# Karolina Antoniak Xyz Homework

## Institute for Theoretical Physics • University of Cologne

## Past Events

Statistical Physics Seminar March 09, 2018, 11:00 Seminarraum Theorie Altbau | Claudio Feliciani, The University of Tokyo, Research Center for Advanced Science and Technology Measurement and numerical modeling of pedestrian flows In the recent years, a growing interest on pedestrian traffic has led to a better knowledge on the movement of large crowds inside public facilities such as transportation hubs and recreational halls. However, there are still many aspects which are unknown and, especially when psychological aspects arise, managing and controlling large crowds is a challenging task. In this talk, classical methods to analyze and measure pedestrian crowds will be introduced and discussed. We will start with the bidirectional flow (for example people moving in a corridor), which, although rather simple, presents most of the characteristics of pedestrian dynamics, in particular regarding the presence of collision avoidance and emergent phenomena (in the form of lanes). Later the discussion will be extended to more complex cases, with the chaotic motion being the least organized. In the presentation a focus will be set on methods to analyze the level of congestion and the intrinsic risk of human crowds by also discussing methods to simulate pedestrian behavior with the aim of predicting future changes. Novel techniques (for example the use of inertial data from portable devices) will be presented showing advantages and limitations compared to classical approaches typically taken for pedestrian traffic and more generally for transportation systems. |

March 06, 2018, 14:00 Seminar room 0.02, ETP | Alvise Bastianello, SISSA, Trieste Moving Impurities & Quantum Sonic Bangs It is a well-known fact that the spreading of information in lattice quantum systems is not instantaneous, but it rather exists a maximum velocity dictated by the Lieb-Robinson bound. The existence of such a lightcone deeply affects equilibrium properties as well as out-of-equilibrium ones, which have been a subject of outstanding interest in the recent years. In this talk, a novel out-of-equilibrium protocol critically affected by the presence of a maximum velocity is proposed and discussed. Specifically, a one dimensional lattice model is considered, where a localized impurity is suddenly created and then dragged at a constant velocity. Focussing on a simple, but far from trivial, free model the response of the system at late times is analyzed, with emphasis on its transport properties. The finite maximum velocity is responsible for a rich phenomenology, for which exact results are provided. Taking into account the experience acquired so far, more general models are discussed and unpublished results presented, with exact predictions in completely generic (non integrable) one dimensional lattice systems. Reference paper: A. Bastianello, A. De Luca, Phys. Rev. Lett. 120, 060602 (2018). |

Condensed Matter Theory seminar February 28, 2018, 10:00 Seminar Room 0.03, ETP | Eddy Ardonne, Stockholm University Exact (ground) states and zero-modes of interacting Spin and Clock models In this blackboard talk, I will first review the old results of Peschel en Emery, who devised a set of interacting spin-1/2 models, for which the degenerate ground states can be written in terms of product states. This model has more interesting properties, such as excited states that can be constructed explicitly, and one can find exact, local operators that swap the ground states. These results can be generalized to three state Clock models, and models with arbitrary spin. If time allows, I will comment on how to adapt the construction to arbitrary lattices. |

SFB 1238 February 27, 2018, 14:30 SR II. Physik | Davide Bossini, TU Dortmund Femtosecond manipulation of magnets via photoconducted magnons at the edge of the brillouin zone Contact Person: Paul van Loosdrecht |

SFB 1238 February 27, 2018, 15:30 SR Physics II | Yuichiro Ando, Kyoto University Spintronic devices based on topological insulators Contact Person: Yoichi Ando |

Condensed Matter Theory seminar February 26, 2018, 14:00 Seminar Room 0.03, ETP | Jan Mueller Magnetic Skyrmions and Topological Domain Walls (PhD Defense) |

QM2 - Quantum Matter and Materials February 26, 2018, 11:00 Seminar Room 0.03, ETP | Jairo Sinova, Mainz University Topological Antiferromagnetic Spin-orbitronics: A Dormant Giant Awakens Antiferromagnets and ferromagnets represent two fundamental forms of magnetism with antiferromagnets being the more abundant of the two. However, it has been notoriously difficult to manipulate and detect antiferromagnetic order by any practical means due to the compensated magnetic moment. This has left antiferromagnets over their hundred-year history poorly explored, in striking contrast to the thousands of years of fascination and utility of ferromagnets. This has changed with the proposal and subsequent discovery of a new relativistic spin-torque phenomenon, the Neél spin-orbit torque, that allow us to efficiently control antiferromagnetic moments in spintronic devices. This allows for antiferromagnets to become active elements in devices. An additional concept that has emerged is that antiferromagnets provide a unifying platform for realizing synergies among three prominent fields of contemporary condensed matter physics: Dirac quasiparticles and topological phases. These recent developments have unlocked a multitude of known and newly identified unique features of this "dormant-giant" class of materials that the community is beginning to explore. |

Master colloquium February 19, 2018, 12:00 Seminarraum Theoretische Physik (Altbau) | Maike Schön Physical properties of entangled Majorana fermion states on textured surfaces of topological insulators Contact Person: Dmitry Bagrets |

UoC Forum on Interacting Particle Systems February 05, 2018, 10:00 TP seminar room 0.03 | Semyon Klevtsov, Mathematical Institute, Cologne Mathematics of the Fractional Quantum Hall wave functions Quantum Hall effect is one of the most interesting examples quantum many-particles systems. It occurs in certain two-dimensional electron systems at low temperatures and in high magnetic fields, which exhibit plateaux with the quantized values of the Hall conductance. The Fractional Quantum Hall effect (FQHE), when the Hall conductance takes on fractional values, is an example of the strongly-interacting quantum many-particles. The standard physics approach to the FQHE is to assign a certain many-body wave function to each plateaux. I will talk about a program as to how one can use a combination of probabilistic, asymptotic and geometric methods to learn more about the physics and mathematics of the FQHE wave functions, in particular, describe the electromagnetic and gravitational responses, asymptotics for a large number particles, novel quantized coefficients for the adiabatic transport. |

UoC Forum on Interacting Particle Systems February 05, 2018, 11:05 TP seminar room 0.03 | Jean-Sebastian Bernier, University of Bonn Propagation of correlations in dissipative systems: ballistic, diffusive and aging dynamics In recent years, considerable experimental efforts have been devoted to dynamically generate complex states and monitor their evolution. Despite remarkable advances, the theoretical principles behind the non-equilibrium dynamics of strongly correlated quantum matter are still far from being fully understood. In particular, very few studies have sought to clarify the influence of environmental couplings on the propagation of correlations. We attempt here to fill this gap. Considering first an interaction quench in the Bose-Hubbard model under the effect of dephasing, we observe that dissipation effectively speeds up the propagation of single-particle correlations while reducing their coherence. In contrast, for two-point density correlations, the initial ballistic propagation regime gives way to diffusion at intermediate times. In a second time, considering the XXZ spin-1/2 model in contact with a similar environment, we find this system to display aging. A dynamical phenomenon characterized by a breakdown of time-translation invariance, a slow non-exponential relaxation of two-time correlations and the presence of dynamical scaling. |

