Physikalisches Kolloquium der Sektion Physik im Wintersemester 2020 / 2021

Termin: dienstags 16.15 (Videokonferenz).

Verantwortlich: Prof. Michael Bonitz






Das Kolloquium wird als Videokonferenz durchgeführt:
ZOOM Meeting ID: 889 1529 8048
Passcode: 429028


  • 10.11.2020: Prof. Fabio Caruso (ITAP, CAU Kiel)

    ** Antrittsvorlesung **

    Electronic-structure theory of two-dimensional quantum materials:
    from fundamental interactions to novel emergent phenomena

    Prof. Fabio Caruso

    A detailed understanding of the quantum-mechanical interactions between the constituents of matter holds the promise of driving the discovery of new functional materials and designing their properties. First-principles electronic-structure theory has evolved into a robust framework to address this challenge, as it allows to establish the inherent relationship between the macroscopic properties of solids and elementary interactions at the nano scale.

    In the Computational Solid-State Theory group, we develop and employ state-of-the-art first-principles approaches for the theoretical description of the electronic and vibrational properties of two-dimensional materials, such as graphene and transition-metal dichalcogenides. These compounds are characterized by a unique interplay of charge confinement, reduced dielectric screening, and strong light-matter coupling. This makes them prone to exhibit a rich spectrum of emergent phenomena, including charge-density waves, polarons, and circular dichroism. Through a few examples taken from our recent research activities, I will illustrate the predictive power achievable by means of modern first-principles techniques based on many-body perturbation theory in the description of these phenomena. I will further discuss how these activities relate and contribute to the open challenges in the field of electronic structure theory.

  • 17.11.2020: Prof. Philipp Werner (Uni Fribourg, CH)

    Nonequilibrium dynamical mean field theory
    Philipp Werner

    Recent experiments on laser-driven solids have revealed interesting nonequilibrium effects such as light-induced superconducting states [1,2] or switching into long-lived metastable states with different structures and electronic properties [3]. To investigate and understand such phenomena, new theoretical and computational tools need to be developed. I will present the dynamical mean field approach, which over the past 15 years has been extended into a powerful framework for the simulation of real-time dynamics in correlated lattice systems [4]. After introducing this nonequilibrium Green's function based technique, I will discuss benchmarks against cold-atom based quantum simulators [5], and present recent applications to laser-driven lattice models. These investigations demonstrate the possibility of effectively cooling correlated electron systems [6], and inducing magnetic, superconducting or excitonic order in long-lived nonequilibrium states. I will comment on the implications of these findings for the experiments on light-induced superconductivity.

    [1] S.Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri, Phys. Rev. B 89, 184516 (2014).
    [2] M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Ricco, S. R. Clark, D. Jaksch, and A. Cavalleri, Nature 530, 461 (2016).
    [3] L. Stojchevska, I. Vaskivskyi, T. Mertelj, P. Kusar, D. Svetin, S. Brazovskii, and D. Mihailovic, Science 344, 177 (2014).
    [4] H. Aoki, N. Tsuji, M. Eckstein, M. Kollar, T. Oka, and P. Werner, Rev. Mod. Phys. 86, 779 (2014).
    [5] K. Sandholzer, Y. Murakami, F. Goerg, J. Minguzzi, M. Messer, R. Desbuquois, M. Eckstein, P. Werner, and T. Esslinger, Phys. Rev. Lett. 123, 193602 (2019).
    [6] P. Werner, M. Eckstein, M. Mueller, and G. Refael, Nature Comm. 10, 5556 (2019).

    Einladender: Prof. Caruso

    A video of the colloquium is available upon request.
  • 24.11.2020: Prof. Bradley J. Siwick (McGill University Montreal)

    Structure and Dynamics with Ultrafast Electron Microscopes: Moving Beyond the Molecular Movie
    Bradley J. Siwick

    In this talk I will describe how combining ultrafast lasers and electron microscopes in novel ways makes it possible to directly ‘watch’ the time-evolving structure of condensed matter on the fastest timescales open to atomic motion.  By combining such measurements with complementary (and more conventional) spectroscopic probes one can develop structure-property relationships for materials under even very far from equilibrium conditions and explore how light can be used to control the properties of materials.

