion collisions. Using a realistic input for the magnetic field and the hydrodynamical description of the bulk modes of QCD plasma, I show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. The transverse momentum spectra and the anisotropy, of photons at sqrt(s) =200 GeV are calculated. This mechanism provides considerable contribution to the anisotropy of photons. The results will be compared to the data from the PHENIX collaboration.

6. 11. 2012 {$\hspace{0.5cm}$}Naseemuddin Khan (Heidelberg)

Title: Production of Particles in Heavy Ion Collisions within the Landau Model

Abstract: A simple hydrodynamic model will be introduced which describes the production of particles in heavy ion collisions. In this model it is assumed that immediately after the collision there is a cylindrical distribution of energy due to the Lorentz contraction, which then rapidly expands into the vacuum. The evolution of the system is given by the solution of simple hydrodynamic equations from which quantities like temperature, entropy and pressure can be deduced. In this model the freeze-out concept plays an important role, according to which the particles stop interacting after they have passed a certain hypersurface corresponding to the freeze-out temperature. Most importantly we will compute the velocity distribution, which is fixed after the freeze-out and can be compared to experimental data.

30. 10. 2012 {$\hspace{0.5cm}$}Stefan Floerchinger (CERN)

Title: Phenomenology of relativistic heavy ion collisions

17.7. 2012 {$\hspace{0.5cm}$}Piotr Piasecki (Darmstadt)

Title: Scaling behavior of the quark-meson-model in (in)finite volume

Abstract: I will motivate the used method (functional RG) and will present basic facts about scaling and Quark-Meson-Model. Then results are given.

15. 5. 2012 {$\hspace{0.5cm}$}Ulrich Linden (Heidelberg)

Title: Renormalization of Many-Fermion Models with Regular and Singular Fermi Surfaces

15. 5. 2012 {$\hspace{0.5cm}$}Martin Trappe (Heidelberg)

Title: Parity-Violating and Parity-Conserving Berry Phases for Hydrogen in an Atom Interferometer

Abstract: We discuss the propagation of hydrogen atoms in static electric and magnetic fields in a longitudinal atomic beam spin echo (lABSE) Interferometer. The atoms acquire geometric (Berry) phases that exhibit a manifestation of parity-(P-)violation effects arising from electroweak Z-boson exchange between electron and nucleus. We provide analytical as well as numerical calculations of the behaviour of the metastable n=2 states of hydrogen. We are able to systematically search for Berry phases with tailored properties. Besides maximizing P-violating geometric phases emerging for the respective states we also find the possibility to modify their decay rates, nearly at the order of a percent, solely through P-conserving geometric phases.

24. 4. 2012 {$\hspace{0.5cm}$}Leonard Fister (Heidelberg)

Title: Yang-Mills Theory at Non-Vanishing Temperature

Abstract: We study the temperature dependence of correlators in Yang-Mills theory. For this purpose we utilise a purely thermal renormalisation group flow equation, and obtain the full thermal propagators. Interestingly, the electric screening mass is sensitive to the confinement-deconfinement phase transition. We also compute thermodynamic quantities such as the pressure.

7. 2. 2012 {$\hspace{0.5cm}$}Christoph Reinhardt (Dresden)

Title: Metal-Organic Microcavity Lasers

Abstract: Optical microcavities are structures that confine light to small volumes in the micron-size range. This confinement is realized by reflecting boundaries, which enables the field inside the resonator to build up standing waves with a size- and shape-dependent resonant frequency spectrum. The microscale volume enables controlling the rate of spontaneous emission of an inserted emitter and the strong coupling of atomic excitations to the vacuum field of the microcavity.
Apart from insight to such "fundamental" processes, optical microcavities open the field to a wide range of applications like microlaser-based novel sources and dynamic filters in optical communication. Although devices based on III-V semiconductor materials are used for years, the realization of an electrically driven microcavity laser based on organic emitters remains challenging. The main restriction from this class of material arises due to the low stability of the amorphous structure, which requires new strategies with respect to the sample's design.
After giving an introduction to optical microcavities, I will present results of our group on the topic of an organic microcavity including a structured metal layer.