UoC Forum on Interacting Particle Systems February 05, 2018, 14:30 TP seminar room 0.03 | Wojciech de Roeck, KU Leuven Dynamics and slowdown of quantum spin systems I will discuss several results (also in the mathematical sense) about the dynamics of quantum spin systems. The main theme will be 'localization' which is a term that has been much in vogue in the last years. Usually, it is understood as 'absence of dissipative behaviour'. I will explain what this means and how it comes about. The phenomenon that is probably to be expected to apply more broadly than 'localization' is 'sparsity of resonances', sometimes also called 'asymptotic localization' or 'quasi-localization'. It manifests itself in quantum and classical systems, where transport and thermalization take place only due to effects that are non-perturbative (smaller than any power) in the natural parameters of the system. |

UoC Forum on Interacting Particle Systems February 05, 2018, 16:20 TP seminar room 0.03 | Sebastian Diehl, ITP From Micro- to Macrophysics in Driven Open Quantum Systems Recent developments in diverse areas - ranging from cold atomic gases over light driven semiconductors to microcavity arrays - move systems into the focus, which are located on the interface of quantum optics, many-body physics and statistical mechanics. These "driven open quantum systems" share in common that coherent and driven-dissipative dynamics occur on an equal footing, creating genuine non-equilibrium scenarios without immediate counterpart in equilibrium condensed matter physics. A case in point are so-called exciton-polaritons in two spatial dimensions. We briefly explain the physical basis, their description, and how to detect the violation of thermal equilibrium conditions in the formalism. We then show that a paradigmatic hallmark of low-temperature equilibrium systems -- the presence of quasi-long range order, i.e. the algebraic decay of spatial correlation functions -- must be absent due to non-equilibrium conditions. This conclusion is drawn based on a connection to the problem of surface roughening. |

UoC Forum on Interacting Particle Systems February 05, 2018, 15:20 TP seminar room 0.03 | Gunter Schütz, FZ Jülich Exact density matrix for the dissipative Heisenberg quantum spin chain We demonstrate that the exact nonequilibrium steady state of the one-dimensional Heisenberg XXZ spin chain driven by boundary Lindblad operators can be constructed explicitly with a matrix product ansatz for the nonequilibrium density matrix. For spin 1/2 the matrices satisfy the quantum algebra U_q[sl(2)]. For the isotropic Heisenberg chain, coupled at the ends to boundary reservoirs polarized in different directions with twist angle theta, we calculate the exact magnetization profiles and magnetization currents in the nonequilibrium steady state of a chain N sites. For large N the in-plane steady-state magnetization profiles are harmonic functions with a frequency proportional to the twist angle. In-plane steady-state magnetization currents are subdiffusive, while the transverse current saturates when the coupling strength is sufficiently large. For the anisotropic chain we find a current resonance at the specific values of the anisotropic interaction strength where the transverse current is independent of system size, even for non-integrable higher-spin chains. |

Theoretisch-Physikalisches Kolloquium February 02, 2018, 16:30 SR 0.03 TP | Paul Busch, University of York Measurement uncertainty relations for qubits: theory and experiment In standard formulations of the uncertainty principle, two fundamental features are typically cast as impossibility statements: two noncommuting observables cannot in general both be sharply defined (for the same state), nor can they be measured jointly. The pioneers of quantum mechanics were acutely aware and puzzled by this fact, and it motivated Heisenberg to seek a mitigation, which he formulated in his seminal paper of 1927. He provided intuitive arguments to show that the values of, say, the position and momentum of a particle can at least be unsharply defined, and they can be measured together provided some approximation errors are allowed. Only now, nine decades later, a working theory of approximate joint measurements is taking shape, leading to rigorous and experimentally testable formulations of associated error tradeoff relations. Here we briefly review this new development, explaining the concepts and steps taken in the construction of optimal joint approximations of pairs of incompatible observables. As a case study, we deduce measurement uncertainty relations for qubit observables using two distinct error measures. We provide an operational interpretation of the error bounds and discuss some of the first experimental tests of such relations. The talk is based on the review paper arXiv:1512.00104v3. |

Condensed Matter Theory seminar February 02, 2018, 14:00 Seminar Room 0.03, ETP | Bjoern Ladewig Non-equilibrium Phase Transitions in the presence of fluctuation less States (Master colloquium) |

January 31, 2018, 16:00 Seminar Room 0.03, ETP | Markus Mueller Could the physical world be emergent instead of fundamental, and why should we ask? In physics, there is the prevailing intuition that we are part of a unique external world, and that the goal of physics is to understand and describe this world. This assumption of the fundamentality of objective reality is often seen as a major prerequisite of any kind of scientific reasoning, delineating science from pseudoscience, and explaining why successful empirical science is possible in the first place. However, here I argue that we should consider relaxing this assumption in a specific way in some contexts. Namely, there is a collection of open questions in and around physics that can arguably be addressed in a substantially more consistent and rigorous way if we consider the possibility that the first-person perspective is ultimately more fundamental than our usual notion of external world. These are questions like: which probabilities should an observer assign to future experiences if she is told that she will be simulated on a computer? How should we think of cosmology’s Boltzmann brain problem or assign probabilities to properties of ‘possible worlds’? What can we learn from the fact that measurements in quantum theory seem to do more than just reveal preexisting properties? Why are there simple computable laws of physics in the first place? In the talk, I sketch a mathematically rigorous approach along these lines, suggesting a simple and unified framework (rooted in algorithmic information theory) to address questions like those above. It is not meant as a ‘theory of everything’ (in fact, it predicts its own limitations), but it shows how a notion of objective external world, looking very much like our own, can provably emerge from a starting point in which (only) the first-person perspective is primary. Based on arXiv:1712.01826 (full version) and arXiv:1712.01816 (short version). |

Großes Physikalisches Kolloquium January 30, 2018, 16:45 HS III | Pierre Le Doussal, ENS Paris Pinning, growth and memory(ies) Starting from examples of experimental systems which exhibit growth and pinning I will give an introduction into the physics of elastic systems with quenched disorder in non-equilibrium situations. We review developments in the description of depinning and avalanches, as well as in the study of the Kardar-Parisi-Zhang (KPZ) class of stochastic growth. We introduce some of the analytical methods which lead to predictions testable (and sometimes tested) in experiments. Our last example is the calculation of memory effects in the KPZ class in expanding geometries which illustrates the remarkable connections between growth and disordered systems. |

