The Cold Quantum Coffee brings together research students of the institute to discuss topics revolving around gauge theories, quantum gravity, cold quantum gases, solid state systems, and everything in between. The seminar is organized by students. In each seminar we have a talk of a member of the institute or an invited speaker. For further questions or in case you want to give a talk, please contact one of the organizers (Anton K. Cyrol, Manuel Reichert, Nicolas Wink and Felix Ziegler).
We are supported by the Heidelberg Graduate School of Fundamental Physics.
Date: Tuesday 16:15
Location: Institute for Theoretical Physics, Philosophenweg 16, seminar room
April 2018 
The CQC seminar continues in the summer term 2018, speakers and dates will be announced shortly.
If you want to give a talk, please write an email to the organizers.

10. 04. 2018 
40th Heidelberg Physics Graduate Days

17. 04. 2018 
Marc Steinhauser (Friedrich Schiller University, Jena) 
24. 04. 2018 
Lukas Kades (Heidelberg University) 
01. 05. 2018 
Holiday (Tag der Arbeit)

08. 05. 2018 
Bruno FaigleCedzich (Heidelberg University) 
15. 05. 2018 
Riccardo Martini (Friedrich Schiller University, Jena) 
22. 05. 2018 
TBA

29. 05. 2018 
TBA

05. 06. 2018 
TBA

12. 06. 2018 
TBA

19. 06. 2018 
TBA

26. 06. 2018 
TBA

03. 07. 2018 
TBA

10. 07. 2018 
Exact Renormalization Group 2018

17. 07. 2018 
TBA

24. 07. 2018 
TBA

31. 07. 2018 
TBA

30. 01. 2018 
Martin Pospiech (TU Darmstadt)
Slides Exploring the QCD phase diagram at finite temperature and density We discuss the structure of the QCD phase diagram at finite temperature and quark chemical potential using the Functional Renormalization Group. In the first part, I will present a Fierzcomplete NJLmodel study and show how the phase boundary is altered when Fierzincomplete ansätze are considered. In the second part of my talk, I will then show first preliminary results for the phase diagram as obtained from a study including gaugefield dynamics. Finally, I discuss future improvements. 
23. 01. 2018 
Alexander Stegemann (Goethe University, Frankfurt)
Slides FRG beyond the local potential approximation at finite temperature The functional renormalisation group (FRG) is a nonperturbative method suited to describe strongly interacting theories like quantum chromodynamics (QCD) at finite temperature and chemical potential. We apply this method to the twoflavour quarkmeson model, an effective lowenergy model for QCD. In the local potential approximation (LPA), which is the lowest order truncation of the derivative expansion, inconsistencies are observed. In this talk, I will present my recent progress on improving this truncation by including mesonic wave function renormalisations. 
16. 01. 2018 
Giovanni Rabuffo (DESY, Hamburg)
Slides Spin foam models and their renormalization: an introduction Spin foam models provide a path integral formulation for quantum gravity. They are defined on a discretization of spacetime, which can be regarded as an irregular lattice. In the first part of the talk we will get a general introduction to the spin foam models, their construction and achievements. While very successful, some important aspects of the models are still not understood. In the second part we will then face the open question of the continuum limit. How can we `zoom out' from Planck scale to large scales? How does the theory change when we build it on different discretizations? To answer these questions, the Renormalization group techniques are an ideal tool, relating theories at different scales. 
09. 01. 2018 
Igor Boettcher (Simon Fraser University, Vancouver)
Slides Complex tensor order and quantum criticality in halfHeusler superconductors A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spinorbit coupling of electrons, such as the halfHeusler compounds YPtBi or LuPdBi. Due to band inversion, the lowenergy degrees of freedom are electrons at a threedimensional quadratic band touching point with an effective spin3/2, which allows for Cooper pairs with spins ranging from 0 to 3. I will illuminate some of the unconventional superconducting properties that arise from this band structure and attractive shortrange interaction: (i) At strong coupling, the system features an swave superconducting quantum critical point with nonFermi liquid scaling of fermions and several other unusual scaling properties. (ii) The system may further undergo a transition into a phase with complex tensor order, which is a superconducting state captured by a complexvalued matrix order parameter describing Cooper pairs having spin2. Here, the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the meanfield phase structure as a function of doping and temperature, and relate our finding to experiments in YPtBi. Further, the critical properties of this new paradigm for superconductivity will be addressed. 
19. 12. 2017 