31. 1. 2012 {$\hspace{0.5cm}$}Naseemuddin Khan (Heidelberg)

Title: The Phase Diagram of two colour QCD with functional methods

Abstract: We apply the Functional Renormalization Group equation to Quantum Choromodynamics with two colours and two quark flavours. Diquark states can form colour singlets and hence a vacuum state with non-zero baryon number can be realized. We explore the phase diagram for various regions of temperature and chemical potential, and evaluate the behavior of the chiral condensate, the diquark condensate and the mass spectrum. The FRG treatment of two colour QCD also yields a phase of pre-condensation of diquarks, which was unobserved until now.

24. 1. 2012 {$\hspace{0.5cm}$}Denes Sexty (Heidelberg)

Title: Bose condensation far from equilibrium

Abstract: The formation of Bose condensates far from equilibrium can play an important role in our understanding of collision experiments of heavy nuclei or for the evolution of the early universe. In the relativistic quantum world particle number changing processes can counteract Bose condensation, and there is a considerable debate about the relevance of this phenomenon in this context. We show that the involved question of Bose condensation from initial over-population can be answered for the example of scalar field theories. Condensate formation occurs as a consequence of an inverse particle cascade with a universal power-law spectrum. This particle transport towards low momenta is part of a dual cascade, in which energy is also transfered by weak wave turbulence towards higher momenta. To highlight the importance of number changing processes for the subsequent decay of the condensate, we also compare to non-relativistic theories with exact number conservation. We discuss the relevance of these results for nonabelian gauge theories.

17. 1. 2012 {$\hspace{0.5cm}$}David Mesterhazy (Heidelberg)

Title: Entanglement entropy in quantum field theory

Abstract: Entanglement measures provide ways to characterize correlations for a given quantum state. That way they serve as a tool to study the ground state properties for quantum many-body theories. At the example of a 1+1-dimensional critical lattice spin model I will illustrate how to calculate entanglement entropy using functional methods. In the continuum limit, results from conformal field theory are applied to extract the form of the von Neumann entropy. Finally, I will extend these ideas to more general cases as, e.g. finite temperature and higher dimensions.

10. 1. 2012 {$\hspace{0.5cm}$}Stefan Floerchinger (CERN)

Title: Analytic continuation of functional renormalization group equations

Abstract: Functional renormalization group equations are analytically continued from imaginary Matsubara frequencies to the real frequency axis. On the example of a scalar field with O(N) symmetry I discuss the analytic structure of the flowing action and show how it is possible to derive and solve flow equations for real-time properties such as propagator residues and particle decay widths. The formalism conserves space-time symmetries such as Lorentz or Galilei invariance and allows for improved, self-consistent approximations in terms of derivative expansions in Minkowski space.

13. 12. 2011 {$\hspace{0.5cm}$}Christian Zielinski (Heidelberg)

Title: The Thirring Model at Finite Density with Stochastic Quantization

Abstract: The introduction of a fnite chemical potential \mu > 0 renders the fermion determinant complex. Usual simulation algorithms for lattice feld theories fail as the integrand of the path integral is highly oscillatory and the measure cannot be interpreted as a probability weight. Possible solutions for the so-called sign problem have been proposed, such as reweighting procedures, Taylor expansions in \mu and the introduction of an imaginary chemical potential. Another approach is stochastic quantization, namely a complex Langevin evolution, which has already been proposed in 1983 by G. Parisi. For complex actions there are known cases where Langevin evolutions describe models correctly, while for others convergence towards the wrong value was observed. Our aim is to check the applicability of stochastic quantization to the Thirring model at finite density and to get an understanding of how and why it works or fails.

6. 12. 2011 {$\hspace{0.5cm}$}Christof Wetterich (Heidelberg)

Title: Introduction to Functional Renormalization

29. 11. 2011 {$\hspace{0.5cm}$}Steven Mathey (Heidelberg)

Title: Effects of initial correlations in the dynamics of a simple model of decoherence

Abstract: A model of dephasing (decoherence) in a two-state quantum system (q-bit) coupled to a bath of harmonic oscillators is introduced. An exact analytic solution for the reduced dynamics of a two-state system has been obtained for two sets of initial states. The first is the usual separated state, $\rho(0) = \rho_S \times {\rm e}^{-\beta H_B}/Z_B$, where the environment is initially in thermal equilibrium independently of the q-bit. The second, however, contains correlations in between the system and it's environment. After a short introduction to the model and formalism, the two solutions are discussed and compared, then the time evolution of the entropy of the q-bit is briefly discussed.