Condensed Matter Theory seminar January 26, 2018, 14:00 Seminar Room 0.03, ETP | Denis Golez, University of Fribourg Relaxation dynamics in Mott insulators: the role of collective modes Strong correlations between spin, charge and orbital degrees of freedom play an important role in materials and a recent development of ultrafast spectroscopies enabled to disentangle these relevant degrees of freedom by their temporal evolution. I will start with a summary of the charge carrier relaxation after the photo-excitation in Mott insulators described within the dynamical mean field theory (DMFT) and continue how this formalism can be extended to including the role of dynamical screening and non-local fluctuations (GW+EDFMT)[1,2]. Then I will open the question how to use the laser pulse to manipulate screening in Mott insulators. As an extreme example I will present a self-trapping of the system in the negative temperature state by a proper manipulation of the screening environment, which leads to the enhanced subgap response in the charge susceptibility. This population inversion leads to the low-energy anti-screening and I will comment on its experimental relevance. In the second part I will shed light on the role of spin fluctuations in the relaxation dynamics, which can be analysed by an extension of DMFT[3]. I will exemplify how optical pump probe techniques can be used to detect some basic theoretical ideas in higher dimensional doped antiferromagnets, like string states, Trugman paths and the lack of spin-charge separation. At the end I will provide an outlook how to extend these tools to an ab-initio description of strongly correlated materials out of equilibrium. [1] D. Golez, M. Eckstein, and P. Werner. Phys. Rev. B, 92:195123, Nov (2015). [2] D. Golez, L. Boehnke, H. U. R. Strand, M. Eckstein, and P. Werner. Phys. Rev. Lett. 118:246402 (2017). [3] N. Bittner, D. Golez, M. Eckstein, P. Werner, in preparation. |

SFB 1238 January 24, 2018, 14:30 Seminar Room of the Institute of Physics II | Istvan Kezsmarki, Uni Augsburg Unidirectional light propagation in multiferroics and multi-antiferroics Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in several multiferroic compounds [1]. In conventional media light propagation is reciprocal, that is counter-propagating beams experience the same refractive index. However, reciprocity can be violated in multiferroic materials, where the refractive index depends not only on the polarization of light but also on the +- k direction of the propagation [1]. Such unidirectional transmission is the consequence of the dynamic magnetoelectric effect emerging in materials with simultaneously broken time reversal and spatial inversion symmetries. This phenomenon, exclusively observed in multiferroic and magnetoelectric materials [2-5], may allow the development of optical diodes transmitting unpolarized light in one, but not in the opposite, direction [5]. Recently, the emergence such unidirectional light propagation, governed the dynamic magnetoelectric effect, has also been demonstrated in multi-antiferroics [6], i.e. in materials with coexisting purely antiferroelectric and antiferromagnetic orders. References [1] D. Szaller et al., Phys. Rev. B 87, 014421 (2014) [2] I. Kezsmarki et al., Phys. Rev. Lett. 106, 057403 (2011) [3] S. Bordacs et al., Nat. Phys. 8, 734 (2012) [4] I. Kezsmarki et al., Nat. Commun. 5, 3203 (2014) [5] I. Kezsmarki et al., Phys. Rev. Lett. 115, 127203 (2015) [6] V. Kocsis et al., arXiv:1711.08124 (2017) |

Großes Physikalisches Kolloquium January 23, 2018, 15:00 HS I | Johan Elf, Lund University Kinetics of dCas9 target search in Escherichia coli Contact Person: Tobias Bollenbach |

Theoretisch-Physikalisches Kolloquium January 19, 2018, 16:30 TP seminar room 0.03 | Jürgen Berges, University of Heidelberg Universality far from equilibrium: From the early universe to ultracold quantum gases In recent years there have been important advances in understanding isolated quantum systems far from equilibrium. Prominent examples include the (pre-)heating process in the early universe after inﬂation, the initial stages in collisions of relativistic nuclei at giant laboratory facilities, as well as table-top experiments with ultracold quantum gases. Even though the typical energy scales of these systems vastly diﬀer, they can show very similar dynamical properties. Certain characteristic numbers can even be quantitatively the same, defining nonthermal universality classes. One may use this universality to learn from experiments with cold atoms aspects about the dynamics during the early stages of our universe. |

Condensed Matter Theory seminar January 19, 2018, 14:00 Seminar Room 0.03, ETP | Enej Ilievski, University of Amsterdam Hydrodynamic equation for thermodynamic classical and quantum soliton gases Exactly solvable nonlinear wave equations -- colloquially known as the soliton systems -- are widely regarded as one of the greatest achievements of mathematical physics. But somehow, aside of several mathematical frameworks and other formal aspects, not a lot seems to be known about statistical properties of classical integrable field theories and, in particular, classical transport properties at finite temperature. In this talk, we present a kinetic equation to deal with dense soliton gases, expressed in terms of a linear integral dressing equation for the soliton spectral function which accounts for renormalization of the soliton velocities due to the interactions with a soliton many-body state. This is accomplished in the framework of the algebro-geometric integration technique which permits to classify all quasi-periodic solutions of an integrable equation of motion in terms of the moduli of finite-genus Riemann surfaces. By identifying soliton excitations, applying Born-Sommerfeld quantization for soliton orbits, extracting the two-body S-matrix, and finally taking the thermodynamic finite-density, the free energy functional is expressed as the saddle point of the action-space path-integral. Our hydrodynamic equations can understood as the thermodynamic analogue of the celebrated Whitham's modulation equations. The equations are universal, and even apply to the quantum theories of solitons. If time permits, we show how to obtain a closed compact formula for the Drude weight in the quantum Heisenberg spin chain, and discuss peculiarities related to it. |

Statistical Physics Seminar January 18, 2018, 17:00 Seminarraum Theoretische Physik (Altbau) | Timothy Halpin-Healy, Barnard College, Columbia University Within & Beyond the Realm of KPZ We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang, a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as non-equilibrium stochastic growth, extremal paths through disordered landscapes, driven lattice gases & dissipative condensates, as well as the statistics of random matrices & permutations. |

SFB 1238 January 17, 2018, 14:30 Seminar room of the Institute of Physics 2 | Mathias Wickleder, Department of Chemistry, University of Cologne Oxoanionic Compounds Syntheses, Properties, Perspectives Contact Person: M. Braden |