    I will give several examples of the remarkable new kinds of information that can be gleaned from such studies and describe how these opportunities emerge from the unique capabilities of the current generation of ultrafast electron microscopy instruments.  For example, in diffraction mode it is possible to identify and separate lattice structural changes from valence charge density redistribution in materials on the ultrafast timescale and to identify novel photoinduced phases that have no equilibrium analogs.   It is also possible to directly probe the strength of the coupling between electrons and phonons in materials across the entire Brillouin zone and to probe nonequilibrium phonon dynamics (or relaxation) in exquisite detail.  

    I will assume no familiarity with ultrafast lasers or electron microscopes.

    [1] Morrison et al Science 346 (2014) 445
    [2] Otto et al, PNAS, 116 (2019) 450
    [3] Stern et al, Phys. Rev. B 97 (2018) 165416
    [4] Rene de Cotret et al, Phys. Rev. B 100 (2019) 214115

    Einladender: Prof. Bauer
  • 08.12.2020: Prof. Ido Kaminer (Technion –­­ Israel Institute of Technology)

    Free-Electron Quantum Optics
    Ido Kaminer

    Research of cavity quantum electrodynamics (CQED) has enabled new capabilities in quantum optics, quantum computation, and various quantum technologies. So far, all the work in this field has included light interacting with bound-electron systems such as atoms, quantum dots, and quantum circuits. In contrast, free-electron systems enable fundamentally different physical phenomena, as their energy distribution is continuous and not discrete, and allow for tunable transitions and selection rules.

    We have developed a platform for studying free-electron CQED at the nanoscale and demonstrated it by observing coherent electron interaction with a photonic cavity for the first time. Our platform includes femtosecond lasers in an ultrafast transmission electron microscope, which created what is, in many respects, the most powerful nearfield optical microscope in the world today. We resolve photonic bandstructures as a function of energy, momentum, and polarization, simultaneously with capturing the spatial distribution of the photonic modes at deep-subwavelength resolution.

    These capabilities open new paths toward using free electrons as carriers of quantum information. As examples, we show how to create free-electron qubits and implement quantum gates with femtosecond lasers. We further show how to measure quantum decoherence in space and time using the free-electron quantum interactions. Such interactions also enable new avenues for tunable X-ray sources, as we demonstrate with theory and experiments.

    Einladende: Prof. Talebi

  • 15.12.2020: Prof. Dr. David Go (University of Notre Dame, USA)

    How Should We Think About Plasma-Catalysis? Insights from Experiments and Simulations David B. Go

    Plasma-catalysis is an emerging field of plasma science and engineering where non-equilibrium plasmas are coupled with catalytic materials to more effectively drive chemical reactions. The field holds significant promise, with the potential to overcome existing challenges for many industrially-relevant processes, such as the reforming of natural gas or the synthesis of ammonia. However, plasma-catalysis systems are extremely complex, consisting of a wide variety of chemical and physical processes that can both synergistically work together and function in opposition to each other. While plasma chemistry and catalysis are both well studied fields in their own right, when they are coupled, the question arises: How should we think about these systems?  That is, should we think about them as catalysis systems that are enhanced by a plasma or a plasma system that is enhanced by a catalyst? Or should we think about them in a completely different way?  

    At the University of Notre Dame, an interdisciplinary team with expertise in plasma science, catalysis, surface science, and atomistic modelling have been trying to answer these questions both at a fundamental level and for what they imply for engineering plasma-catalysis systems. This colloquium talk will present a holistic perspective on our team’s work in this area. I will discuss how our findings have shown how plasma-catalysis diverges from ‘conventional’ thermal catalysis, how plasmas can drive chemical conversion ‘beyond equilibrium’, and the evidence we have that molecular processes – rather than macroscopic effects – help drive these behavior. This talk will set the stage for understanding how to design both catalysts and reactor systems that capitalize on the non-equilibrium conditions in a plasma to enhance chemical conversion.

