Program winter term 2019/2020
Monday, October 14, 2019, 4:15pm, Hörsaal 28 D 001
Claudia Felser (MPI Dresden)
Topological materials science
Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. Since there is a direct connection between real space: atoms, valence electrons, bonds and orbitals, and reciprocal space: bands, Fermi surfaces and Berry curvature, a simple classification of topological materials in a single particle picture should be possible. Binary phosphides are an ideal material class for a systematic study of Dirac, Weyl and new Fermion physics, since these compounds can be grown as high-quality single crystals. A new class of topological phases that have Weyl points was also predicted in the family that includes NbP, NbAs. TaP, MoP and WP2. Beyond Weyl and Dirac, new fermions can be identified in compounds that have linear and quadratic 3-, 6- and 8- band crossings that are stabilized by space group symmetries. Crystals of chiral topological materials CoSi, AlPt and RhSi were investigated by angle resolved photoemission and show giant unusual helicoid Fermi arcs with topological charges of ±2. In agreement with the chiral crystal structure two different chiral surface states are observed. In magnetic materials the Berry curvature and the classical anomalous Hall (AHE) and spin Hall effect (SHE) helps to identify potentially interesting candidates. As a consequence, the magnetic Heusler compounds have already been identified as Weyl semimetals: for example, Co2YZ, and Co3Sn2S2. The Anomalous Hall angle also helps to identify materials in which a QAHE should be possible in thin films. Heusler compounds with non-collinear magnetic structures also possess real-space topological states in the form of magnetic antiskyrmions, which have not yet been observed in other materials.
Monday, October 28, 2019, 4:15pm, Hörsaal 28 D 001
Thomas Lippert (FZ Jülich)
Taming Complexity in Computational Physics Simulations
In the last ten years, simulation and data analysis have become increasingly complex in computational physics, chemistry and biology. Examples for such multi-physics workflows are found in particle physics, astronomy, earth system research, civil security, neuroscience, medicine, etc. The advent of Deep Learning in Data Analysis and its integration into simulation processes has added a further dimension of complexity.
In contrast, all supercomputers in the Top 500 are based on a monolithic architecture. They assemble heterogeneous nodes consisting of elements such as CPUs and GPUs sharing I/O. They are significantly underutilized as complex nodes make the overall system prone to inefficiencies. Secondly, scalability is costly, as a node needs to perform very complex computations for problems often not being scalable, and the same node must perform scalable computations for problems being easily scalable. A third difficulty is the inclusion of future technologies in workflows, such as quantum computers.
To master these challenges, we propose a disaggregation of resources and their dynamic recomposition through a programming paradigm called modular supercomputing. It gives a new degree of freedom in supercomputing, the dynamical optimal adaptation of program parts to different architectures within a joint high-speed network. Modularity is motivated by a computer theoretical generalization of Amdahl's Law.
Modular supercomputing helps to tame the increasing complexities in computational science: It offers energy-efficient exascale computing, optimized workflows in data analysis or interactive supercomputing, but also the simple integration of future quantum computers and neuromorphic computers.
Monday, November 11, 2019, 4:15pm, Hörsaal 28 D 001
Matthias Wuttig (RWTH Aachen University)
Novel Materials by Design: The Power and Potential of Maps
It has been a long-time dream of mankind to design materials with tailored properties. Such a material design is particularly interesting, if one wants to realize advanced functional materials with a demanding set of properties. In recent years, the focus of our work has been the design of phase change materials for applications in data storage. In this application, the remarkable property portfolio of phase change materials (PCMs) is employed, which includes the ability to rapidly switch between the amorphous and crystalline state. Surprisingly, in PCMs both states differ significantly in their properties. This material combination makes them very attractive for applications in rewriteable optical data storage, where the pronounced difference of optical properties between the amorphous and crystalline state is employed. This unconventional class of materials is also the basis of a storage concept to replace flash memory. This talk will discuss the unique material properties, which characterize phase change materials. In particular, it will be shown that only a well-defined group of materials utilizes a unique bonding mechanism (‘Bond No. 6’), which can explain many of the characteristic features of crystalline phase change materials. Different pieces of evidence for the existence of this novel bonding mechanism, which we have coined metavalent bonding, will be presented. In particular, we will present a novel map, which separates the known strong bonding mechanisms of metallic, ionic and covalent bonding. This map provides further evidence that metavalent bonding is a novel and fundamental bonding mechanism. Subsequently, this insight is employed to design phase change materials as well as thermoelectric materials.
Monday, November 25, 2019, 4:15pm, Hörsaal 28 D 001
Tommaso Calarco (FZ Jülich)
Monday, December 9, 2019, 4:15pm, Hörsaal 28 D 001
Christian Weinheimer (Universität Münster)
First results from the neutrino mass experiment KATRIN
Since the discovery of neutrino oscillation we know that neutrinos have non-zero masses, but we do not know the absolute neutrino mass scale, which is as important for cosmology as for particle physics. The direct search for a non-zero neutrino mass from endpoint spectra of weak decays is complementary to the search for neutrinoless double beta-decay and analyses of cosmological data. Today the most stringent direct limits on the neutrino mass originate from investigations of the electron energy spectra of tritium beta-decay.
The next generation experiment KATRIN, the Karlsruhe Tritium Neutrino experiment, is improving the sensitivity from the tritium beta decay experiments at Mainz and Troitsk of 2 eV/c^2 by one order of magnitude probing the region relevant for structure formation in the universe. KATRIN uses a strong windowless gaseous molecular tritium source combined with a huge MAC-E-Filter as electron spectrometer. To achieve the sensitivity, KATRIN has been putting many technologies at their limits. The full 70m long setup has been successfully commissioned. From early 2019 on KATRIN is taking high statistics tritium data hunting for the neutrino mass.
In this talk an introduction into the necessity to determine the neutrino mass and the status in the field will be given, followed by a detailed presentation of KATRIN and its results from the first KATRIN science run. The new results are already bringing KATRIN into the lead position of the field. In the outlook the perspectives of KATRIN for the coming years and new technologies to potentially improve further the sensitivity on the neutrino mass will be presented.
Monday, January 13, 2020, 4:15pm, Hörsaal 28 D 001
Heino Falcke (Radboud-Universität Nijmegen)