Theoretisch-Physikalisches Kolloquium January 12, 2018, 16:30 TP seminar room 0.03 | Michael Klatt, Karlsruhe Institute of Technology Universal hidden order in amorphous cellular geometries Starting from an amorphous partitioning of space into cells, we iteratively optimize the `centrality' of the cells, minimizing the so-called Quantizer energy. The energy landscape is replete with local minima to which the system converges despite the existence of lower-energy crystalline configurations. Irrespective of the level and type of disorder in the initial configurations, the tessellations converge to the same amorphous state, as measured by the same structure factor and energy distributions. The final disordered configurations exhibit an anomalous suppression of long-wavelength density fluctuations, known as hyperuniformity. For systems related to the Quantizer problem, such as selfassembled copolymeric phases, our findings suggest the possibility of stable disordered hyperuniform phases. |

Condensed Matter Theory seminar January 12, 2018, 14:00 Seminar Room 0.03, ETP | Carolin Wille, FU Berlin A tensor network approach to topological quantum phases Tensor network states, and in particular projected entangled pair states, play an important role in the description of strongly correlated quantum lattice systems. They do not only serve as variational states in numerical simulation methods, but also provide a framework for classifying phases of quantum matter and capture notions of topological order in a stringent and rigorous language. In this talk I will present how virtual symmetries in tensor networks, summarized by the framework of matrix product operator (MPO) injectivity, are substantial to the classification of not only bosonic but also fermionic topological order in two dimensional systems. I will briefly discuss how this fact relates to Levin-Wen string net models and Tuarev-Viro state sum constructions. For the sake of concreteness, two examples of fermionic topological order, the fermionic toric code and the Majorana dimer model, are discussed using the language of fermionic tensor networks. |

QM2 - Quantum Matter and Materials January 10, 2018, 14:30 Seminar room of the Institute of Physics II | Stefan Floerchinger, University of Heidelberg Dynamics of entanglement in expanding quantum fields We develop a novel real-time approach to computing the entanglement between spatial regions for Gaussian states in quantum field theory. The entanglement entropy is characterized in terms of local correlation functions on space-like Cauchy hypersurfaces. The framework is applied to explore an expanding light cone geometry in the particular case of the Schwinger model for quantum electrodynamics in 1+1 space-time dimensions. We observe that the entanglement entropy becomes extensive in rapidity at early times and that the corresponding local reduced density matrix is a thermal density matrix for excitations around a coherent field with a time dependent temperature. Since the Schwinger model successfully describes many features of multiparticle production in electron-positron collisions, our results provide an attractive explanation in this framework for the apparent thermal nature of multiparticle production even in the absence of significant final state scattering. |

Großes Physikalisches Kolloquium January 09, 2018, 16:45 HS III | Rupert Huber, Regensburg Faster than a cycle of light Contact Person: Paul van Loosdrecht |

Condensed Matter Theory seminar December 22, 2017, 15:00 Seminarraum E0.03 | Aris Alexandradinata, Yale Unveiling the hidden topology in the Fermi-surface wavefunction of metals A metal is a solid with a Fermi surface. It is known how to reconstruct the shape of the Fermi surface – by immersing the metal in a magnetic field and measuring the period of field-induced oscillations of the magnetization/resistivity. I will show how to extract information about the quantum-mechanical wavefunction of the Fermi surface – from measuring the phase offset of these same oscillations. In some metals, this information is robust against deformations of the Hamiltonian (describing the metal), and may therefore be viewed as a topological invariant. |

SFB 1238 December 21, 2017, 11:00 SR II. Physik | Philip Hofmann, University Aarhus ELECTRONIC STRUCTURE AND ELECTRON DYNAMICS IN TWO-DIMENSIONAL DIRAC MATERIALS Contact Person: Thomas Michely |

Condensed Matter Theory seminar December 21, 2017, 16:00 Seminarraum E0.03 | Leonardo Mazza, ENS Paris Majorana fermions in particle-conserving settings The paradigmatic condensed-matter models where zero-energy localized Majorana fermions have been studied so far have the distinguishing feature of not conserving the number of fermions. Moreover, the accepted definition of "Majorana fermion" naturally belongs to this scenario. Is it possible to discuss Majorana fermions in "canonical" particle-conserving settings? In this seminar I will present several exact and numerical results on Majorana fermions in particle-conserving scenarios. I will start from the discussion of a model for bosons and fermions where, in a proper limit, the physics of the celebrated Kitaev's chain appears. I will continue by presenting exact results on Majorana fermions in ladder models where the two legs of the system can only exchange pair of particles. Finally, I will comment on the possibility of making experiments with Majorana fermions in particle-conserving settings using cold atoms. |

QM2 - Quantum Matter and Materials December 20, 2017, 14:30 Seminar Room of the Institute of Physics II | Igor Boettcher Complex tensor order and quantum criticality in spin-orbit coupled superconductors A 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. |

Großes Physikalisches Kolloquium December 19, 2017, 16:45 HS III | Christof Wetterich, Heidelberg Quantum Gravity, Dark Energy, and the Origin of the Universe Contact Person: Sebastian Diehl |

Condensed Matter Theory seminar December 18, 2017, 14:00 Seminar Room 0.03, ETP | Pieter Naaijkens, RWTH Aachen The mathematics of topologically ordered phases One of the most interesting properties of topologically ordered models with long ranged entanglement, such as Kitaev's toric code, is that they have anyonic excitations. The properties of these anyons can be recovered from first principles using techniques from mathematical physics, making it possible to put conjectures about such systems in a precise mathematical language. After explaining the main ideas behind this construction, I will discuss recent results on the stability of the excitation structure and on applications to quantum information theory. |

Condensed Matter Theory seminar December 18, 2017, 16:00 Seminar Room 0.03, ETP | Christian Gogolin, ICFO Barcelona Pure state quantum statistical mechanics - an overview In this talk I will given an overview of pure state quantum statistical mechanics, which is a new way of understanding issues at the foundation of statistical mechanics and thermodynamics. In particular, I will explain recent developments concerning equilibration and thermalization in closed quantum many-body systems. We will see how equilibration and thermalization can be defined in unitarily evolving and finite dimensional quantum systems, under which conditions they can be proved to happen, and what we know about the apparent equilibrium states. I will further speak about general results on structural properties both short- and long-range interacting many-body systems, with a focus on the decay of correlations in their thermal states. Finally, I will provide a glimpse of ongoing work concerning the use of machine learning tools form many-body physics and point out opportunities for future work. |

