Program winter term 2018/2019


Monday, October 15, 2018, 4:15pm, Hörsaal 28 D 001

Markus Ternes (RWTH Aachen University)

Using scanning probe methods to engineer spin structures and to detect correlations and entanglement with atomic precision

Scanning probe microscopes have been very successful tools for studying individual atoms and molecules as well as complex structures. Systems which bear magnetic spin moments can be build with them on surfaces and stabilized in junctions. When such spins interact with each other or with the supporting electron baths, correlated many-particle states can emerge, making them ideal prototypical quantum systems.
My presentation will discuss how transition metal atoms and hydrates can be used as model systems to explore this quantum world. Specially crafted tips, in which the apex is functionalized can be used to detail the manipulation of the spin moment or the transition mechanism between different quantum phases. Furthermore, controlling the couplings enables the quantification of spin-spin correlations, the detection “dark” moments as well as the emergence of entanglement.

Markus Morgenstern



Monday, October 29, 2018, 4:15pm, Hörsaal 28 D 001

Lukasz Plucinski (Forschungszentrum Jülich)

Band Structure Engineering in 3D Topological Insulators

In this talk I will present an introduction to the physics of three-dimensional (3D) topological insulators (TIs), examine experimentally-relevant material classes, and discuss recent contributions to the field by my group.
After giving a brief historical perspective I will start the description of 3D TIs with introducing the quantum anomalous Hall (QAH) phase, that can be described by a two-band model Hamiltonian. The two uncoupled counterpropagating copies of that Hamiltonian describe the 2D topological insulator phase, also known as the quantum spin Hall (QSH) phase, while 3D TI phase requires off-diagonal linear coupling between the two QAH copies.
I will introduce various experimental realizations of topological insulators, in particular the most important 3D TIs, which are Bi2Te3, Bi2Se3, and Sb2Te3 and their alloys. I will describe their growth methods and discuss challenges in preparing truly insulating thin films in which topological properties could be explored experimentally. Subsequently, I will present recent combined experimental and theoretical results of my group on band structure engineering in 3D TI bilayers and superlattices. These studies show how new topologies emerge in complex structures, as compared to the routine Fermi level control by alloying.
Encouraged by these results I will propose new vistas to employ topological mechanisms in the design of novel spintronic devices. This encompasses not only topological insulators but also Weyl and Dirac phases, where, in the intrinsic regime, Fermi arc boundary modes contribute to the electronic transport.

Markus Morgenstern


Monday, November 12, 2018, 4:15pm, Hörsaal 28 D 001

Prof. Zoltan Fodor (University of Wuppertal)

Axions as Dark Matter?

The well-established theories of the strong interaction (QCD) and the electroweak theory determine the evolution of the early universe. The Hubble rate and the relationship between the age of the universe and its temperature are determined by the equation of state (EoS). Since QCD is highly non-perturbative, the calculation of the equation of state is a particularly difficult task. The only systematic way to carry out this calculation is based on lattice QCD. Here we present complete results of such a calculation. QCD, unlike the rest of the Standard Model, is surprisingly symmetric under time reversal, leading to a serious fine tuning problem. The most attractive solution for this is a new particle, the axion –a promising dark matter candidate. Assuming that axions are the dominant component of dark matter we determine the axion mass. The key quantity of the calculation is the topological susceptibility of QCD, a quantity notoriously difficult to calculate.

Robert Harlander


Monday, December 03, 2018, 4:15pm, Hörsaal 28 D 001

Prof. Roger Blandford (Stanford University)

On the Formation, Propagation and Emission of Relativistic Jets in Active Galactic Nuclei

Recent radio interferometric imaging and gamma ray observation of relativistic jets in active galactic nuclei indicate that they are formed electromagnetically close to massive, spinning black holes orbited by accretion disks. However, this interpretation is hard to reconcile with the existence of radio-quiet sources, the observation of gamma ray variability on timescales of minutes and apparent observation of orbital motion close to the black hole at the centre of our Galaxy. These challenges, some possible remedies and upcoming observational tests will be discussed.

Philipp Mertsch


Monday, December 10, 2018, 4:15pm, Hörsaal 28 D 001

Prof. Martino Poggio (University of Basel)

New Scanning Probes for Nanomagnetic Imaging

I will discuss recent experiments in our group aimed at developing and applying two promising new magnetic scanning probes.
The first probe is based on newly developed nanowire (NW) force sensors, which have recently enabled a form of AFM capable of mapping both the size and direction of tip-sample forces.  I will present first results in which we functionalize such NWs with nanometer-scale magnetic tip and characterize their behavior.  Using these NW sensors, we intend to realize a form of vectorial MFM capable of mapping stray magnetic fields with enhanced sensitivity and resolution compared to the state of the art.
The second scanning probe consists of a sharp quartz tip with a nanometer-scale superconducting quantum interference device (SQUID) integrated on its end.  This SQUID-on-tip (SOT) sensor achieves record sensitivity to both stray magnetic flux and local thermal dissipation. I will discuss experiments using this device to map the stray magnetic field produced by individual nanomagnets and superconducting vortices.
The unique capabilities of both of these scanning probes may provide new types of imaging contrast for in a variety of physical systems. These include nanometer-scale magnetic structures such as domain walls, magnetic vortices, and magnetic skyrmions.  The ability to map mesoscopic current flow in two-dimensional materials and topological insulators or image magnetic field produced by superconducting films and nanostructures could shed light on a number of open questions.

Christoph Stampfer


Monday, January 14, 2019, 4:15pm, Hörsaal 28 D 001

Prof. Ralph Engel (Karlsruhe Institute of Technology)

Ultra-High-Energy Cosmic Rays

Ultra-high-energy cosmic rays are the highest energy particles in the universe. The sources of these particles must be extreme astrophysical objects, or new particles physics processes, as their energy exceeds that of man-made accelerators by more than seven orders of magnitude. In this talk, a review of our current understanding of these particles will be given, emphasizing the progress made in recent years thanks to the data collected with the Auger Observatory and Telescope Array. A number of unexpected observations has led to a new picture of ultra-high-energy cosmic rays with increasingly strong constraints on the properties of their sources and their interactions in the atmosphere. The talk will conclude with a discussion of observations planned for the near future, which will address fundamental questions that have emerged from the cosmic-ray observations made so far.

Christopher Wiebusch


Monday, January 28, 2019, 4:15pm, Hörsaal 28 D 001

Prof. Claus Ropers (University of Göttingen)

Ultrafast Electron Imaging and Diffraction

Time-resolved electron imaging, diffraction and spectroscopy are emerging laboratory-based tools to trace non-equilibrium dynamics in materials with a sensitivity to structural, electronic and electromagnetic degrees of freedom. A particularly versatile method is Ultrafast Transmission Electron Microscopy (UTEM), which combines the high spatial resolution of electron microscopy with the temporal resolution of optical spectroscopy. Moreover, UTEM also provides for a unique test bench to study the interaction of intense optical fields with free-electron beams. This talk will introduce the experimental realization of UTEM and will discuss applications in structural and magnetization dynamics as well as in free-electron quantum optics.

Joachim Mayer