22. 11. 2011 {$\hspace{0.5cm}$}Christof Wetterich (Heidelberg)

Title: Introduction to Functional Renormalization

15. 11. 2011 {$\hspace{0.5cm}$}Dorival Goncalves Netto (Heidelberg)

Title: SUSY particles at the LHC

8. 11. 2011 {$\hspace{0.5cm}$}Jan M. Pawlowski (Heidelberg)

Title: A basic introduction to non-equilibrium dynamics from temporal flows

19. 7. 2011 {$\hspace{0.5cm}$}Daniel Scherer (Jena)

Title: Low-Dimensional Chiral Physics: Gross-Neveu Universality & Magnetic Catalysis

5. 7. 2011 {$\hspace{0.5cm}$}Richard Schmidt (Munich)

Title: Functional renormalization group flow of spectral functions and rf spectroscopy

28. 6. 2011 {$\hspace{0.5cm}$}Mario Mitter (Graz, Darmstadt)

Title: Axial Anomaly in a U(2)\times SU(2) Quark-Meson Model with Renormalization Group

Abstract: An extension of the two-flavor quark-meson (QM) model that includes the violation of the axial U_A(1) symmetry on the level of the effective potential is studied within the functional renormalization group approach. The dynamical generation of couplings which violate the axial symmetry is observed. Effects on the mesonic spectrum at non-vanishing temperature are investigated, where a partial restoration of the anomalous \eta'-meson mass-splitting from the pions is found at approximately the chiral critical temperature.

21. 6. 2011 {$\hspace{0.5cm}$}Gerit Barth (Heidelberg)

Title: Dimensionality of Spinmodulation in Superconducting Cuprates

14. 6. 2011 {$\hspace{0.5cm}$}Konrad Schade (Heidelberg)

Title: Applications of the AdS/CFT correspondence to strongly coupled plasmas

7. 6. 2011 {$\hspace{0.5cm}$}Tina K. Herbst (Graz)

Title: The impact of fluctuations on QCD matter

31. 5. 2011 {$\hspace{0.5cm}$}Boris Nowak (Heidelberg)

Title: A life cycle of ultracold turbulence

10. 5. 2011 {$\hspace{0.5cm}$}Henning Soller (Heidelberg)

Title: Full Counting Statistics for Superconductor-Ferromagnet hybrids

Abstract: Modern electronic fabrication techniques have given us the possibility to realize various nanometer sized junctions between a vast number of different materials. Often one is not just interested in the current-voltage characteristics but also its fluctuations. These can be characterised by the full counting statistics of charge transfer. I will introduce this concept and show its application to hybrids of superconductors and ferromagnets. These are of special interest not just due to the different ordering phenomena but also due to pronounced effects of the interface. I will discuss these effects and how they can be observed in experiment. Finally I will discuss the possibility of superconducting beamsplitters with two ferromagnets.

3. 5. 2011 {$\hspace{0.5cm}$}Albrecht Werner (Jena)

Title: Ghost-curvature couplings in Quantum Einstein Gravity

Abstract: Weinberg's asymptotic-safety scenario is a candidate for a non-perturbatively renormalizable theory of quantum gravity. Since the functional renormalization group describes the (non-perturbative) flow of the quantum effective action we approach Weinberg's proposal in this spirit where the crucial step is to include all relevant couplings into the ansatz of the effective action. Therefore we investigate Faddeev-Popov ghosts arising from the gauge-fixing procedure. Although ghosts do not represent measurable physical quantities, they may influence the gauge-fixing sector at the UV fixed point. In particular we focuse on couplings of these ghosts to polynomials in the curvature scalar. We are able to formulate conditions for the fixed-point values of gravitational and cosmological constant that have to be fulfilled to allow for asymptotic safety.