Condensed Matter Theory seminar December 15, 2017, 14:00 Seminar Room 0.03, ETP | Yuta Murakami, University of Fribourg Periodic driving of correlated systems: high-harmonic generation in the Mott insulator and parametric phonon excitation in superconductors Driving systems out of equilibrium can provide new ways to control properties and extract new functions. Recent development of intense laser in a wide frequency range has led to intriguing observations such as light-induced superconductivity-like behaviors above the transition temperature [1], enhancement of the excitonic condensation [2] and high-harmonic generation in solid states [3]. Motivated by this intriguing situation, we have been developing nonequilibrium methods based on the dynamical mean-field theory (DMFT). Here we would like to present recent two applications of the nonequilibrium DMFT for the time-periodic steady states (Floquet DMFT). In the first part, we discuss the high-harmonic generation (HHG) in the Mott insulator under periodic AC driving [4]. We show that qualitative behavior of the HHG spectrum is different between weak and strong field regimes, which originates from qualitative difference in doublon/holon dynamics under the driving. We also discuss the similarity and the difference of the HHG intensity in the Mott insulator compared to that of semiconductors and disordered systems. In the second part, we discuss the effects of the parametric-phonon driving on conventional SC, which has been considered to enhance SC[5][6]. By studying transient dynamics and steady states, we demonstrate that even though the attractive interaction can be enhanced by the driving, SC is always suppressed, in particular, at the parametric resonance. Our systematic analysis shows that, in a wide parameter range, the heating of the system is the dominant effect and the parametric phonon driving has a negative effect on SC. [1] M. Mitrano et al., Nature 530, 461 (2016). [2] S. Mor et al., PRL 119, 086401 (2017). [3] S. Ghimire et al., Nat. Phys. 7, 138 (2011). [4] Y. Murakami, M. Eckstein, P. Werner to be published. [5] M. Knap et al., PRB 94, 214504 (2016); M. Babadi et al., PRB 96, 014512 (2017). [6] Y. Murakami et al, PRB 96, 045125 (2017). |

Condensed Matter Theory seminar December 13, 2017, 10:00 Seminar Room 0.03, ETP | Gabor Halasz, KITP Parton constructions and realistic spin models of fracton topological orders Fracton phases are gapped quantum phases in three dimensions that go beyond the standard paradigm of topological order; they have fractionalized excitations that are immobile or only mobile along lower-dimensional subsystems, such as lines or planes. While there are several exactly solvable models of such fracton phases, these models are far from realistic because they involve interactions between many spins at the same time. By generalizing the fermionic parton construction, a standard phenomenological description of fractionalization in quantum phases, we provide simple variational states capturing fracton phases. Moreover, by showing that each variational state is the asymptotic strong-coupling ground state of a corresponding coupled-spin-chain model, we demonstrate that a large class of fracton phases can be realized in more realistic models involving only two-spin interactions. |

QM2 - Quantum Matter and Materials December 13, 2017, 14:30 Seminar Room of the Institute of Physics II | Francois Dubin Imaging the Superfluid Crossover of Trapped Two-Dimensional Dipolar Excitons State-of-the-art nano-fabrication process allow to control the spatial confinement of electronic carriers, with close to atomic precision. Structures are thus designed to separate spatially oppositely charged electrons and holes, notably bilayer heterostructures. When electrons and hole are separated in such adjacent layers they experience a strong Coulomb attraction pairing them into spatially indirect excitons, i.e. boson-like particles, which exhibit a giant electric dipole of about 500 Debye. Excitons then experience a repulsive dipolar potential stabilizing cold gases against collapse while providing access to a rich variety of collective quantum phases, possibly ranging from superfluidity to supersolidity [1]. Here, first signatures for the superfluid crossover of such dipolar excitons are reported [2]. When confined in a microscopic trap we show that they realize a four-component superfluid at sub-Kelvin temperatures, distributed between two optically active and two optically inactive spin states. By imaging the condensate bright part, we study in-situ the profiles of the exciton density and the phase coherence in the trap. We thus reveal quantum spatial coherence, in a sub-Kelvin regime bound to very dilute densities and probably limited by the strength of dipolar interactions. Also, we evidence quantized vortices, efficiently trapped in the slight electrostatic disorder of our trapping potential [2]. Analyzing the interplay between quasi long-range order, vortex formation, and density profiles across the range of explored parameters it is finally shown that our experimental findings provide a direct evidence for a Berezinskii-Kosterlitz-Thouless crossover [3]. The work presented here results from contributions of S.Dang, R.Anankine, M.Beian, M.Alloing, E.Cambril, A.Lemaitre and M.Holzmann. |

Condensed Matter Theory seminar December 07, 2017, 14:00 Seminarraum Alte Theorie | Adam Smith, University of Cambridge Disorder-Free Localization Contact Person: Achim Rosch |

QM2 - Quantum Matter and Materials December 06, 2017, 14:30 Seminar Room of the Institute of Physics II | Lukasz Plucinski, FZ Juelich Band structure engineering in 3D topological insulators Contact Person: Alexander Grueneis |

Großes Physikalisches Kolloquium December 05, 2017, 16:45 HS III | Satya Majumdar, Paris-Sud KPZ story The celebrated KPZ equation (Kardar, Parisi, Zhang, 1986) is an important milestone in statistical physics, originally introduced to describe the late time dynamics in two dimensional growth models. Over the last 30 years, the KPZ story has evolved in various interesting directions, making links on the way to different areas of physics and mathematics. This includes in particular the link to the famous Tracy-Widom distribution in random matrix theory. The story of KPZ is a very successful one, involving theoretical physics, mathematics and experiments--a fertile playground for interdisciplinary science. In this talk, I will review the evolution of the KPZ story, pointing out the important landmarks as I go along. At the very end, I will discuss some recent developments establishing a nice link between the KPZ height fluctuations and the edge physics in cold atom systems. |

Condensed Matter Theory seminar December 01, 2017, 14:00 Seminar Room 0.03, ETP | Tobias Meng, TU Dresden The gravity of Weyl: tabletop signatures of the mixed axial-gravitational anomaly Topological semimetals have recently been one of the most active topics in solid state physics, and provide an exceptionally fruitful meeting point for fields as diverse as chemistry, band structure calculations and high-energy field theory. This talk will discuss the observation of quantum anomalies in Weyl semimetals. These anomalies are rooted in an incompatibility of quantum mechanics and classical conservation laws. In particular, I will focus on recent measurements demonstrating the presence of the mixed axial-gravitational anomaly in Weyl semimetals. In contrast to earlier expectations, which would for example search for this phenomenon in the hydrodynamics of neutron stars, we used a conceptually simple thermoelectric transport experiment to find signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP. |