Daniel Goeschl (KarlFranzensUniversitaet Graz) Dual formulation of the SU(2) principal chiral model at finite density Monte Carlo simulations are a powerful quantitative tool for obtaining nonperturbative insight into quantum field theories. However, in the conventional lattice formulation of a QFT the sign problem restricts the applicability of Monte Carlo techniques to zero density. In recent years, dual representations have proven to circumvent the sign problem in various lattice field theories by an exact mapping of the theory to new degrees of freedom. In this dual formulation, matter fields are represented by loops and gauge fields by sheets of plaquettes. In this talk we derive the dualization of the SU(2) principal chiral model by using the recently developed Abelian color flux approach. We show that the inclusion of chemical potentials does not give rise to a sign problem in the dual formulation, hence, allowing for a simulation at finite density. 
12. 12. 2017 
Esther Weil (JLU Gießen)
Slides DysonSchwinger 101  Research in the DSE approach 
05. 12. 2017 
Alexander Lehmann (Heidelberg University)
Slides NRQCD + Classical Statistical Fields: RealTime Evolution of Heavy Quarkonium Bound States In order to investigate the timeevolution of the fireball in heavy ion collision from the initial glasma state to the quark gluon plasma over the hadronization until the final detection, usually several subsequent schemes are used, where one method provides the initial conditions for the subsequent one. But yet for the earliest time evolution after the collision assumptions are made for the "second" to be used scheme  a working very first scheme still has to be provided. We aim at providing such a scheme for the time evolution of heavy quarkonia, namely bottomonium. We argue that the highoccupancy of the gluon fields enables the use of classical statistical evolution schemes of the gauge fields. On top of that, we use combine it with NRQCD, an effective field theory for nonrelativistic particles. I present the physical setup, the solution idea, some technical details for solving the appearing differential equation systems and some test results of our integration schemes. 
21. 11. 2017 
Eduardo Grossi (Heidelberg University)
Slides Causality of fluid dynamics for highenergy nuclear collisions Dissipative relativistic fluid dynamics is not always causal. We discuss the causality structure of high energy nuclear collision. When the fluid evolution equations are hyperbolic, one can bring them to a characteristic form describing the radial expansion of a fireball. This dynamics is causal if the characteristic velocities are smaller than the speed of light. We obtain a concrete inequality from this constraint and discuss how it can be violated for certain initial conditions. We argue that causality poses a bound to the applicability of relativistic fluid dynamics. 
14. 11. 2017 
Johannes Lumma (Heidelberg University)
Slides Higgs portal to scalar dark matter in asymptotically safe quantum gravity We investigate asymptotic safety of a singletscalar extension of a toy model of the Higgs sector including two real scalar fields under the impact of quantum gravity fluctuations. Employing functional renormalization group techniques, we search for fixed points of the system which feature quantum scale invariance and provide a tentative ultraviolet completion of the system. We find that in a particular regime of the gravitational parameter space all couplings in the scalar sector including the mass parameters become irrelevant at the ultraviolet fixed point. The infrared physics that can be reached from that fixed point is fully predicted and features no free parameters. In the remainder of the gravitational parameter space, the values of all quartic couplings in our model are predicted in terms of the two mass parameters. In light of these results, we discuss possible scenarios where the singletscalar can be a dark matter candidate. 
24. 10. 2017 
Andreas Elben (IQOQI Innsbruck)
Slides Renyi Entropies from Random Quenches in Atomic Hubbard and Spin Models In this talk, I discuss a technique for measuring nonlinear functionals of a manybody density matrix, such as Renyi entropies with direct connection to entanglement, without measuring and reconstructing the whole density matrix (i.e. without performing full quantum state tomography). The approach, which has direct connection to Random Matrix Theory and quantum chaos, consists in implementing an ensemble of random unitary evolution operators, applying them on the measured manybody state and extracting the desired functions from ensemble averaged observables [1]. Investigating the generation of such random unitary evolution operators and the scaling of errors in possible experiments, I show that our approach is readily implementable with current technology and widely applicable, in particular in systems where full state tomography is not available. Concretely, I present applications in one and twodimensional Fermi (Bose) Hubbard models and Spin models as realized by Rydberg atoms or trapped Ions. [1] S. J. van Enk and C. W. J. Beenakker, Phys. Rev. Lett. 108, 110503 (2012) 
17. 10. 2017 
Nicolas Wink (Heidelberg University)
Slides Finite temperature npoint functions from analytic continuation A formalism for the self consistent calculation of general nonperturbative npoint functions at real times from analytic continuation within the FRG framework is presented. Specifics concerning the analytic continuation of vertices and their spectral representation are discussed at the example of a scalar theory. It is shown how they can be solved numerically in a convenient manner by reformulating the problem as an integral equation for the spectral densities. 