26. 4. 2011 {$\hspace{0.5cm}$}Igor Boettcher (Heidelberg)

Title: Precision observables for ultracold quantum gases

19. 4. 2011 {$\hspace{0.5cm}$}Nicolai Christiansen (Heidelberg)

Title: Quantum Gravity and the Functional Renormalization Group

12.04.2011 {$\hspace{0.5cm}$}Roman Hennig (Heidelberg)

Title: Viscosity in BEC

25. 1. 2011 {$\hspace{0.5cm}$}Oliver Viehmann (Munich)

Title: On Superradiant Phase Transitions and Effective Models in Circuit QED

Abstract: Circuit QED systems of artificial atoms interacting with microwaves have been proved to behave in many respects analogously to their counterparts with real atoms in cavity QED. However, it has been predicted recently that the analogy fails if a large number of (artificial) atoms couple strongly to the electromagetic radiation [1]: Whereas for real atoms a no-go theorem rules out the possibility of a superradiant quantum phase transition as the coupling is increased [2], the standard description of circuit QED systems by an effective model based on macroscopic quantities [1,3] does allow it. We investigate the possibility of a superradiant quantum phase transition in circuit QED systems from a microscopic point of view. Our analysis shows that also circuit QED systems are subject to the no-go theorem. It hence restores the analogy of circuit QED and cavity QED and challenges the applicability of the standard description of circuit QED systems in the regime under concern. In the light of this analysis, the no-go theorem is scrutinized and confirmed in a way more adequate for realistic physical systems.

[1] P. Nataf and C. Ciuti, Nature Commun. 1, 72 (2010).

[2] K. Rzazewski, K. Wodkiewicz, and W. Zakowicz, Phys. Rev. Lett. 35, 432 (1975).

[3] A. Blais et al., Phys. Rev. A 69, 062320 (2004).

11. 1. 2011 {$\hspace{0.5cm}$}Simon Schettler (Heidelberg)

Title: Imprints of the QCD phase transition on the spectrum of gravitational waves

14. 12. 2010 {$\hspace{0.5cm}$}Denis Ludwig (Heidelberg)

Title: Loop expansion for resonant Bose-Fermi mixtures

7. 12. 2010 {$\hspace{0.5cm}$}Conrad Albrecht (Heidelberg)

Title: Induced Delocalization by Correlation and Interaction in the 1D Anderson Model

Abstract: Since P.W. Anderson formulated a disordered model where all quantum states become localized in 1D there has been various theoretical effort to properly understand this phenomenon and open questions remain up to now. Recent experiments with BECs in optical lattices provide access to proof theoretical predicitons today and therefore we study bosons in a 1D random potential with algebraic power spectrum, i.e. varying the degree of randomness in contrast to the white-noise disorder Anderson assumed for his original work. As a result, we observe the loss of localized states for sufficiently longe-ranged correlation. On the other hand we explore the impact of two-particle interactions on the localization property and discover the complex interplay between correlation and interaction in the end which can be used in experiments to engineer the localization-delocalization transition.

30. 11. 2010 {$\hspace{0.5cm}$}Joschka Beyer (Heidelberg)

Title: Cosmon and Geon: Coupled Quintessence and dark matter from dilatation anomalies?

23. 11. 2010 {$\hspace{0.5cm}$}Bertram Klein (Munich)

Title: Finite-size effects and critical phenomena in QCD

Abstract: The behavior of Quantum Chromodynamics (QCD) at large temperatures and densities is currently a very actively researched topic. Experimental efforts in heavy-ion collisions are undertaken at several large-scale facilities (RHIC at BNL, LHC at CERN and the future FAIR at GSI) to explore hadronic matter under extreme conditions.
On the theoretical side, an important method for the investigation of the thermodynamics of QCD is the simulation of the theory on a finite space-time lattice. When one investigates critical phenomena, a very important aspect is the so-called scaling behavior, which is due to long-range fluctuations at the critical point. In simulations in a finite volume, the critical behavior is affected by the volume size, and additional analysis is needed to classify the behavior.
I will discuss the analysis of critical phenomena in a finite volume, with a particular emphasis on the the phase transition in QCD with two light flavors.