Theoretisch-Physikalisches Kolloquium December 01, 2017, 16:30 TP seminar room 0.03 | Rainer Klages, Queen Mary University London Statistical Physics and Anomalous Dynamics of Foraging A question that attracted a lot of attention in the past two decades is whether biologically relevant search strategies can be identified by statistical data analysis and mathematical modeling. A famous paradigm in this field is the Levy Flight Foraging Hypothesis. It states that under certain mathematical conditions Levy dynamics, which defines a key concept in the theory of anomalous stochastic processes, leads to an optimal search strategy for foraging organisms. This hypothesis is discussed very controversially in the current literature. I will review examples and counterexamples of experimental data and their analyses confirming and refuting it. Related to this debate is own work about biophysical modeling of bumblebee flights under predation thread and biological cell migration, both based on experimental data analysis, which I briefly outline. |

SFB 1238 November 29, 2017, 14:30 Seminar Room of the Institute of Physics 2 | Cornelius Krellner, Universität Frankfurt Correlated matter: Insights from new materials Within the field of solid-state physics, the discovery of remarkable phases and transitions is often tightly coupled to the design, growth and characterization of novel materials. Therefore, the past several decades of work in the field of correlated electron physics can be described by a list of materials that have defined new states of matter at extreme conditions, e.g. low temperatures, high magnetic field or pressure. In most cases, a thorough understanding of the underlying physical mechanisms is accessible only, if high-quality single crystals with sufficient sizes are available. In this lecture, I shall give an overview about my own contributions to this research area in recent years focusing on three very different material classes (i) quantum criticality and superconductivity in heavy-fermion metals [1-3], (ii) crystal growth of LnFeAsO iron-pnictide superconductors [4,5], and (iii) spin-liquid phases in Cu-based frustrated spin systems [6]. References [1] A. Steppke et al., Science 339, 933 (2013). [2] E. Schuberth et al., Science 351, 6272 (2016). [3] H. Pfau et al., Phys. Rev. Lett. 119, 126402 (2017). [4] A. Jesche et al., Phys. Rev. B 86, 020501(R) (2012). [5] A. Adamski et al., Phys. Rev. B 96, 100503(R) (2017). [6] P. Puphal et al., J. Mater. Chem. C 5, 2629 (2017). |

Condensed Matter Theory seminar November 28, 2017, 14:00 Seminarraum Kernphysik | Frank Schindler, University of Zurich Higher-order topolectrical circuits Higher-order topological insulators have been recently proposed as unprecedented quantum states of matter. In two spatial dimensions, they incorporate quantized electric quadrupole insulators, which feature zero-dimensional topological corner midgap states. These states are protected by the bulk gap, reflection symmetries, and a spectral symmetry, while their edges are gapped. Although such novel topological phases are often hard to come by in real materials, there are various analogue platforms that enable us to realize the corresponding band theory concepts in classical systems. I will talk about how we developed and measured a topolectrical circuit design for a quadrupole insulator, the midgap states of which manifest themselves as topological boundary resonances in the corner impedance profile of the circuit. This establishes an instance where topolectrical circuitry is employed to bridge the gap between quantum theoretical modeling and the experimental realization of topological band structures. |

Theoretisch-Physikalisches Kolloquium November 24, 2017, 16:30 TP seminar room 0.03 | Peter Mörters, Mathematical Institute, Cologne Metastability of the contact process on evolving scale-free networks We study the contact process in the regime of small infection rates on scale-free networks evolving by stationary dynamics. A parameter allows us to interpolate between slow (static) and fast (mean-field) network dynamics. For two paradigmatic classes of networks we investigate transitions between phases of fast and slow extinction and in the latter case we analyse the density of infected vertices in the metastable state. This is joint work with Emmanuel Jacob (ENS Lyon) and Amitai Linker (Universidad de Chile). |

SFB 1238 November 22, 2017, 14:30 Seminar Room of the Institute of Physics 2 | Harold Zandvliet, University of Twente 1D electron systems studied with scanning tunneling microscopy and spectroscopy Scanning tunneling microscopy is an ideal technique to explore the structural and electronic properties of low-dimensional electron system. In this talk I will focus on the study of physical properties self-organizing metallic nanowires on semiconductor surfaces and transport through single molecules. The metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these properties are intimately related to electron-electron interactions, which play a much more prominent role in one dimension than in two or three dimensions. I will present our work on Pt, Au and Ir atom chains on Ge(001) surfaces and provide a few examples of their unique properties, such as Peierls instability, quantum confinement, suppression of the density of density states near the Fermi level and nanowire length quantization. If time allows I will also elaborate on the transport through single molecules. |

QM2 - Quantum Matter and Materials November 22, 2017, 09:30 Seminar Room 147, Institute for Physical Chemistry | Svetlana Mansurova, INAOE Hybrid device structures based on amorphous silicon and organic semiconductors In the first part of the talk a brief overview of the research activities in area of fabrication and characterization of hybrid device structures based on amorphous silicon and organic semiconductors will be presented. The second part of the talk will be devoted to the discussion of the dynamic grating technique based on non-steady-state photo-EMF effect and its application for characterization of the photo-physical properties of organic, inorganic and hybrid semiconductors. |

Großes Physikalisches Kolloquium November 21, 2017, 16:45 HS III | Nina Mueller, Institut für Theoretische Physik, Köln Cancer evolution and stochastic modeling of resistance to therapy |

Großes Physikalisches Kolloquium November 21, 2017, 17:15 HS III | Oliver Zingsheim, I. Physik, Uni Köln Rotational spectroscopy: A powerful analytical tool |

Statistical Physics Seminar November 21, 2017, 12:00 Seminar room TP 0.02 | Tirthankar Banerjee, Saha Institute of Nuclear Physics, Kolkata Universality in directed single-file motion in closed heterogeneous landscapes Single-file motion in quasi one-dimensional geometry appears in wide-ranging systems, ranging from ion transport in biological channels, fluids absorbed in materials with nanopores, colloidal particles moving along circular grooves to pedestrian dynamics regulated along a line. In this talk, we focus on the steady state density profiles in directed single-file motions of a collection of particles along closed heterogeneous landscapes. Modeling these as asymmetric exclusion processes with heterogeneous hopping rates along a closed ring, we explore generic localised (LDW) and delocalised (DDW) domain walls and delocalisation of LDWs in the steady states. We further show how the interplay between heterogeneity and particle non-conserving Langmuir kinetics leads to a phase transition between a two-phase coexistence and a three-phase coexistence state. We eventually discuss the more generic problem of heterogeneities with arbitrary spatial variation and extract a notion of universality, hitherto unknown, for the steady state density profiles and the associated phase diagrams. |

