Das physikalische Kolloquium der Universität Göttingen und das Institut für Materialphysik laden ein zur Verleihung der Goldenen Promotionsurkunde an Herrn Prof. Dr. Richard Wagner.
Gastredner: Prof. Dr. Ludwig Schultz (TU Dresden)
Vortrag des Ehrengastes: „Zeitreise durch 50 Jahre Physik: Von Göttingen über Oxford, Pittsburgh, Geesthacht, Jülich nach Grenoble“
The past three decades of exoplanet discovery have revealed a bewildering diversity of planetary systems, as well as tantalizing correlations between their features and other properties such as the mass of the host stars. To understand the origin of these patterns, we must study the emergence of planets from circumstellar disks of gas and dust, and the prior evolution of those disks. These too exhibit remarkable differences in structure, evolution, and lifetime which are poorly understood. The dissipation of disks under the influence of high-energy emission from the central star is considered important; I show how the interaction of the partially ionized disk with the stellar magnetic field will modulate that emission and can explain observations. This suggests that the outcome of planet formation is significantly influenced by the large-scale magnetic field of the star, among other variables. Future observations by ground- and space-telescopes of the fields of very young stars, the structures and compositions of their disks, and the planets that form from them will more fully explore this connection. In a real way, the ancient Sun’s magnetic field could be one reason these words are written on a world that orbits it.
The present-day expansion rate of the Universe, or Hubble constant (H0),
is a fundamental parameter of cosmology. H0 sets the age of the
Universe, its critical density, and the absolute scale for a wide array
of cosmological observables that probe the laws of physics on extremely
large scales and across time. However, the last decade has brought about
an increasing disagreement between the values of H0 measured directly in
today's Universe and the values of H0 determined indirectly from
observations of the very early Universe. This 5-6 sigma disagreement has
come to be known by the term "Hubble constant tension" and is one of the
most pressing issues in cosmology. Yet, the Tension has thus far escaped
a satisfactory resolution.
Following a brief summary of the status quo, I will describe the
observational setup that achieves the most accurate direct H0
measurement using an extragalactic distance ladder in which pulsating
stars anchor type-Ia supernovae to geometrically measured distances. In
turn, I will present key developments that have slashed the uncertainty
on H0 by nearly a factor of 10 since the 2001 Hubble key project as well
as the ruthless efforts ongoing to identify and mitigate even the
smallest systematics. Third, I will discuss the usefulness of the James
Webb Space Telescope for cross-checking the state of the art as well as
its limitations for doing better. In closing, I will offer some thoughts
on the possibly far-reaching consequences of the Hubble constant tension
and useful ways forward.
Accessing theoretically the ground state of interacting quantum matter is a long-standing challenge, especially for complex two-dimensional systems. Recent developments have highlighted the potential to solve the quantum many-body problem by means of so-called neural quantum states. The enabling idea of this approach is to harness the power of machine learning by encoding the many-body wave function into an artificial neural network. In this talk, I will aim at introducing the main idea and central developments of this computational approach. In particular, I will outline one of the critical limitations, which until recently has prohibited the training of modern large-scale deep network architectures. I will show how this key limitation has been now resolved through the so-called minimum-step stochastic reconfiguration method. I will demonstrate for paradigmatic frustrated quantum magnets that this enables the neural quantum states method to reach regimes and accuracies beyond what is accessible by other computational approaches. Further, I will highlight the recent results on solving the real-time dynamics of correlated quantum matter, which has allowed us to verify for instance for the first time the quantum Kibble-Zurek mechanism for interacting quantum many-body systems in two spatial dimensions.
One of the most fascinating areas of condensed matter research is understanding how electrons in a solid interact with one another and the underlying atoms. This intricate interplay gives the system the amazing ability to have drastically different properties. It can for example be either insulating or superconducting. It is incredible to think that how electrons interact with one another, they can behave very differently. With so many electrons and atoms involved, it is no surprise that developing a general understanding of this interplay is so complex. That is why it is so exciting to find experimental systems that allow us to systematically control things like electron density and their mutual interaction. The novel class of van-der-Waals quantum materials offers such tunability, opening up a new era of condensed matter research.
This talk will give a general introduction in the fascinating class of van-der-Waals materials, how to prepare them and why these quantum materials are of interest for fundamental sciences but also potential applications in fields as diverse as quantum computation and energy science. One specific material will be discussed in greater detail, so-called bilayer graphene. This is a material that is just two carbon atoms thick. It is a truly fascinating system where we can control whether electrons behave more as individuals or strongly interact. When they interact strongly, one can observe either insulating or superconducting phases.
Last but not least, the talk will give a brief look into our laboratory, our work beyond graphene and why we have a little Oktoberfest on a daily basis.
Since the formation of atomic Bose-Einstein condensates (BEC) nearly 30 years ago, systems of cold atoms have developed into a versatile playground for the investigation into quantum many-body physics, with the promise to shed light onto dynamical processes in the spirit of quantum simulation efforts. With exquisite parameter control and a broad range of detection methods, a multitude of phenomena, ranging from superfluidity and quantum phase transitions to supersolidity and quantum transport, can be probed with high precision. I will give a broad introduction to the field of cold atoms and its techniques, highlighting a few of the milestone results of the past, and then turn to a selected set of recent experiments that address various dynamical quantum many-body processes such as dynamical localization, and the breaking thereof, and anyonization of bosons.
