# 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

Abstract:

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.

Host:

Markus Morgenstern

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

Lukasz Plucinski (Forschungszentrum Jülich)

Band Structure Engineering in 3D Topological Insulators

Abstract:

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.

Host:

Markus Morgenstern

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

Prof. Zoltan Fodor (University of Wuppertal)

Axions as Dark Matter?

**Abstract:**

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.

Host:

Robert Harlander

### Monday, November 26, 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

Abstract:

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.

Host:

Philipp Mertsch

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

Prof. Martino Poggio (University of Basel)

TBA

Host:

Christoph Stampfer

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

Prof. Ralph Engel (Karlsruhe Institute of Technology)

TBA

Host:

Christopher Wiebusch

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

Prof. Claus Ropers (University of Göttingen)

TBA

Host:

Joachim Mayer