Theoretisch-Physikalisches Kolloquium November 17, 2017, 16:30 TP seminar room 0.03 | John Bechhoefer, Simon Fraser University, Vancouver The details are in the devil: Experiments on Maxwell's demon and the role of information in thermodynamics One hundred and fifty years ago, Maxwell's demon was first posed as a fundamental challenge to the newly developed field of statistical physics. Just two months later, Maxwell's paper "On governors" gave the first analysis of a feedback system. These two foundational works reflect the fundamental and practical aspects of control. Here, I will present an experiment that unites the two: using feedback to create `impossible' dynamics, we make a Maxwell demon that can reach the fundamental limits to control set by thermodynamics. We test - and then extend - Rolf Landauer's 1961 prediction that information erasure requires at least as much work as can be extracted from a system by virtue of information. These fundamental thermodynamic limits are benchmarks for evaluating the performance of practical information engines, such as those active within cells and other complex systems. |

Condensed Matter Theory seminar November 17, 2017, 14:00 Seminar Room 0.03, ETP | Lukas Sieberer, University of Innsbruck Designing quantum paths in adiabatic quantum computing The computational superiority of a computer that operates according to the laws of quantum mechanics relies crucially on quantum parallelism: different sets of data stored as a superposition of quantum states can be processed in parallel. To play out this advantage, however, one has to be able to prepare such superpositions in the first place --- using only a minimum of computational resources. We present a framework to efficiently prepare superpositions of many-body spin configurations with programmable squared amplitudes. The spin configurations are encoded in the degenerate ground states of the lattice-gauge representation of an all-to-all connected Ising spin glass. The ground state manifold is invariant under variations of the gauge degrees of freedom, which take the form of four-body parity constraints. Our framework makes use of these degrees of freedom by individually tuning them to dynamically prepare programmable superpositions. The dynamics combines an adiabatic protocol with controlled diabatic transitions. |

Condensed Matter Theory seminar November 16, 2017, 14:00 Seminarraum Altbau Theorie | Ipsita Mandal, MPI-PKS Dresden Critical Fermi Surface: UV/IR mixing and Superconducting Instability We discuss the perturbative control of low-energy effective theory of strongly-interacting systems which cannot be treated within the Landau Fermi liquid framework. These are generically called non-Fermi liquids where quasiparticles do not exist. In particular, we focus on critical Fermi surface states where there is a well-defined Fermi surface, but no quasiparticle resulting from the strong interaction between the Fermi surface and a massless boson. We will show that for Fermi surface having a dimension m>1, the Fermi momentum k_F enters the expressions for physical quantities as a dimensionful parameter leading to UV/IR mixing, thus modifying the naive scaling arguments, whereas for a one-dimensional Fermi surface there is an emergent locality with no such k_F dependence. |

QM2 - Quantum Matter and Materials November 15, 2017, 14:30 Seminar Room of the Institute of Physics 2 | Sabrina Disch, Department Chemie, University of Cologne Magnetic structure on the Nanoscale: Intraparticle Magnetization and Interparticle Interactions Magnetic nanoparticles reveal unique magnetic properties and relaxation phenomena which make them relevant for data storage, electronic and mechanical engineering, and biomedical applications1,2. Whereas the implementation of nanomagnetic properties into technological applications is progressing rapidly, understanding the microscopic origin of phenomena such as magnetization enhancement or decrease, magnetic anisotropy and the related magnetization distribution in individual nanoparticles, and interparticle interactions leading to aggregation or even ordered assemblies of nanoparticles is fundamentally challenging and needs intensive research. Polarized neutrons are an excellent, microscopic probe for spatial and time-resolved studies of magnetism. In this contribution, I will present examples of our recent studies of magnetic nanoparticles on different scales, ranging from atomic magnetic structures to the mesoscale intraparticle magnetization, and interparticle interactions in well-ordered arrays of magnetic nanocubes. References: [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006); DOI: 10.1103/RevModPhys.78.1 [2] Q. A. Pankhurst et al., J. Phys. D: Appl. Phys. 36, R167 (2003); DOI: 10.1088/0022-3727/36/13/201 |

Großes Physikalisches Kolloquium November 14, 2017, 16:45 HS III | Bahram Mashhoon, University of Missouri Nonlocal gravity and dark matter The conceptual basis for a classical nonlocal generalization of Einstein's theory of gravitation is presented. The framework of general relativity is enlarged by the introduction of a preferred frame field; then, history dependence is introduced. Nonlocality --- in the sense of an influence ("memory") from the past that endures --- could be a natural feature of the universal gravitational interaction. Nonlocal gravity is formally analogous to the nonlocal electrodynamics of media. The nonlocal aspect of gravity can simulate dark matter. The implications of nonlocal gravity theory for the problem of dark matter in galaxies, clusters of galaxies, and cosmology are discussed. Special Colloquium honoring Friedrich Hehl's 80th birthday. |

SFB 1238 November 13, 2017, 14:00 Seminarroom New Theorie 0.03 | Marko Kralj, Institute of Physics, Zagreb Transferring millimeter sized atomically thin layers: atomic-scale defects and structural modifications Contact Person: Thomas Michely |

Theoretisch-Physikalisches Kolloquium November 10, 2017, 16:30 Seminar Room 0.03, ETP | Zi Yang Meng, Institute of Physics, Chinese Academy of Sciences Itinerant quantum criticality, self-learning Monte Carlo, duality and all that Based on the recent conceptual and technical developments of quantum Monte Carlo simulations in correlated electron systems, I will present and discuss fresh results of itinerant quantum critical points, i.e., the critical phenomena arising from the strong coupling between Fermi surface and bosonic fluctuations, which are made possible by the self-learning Monte Carlo scheme. After which, I will also discuss the lately proposed duality relations between interaction-driven topological phase transitions and deconfined quantum critical points, which is verified via unbiased large-scale quantum Monte Carlo simulations. |

Großes Physikalisches Kolloquium November 07, 2017, 16:45 HS III | Eva Grebel, Heidelberg Dwarf Galaxies - Fossils of Galaxy Evolution Contact Person: Stephanie Walch |