    Einladender: Prof. Benedikt

  • 12.01.2021: Prof. Dr. Mathieu Kociak (Université Paris Sud, France)

    Nanooptics in the electron microscope
    Prof. M. Kociak

    Hunting optical phenomena at the nanometer scale, namely performing nanooptics, is paradoxical. On the one hand, the typical length-scale relevant for optics is of the order of a visible radiation wavelength: few hundred of nanometers. On the other hand, optical properties of nano-objects become to depart from bulk properties when the nano-objects become smaller than a few hundred of nanometers. In this case, the optical properties depend on minute variations of the size and shape of the nano-objects, and sometimes the morphology or structure of nano-objects have to be known with atomic resolution. Therefore, techniques that are not limited by the optical diffraction limit have been developed in the last 15 years to make possible to study novel optical nanomaterials and the novel physics they brought.

    In this talk, I will present the use of free electron beams – such as delivered in a transmission electron microscope – to perform optical spectroscopy at the nanometer scale. I will show how they can be used to map phonons, plasmons and excitons with unbeatable spatial resolution, and even in 3D. Beyond their impressive success in generating nice images, I will show how it is now possible to quantitatively understand such experiments in pure optical terms, such as extinction and scattering cross-sections or electromagnetic local density of states. I will also emphasize recent developments in ultra-high spectral resolution that makes it possible to tackle new field of nanooptics, such as the study of strong coupling between plasmons and excitons or phonons.

    Einladende: Prof. Talebi

  • 19.01.2021: Prof. Dr. Alexander Ako Khajetoorians (Radboud University, Netherlands)

    What can we “learn” from atoms?Prof. Dr. Alexander Ako Khajetoorians

    In machine learning, energy-based models are rooted in concepts common to magnetism, like the Ising model. Within these models, plasticity, learning, and ultimately pattern recognition can be linked to the dynamics of coupled spin ensembles that exhibit complex energy landscapes akin to behavior seen in spin glasses. While this behavior is commercially emulated in software, there are strong pursuits to implement these concepts directly and autonomously in solid-state materials. To date, hybrid approaches, which often use the serendipitous electric, magnetic, or optical response of materials, emulate machine learning functionality with the help of external computers. Yet, there is still no clear understanding of how to create machine learning functionality from fundamental physical concepts in materials, like hysteresis, glassiness, or spin dynamics. This motivates new fundamental investigations of complex spin systems, and how their behavior can be manipulated to potentially new paradigms.

    Based on scanning tunneling microscopy, magnetic atoms and films on surfaces have become a model playground to understand and design magnetic order. However, these model systems historically have been probed in limits for robust memory applications, namely strong double-well regimes. In this talk, I will illustrate new model platforms to realize machine learning functionality directly in the dynamics of coupled spin ensembles that exhibit multi-modal landscapes. I will first review the concept of energy- based neural networks and how they are linked to the physics of spin glasses. I will then highlight new examples based on the recent discovery of the so-called spin Q glass and the atomic Boltzmann machine. I will illustrate the creation of atomic-scale neurons and synapses, in addition to new learning concepts based on the separation of time scales and self-adaptive behavior. I will also discuss recent cutting-edge developments that enable magnetic characterization in new extreme limits and how this platform may be applied toward autonomous adaption and quantum machine learning.

    Einladender: Prof. Heinze

  • 26.01.2021: Dr. Hanno Kählert (ITAP, CAU Kiel)

    ** Antrittsvorlesung **

    Theory and simulation of strongly correlated plasmas and dense matter

    High density plasmas are fundamentally different from their low density counterparts in space or high temperature fusion reactors. Strong interactions give rise to novel phenomena such as liquid-like ordering and affect their thermodynamic and transport properties. The latter are important for the modeling of dense plasmas in inertial confinement fusion or warm dense matter in planetary interiors. Similar conditions occur at low temperatures or when highly charged ions are involved, which makes it possible to study strong coupling physics on a smaller scale with laser-cooled plasmas or charged dust particles. This talk gives an introduction to the physics of strongly correlated plasmas, their occurrence, and theoretical description. In particular, simulation methods have become indispensable tools to obtain reliable data for their properties in a regime where traditional plasma theory fails. As an example, I present results for the dynamic structure factor - a key quantity that is accessible experimentally and provides deep insight into the thermodynamic, transport, and dielectric properties of strongly correlated plasmas. Future challenges and possible directions for the field will be addressed.