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

## 23. 01. 2018 |
Alexander Stegemann (Goethe University, Frankfurt) ## FRG beyond the local potential approximation at finite temperatureThe functional renormalisation group (FRG) is a non-perturbative method suited to describe strongly interacting theories like quantum chromodynamics (QCD) at finite temperature and chemical potential. We apply this method to the two-flavour quark-meson model, an effective low-energy 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. |

## 30. 01. 2018 |
Martin Pospiech (TU Darmstadt) ## Exploring the QCD phase diagram at finite temperature and densityWe 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 Fierz-complete NJL-model study and show how the phase boundary is altered when Fierz-incomplete 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 gauge-field dynamics. Finally, I discuss future improvements. |

## 16. 01. 2018 |
Giovanni Rabuffo (DESY, Hamburg)
Slides ## Spin foam models and their renormalization: an introductionSpin 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 half-Heusler superconductorsA revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling of electrons, such as the half-Heusler compounds YPtBi or LuPdBi. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin-3/2, 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 short-range interaction: (i) At strong coupling, the system features an s-wave superconducting quantum critical point with non-Fermi 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 complex-valued matrix order parameter describing Cooper pairs having spin-2. Here, the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the mean-field 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 (Karl-Franzens-Universitaet Graz) ## Dual formulation of the SU(2) principal chiral model at finite densityMonte Carlo simulations are a powerful quantitative tool for obtaining non-perturbative 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 ## Dyson-Schwinger 101 - Research in the DSE approach |

## 05. 12. 2017 |
Alexander Lehmann (Heidelberg University)
Slides ## NRQCD + Classical Statistical Fields: Real-Time Evolution of Heavy Quarkonium Bound StatesIn order to investigate the time-evolution 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 high-occupancy 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 non-relativistic 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 high-energy nuclear collisionsDissipative 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 gravityWe investigate asymptotic safety of a singlet-scalar 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 singlet-scalar can be a dark matter candidate. |

## 24. 10. 2017 |
Andreas Elben (IQOQI Innsbruck)
Slides ## Renyi Entropies from Random Quenches in Atomic Hubbard and Spin ModelsIn this talk, I discuss a technique for measuring nonlinear functionals of a many-body 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 many-body 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 two-dimensional 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 n-point functions from analytic continuationA formalism for the self consistent calculation of general non-perturbative n-point 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. |