SFB 1238 November 07, 2017, 15:00 SR Kernphysik | Felix Gunkel, FZ Juelich Thermodynamic processes and defect concentration profiles at engineered complex oxide interfaces and surfaces The properties of thin films and heterostructures of complex and strongly correlated oxides are one of the most fascinating and demanding topics in today´s solid state physics research. Oxide heterostructures give rise to novel and unexpected physical phenomena such as metallicity in nominally non-metallic materials, magnetism in nominally non-magnetic materials, ferroelectricity in nominally non-ferroelectric materials, or so far unobserved topological effects. Many of these phenomena are affected or even more driven by the nanoscale defect structure established during material synthesis and processing, ultimately limited by the thermodynamics of the system. Tuning the thermodynamic processes by engineering dedicated heterostructures may be used to tailor their defect structure in a desired way, hence, to deplete or accumulate defects at interfaces, to spatially separate electronic and ionic charge carriers, or to confine defects to dedicated regions. In this talk, I discuss the low-dimensional electron transport observed along complex oxide heterointerfaces, such as the one in LaAlO3/SrTiO3 heterostructures. While the formation of these 2-dimensional electron gases is attributed to electronic charge transfer triggered by a built-in electric field, the ionic defect structure at these interfaces is still being discussed controversially. I address the thermodynamic processes associated with built-in electric fields and derive ionic defect concentration profiles established at polar/non-polar oxide interfaces. [1] The specific ionic-electronic defect structure stabilized within such interfacial space charge layers strongly depends on ambient oxygen partial pressure applied during sample fabrication and on the strength of the built-in electric field. As will be discussed, the low-temperature behavior of these novel electron system is unambiguously correlated to the adjacent ionic structure typically set at high temperature, making thermodynamic considerations indispensable in order to understand novel phenomena occurring at cryostatic temperatures. Therefore, a comprehensive study of low temperature physics on the one hand and high temperature thermodynamics on the other hand is essential for a unified understanding of the manifold (and sometimes contradictory) phenomena observed in these low-dimensional electron systems. Here, we explicitly compare the thermodynamic ground states obtained for various oxide heterostructure systems [2] and discuss resulting implications for important measures characterizing the electron gas, such as electron mobility [3] as well as its magnetic signature [4], both controllable by thermodynamic means. The thermodynamic model obtained for oxide heterointerfaces is furthermore linked to kinetic space charge formation occurring at complex oxide surfaces [5, 6]. [1] F. Gunkel et al., “Defect concentration profiles at complex oxide interfaces”, Physical Review B 93, 245431 (2016) [2] F. Gunkel et al., “Thermodynamic ground states of complex oxide heterointerfaces”, ACS Applied Materials & interfaces, ACS Appl. Mater. Interfaces, 9 (1), 1086 (2017) [3] C. Xu et al., “Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures”, Scientific Reports 6, 22410 (2016) [4] F. Gunkel, et al., “Defect-control of anomalous and conventional electron transport in NdGaO3/SrTiO3 heterostructures”, Physical Review X, 6, 031035 (2016) [5] R. Meyer et al., “Dynamics of the metal-insulator transition of donor-doped SrTiO3”, Physical Review B 94, 115408 (2016) [6] M. Andrae et al., “Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3”, APL Materials 5, 056106 (2017) |

Condensed Matter Theory seminar November 03, 2017, 14:00 Seminar Room 0.03, ETP | Joaquin E. Drut, University of North Carolina Signal-to-noise issues in non-relativistic matter: from entanglement to thermodynamics Non-relativistic quantum matter, as realized in ultracold atomic gases, continues to be a remarkably versatile playground for many-body physics. Experimentalists have exquisite control over temperature, density, coupling, and shape of the trapping potential. Additionally, a wide range of properties can be measured: from simple ones like equations of state to more involved ones like the bulk viscosity and entanglement. The latter has received much attention due to its connection to quantum phase transitions, but it has proven extremely difficult to compute: stochastic methods display exponential signal-to-noise issues of a very similar nature as those due to the infamous sign problem affecting finite-density QCD. In this talk, I will present an algorithm that solves the signal-to-noise issue for entanglement, and I will show results for strongly interacting systems in three spatial dimensions that are the first of their kind. I will also present a few recent explorations of the thermodynamics of polarized matter and other cases that usually have a sign problem, using complexified stochastic quantization. |

Condensed Matter Theory seminar October 27, 2017, 14:00 Seminar Room 0.03, ETP | Masahiko Yamada, ISSP, University of Tokyo Crystalline spin-orbital liquids with an emergent SU(4) symmetry A promising approach to realize quantum spin liquids is to enhance the spin-space symmetry from usual SU(2) to SU(N). While the SU(N) symmetry with a general N is proposed in ultracold atoms using nuclear spin degrees of freedom, its realization in magnetic materials is challenging. Here we propose a new mechanism by which the SU(4) symmetry emerges in the strong spin-orbit coupling limit. The spin-orbit coupling in d^1 transition metal compounds with edge-sharing anion octahedra leads to strongly bond-dependent hopping, which is apparently not SU(4)-symmetric. However, in the honeycomb structure, a gauge transformation maps the system to an SU(4)-symmetric Hubbard model. In the strong repulsion limit at quarter filling, the low-energy effective model is the SU(4) Heisenberg model on the honeycomb lattice, which cannot have a trivial gapped ground state and is expected to host a gapless spin-orbital liquid. By generalizing this model to other three-dimensional lattices, we also propose crystalline spin-orbital liquids protected by this emergent SU(4) symmetry and space group symmetries. |

SFB 1238 October 25, 2017, 14:30 Seminar Room of the Institute of Physics II | Dirk Sander, Max-Planck-Institut für Mikrostrukturphysik, Halle New insights into nanomagnetism and superconductivity by low-temperature scanning tunneling microscopy Spin-polarized scanning tunneling microscopy at low temperature (8 K) and in high magnetic fields (6 T) is a powerful technique to investigate magnetic properties of individual nanoscale objects ranging in size form single atoms to several thousand atoms. I focus on the magnetization reversal and the spin-dependent electronic properties of bilayer Co, Fe-decorated Co and Fe islands on Cu(111). We find a novel noncollinear, helical magnetic order in the Fe islands, which is identified by a magnetic stripe contrast with a period of 1.28 nm. The high spatial resolution of the scanning tunneling spectroscopy reveals the significance of structural and electronic relaxation for the magnetic anisotropy and the spin-dependent transport properties of single islands. In an outlook I present first results from our new STM, which operates at 0.3 K in a vector magnetic field, on superconductivity of Pb monolayers and on the proximity effect. |

SFB 1238 October 24, 2017, 13:00 Seminar Room Kernphysik | Peter Milde, TU Dresden Scanning Force Microscopy Investigations of Skyrmions Contact Person: I. Lindfors-Vrejoiu |

Condensed Matter Theory seminar October 20, 2017, 14:00 Seminar Room 003, ETP | Lukas Janssen A fermionic gauge theory for bosonic deconfined criticality |

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