Department of Physics • University of Cologne

Valleys, defined as maxima or minima in the energy-momentum relation where carriers reside isolated by momentum mismatch, present a promising material degree of freedom, akin to electron charge or spin. Despite early theoretical interest, practical utilization of valleys has been hindered by the challenge of measurable valley contrast in natural systems. The advent of two- dimensional (2D) materials, particularly transition metal dichalcogenides (TMDs), has revolutionized valleytronics due to their intrinsic valley contrast. Specifically, monolayer TMDs exhibit a direct band gap, enabling straightforward access to valley degrees of freedom via photoluminescence experiments and valley-contrasting optical selection rules (see inset in Fig. 1 (a)). Circularly polarized photons can selectively address each valley type, with emitted photons being circularly polarized as well retaining valley information. Although significant advances have been made in addressing, reading out, and tuning valley degrees of freedom, these processes remain challenging, requiring sophisticated and costly experiments. The optical addressability of valleys in 2D TMDs holds potential for integration with nanophotonics, such as resonant nanoantennas and structured light. This talk will present a comprehensive experimental and numerical study of a hybrid system where valley-polarized photoluminescence from a MoS2 monolayer is coupled with a plasmonic nanosphere (see Fig. 1 (a)), aiming to refine simulation approaches and enhance understanding of nanoscale light-matter interactions. Additionally, the talk will explore the interaction of vortex beams with valley excitons, examining valley depolarization effects, and the potential for directional exciton transport (see Fig. 1 (b)). Figure 1 Optically pumped hybrid system consisting of a gold nanoparticle resonantly interacting with photoluminescence emitted by a monolayer MoS2. Inset: valley-contrasting optical selection rules. (b) Circularly polarized vortex beam incident on a monolayer TMD transfers momentum to valley excitons.

Acinetobacter baumannii is frequently found colonising patients in the hospital. Many of these isolates are wild-type and susceptible to antibiotics. However there are some isolates that have become multi-drug resistant, with carbapenem resistant isolates particularly difficult to treat. Carbapenem-resistance in A. baumannii is acquired mainly through the acquisition of a carbapenemase, however, the acquisition seems to be restricted mostly to a few international clonal lineages. The advent of genomics has enriched our understanding of how clonality and antimicrobial resistance are linked. Genomes from different lineages are distinct and this impacts how we use genomic data to identify resistance mechanisms. In this presentation, the mechanisms of antimicrobial resistance will be discussed within the background of the different lineages of A. baumannii, using data obtained from routine hospital surveillance and global collections of this organism.

I have borrowed the title from the 1935 paper in which Schrödinger introduced his cat example. I will explain why present-day critics still, like Schrödinger, see problems with the orthodox Copenhagen interpretation of quantum mechanics, and what the alternatives are. I will also discuss whether these alternatives are compatible with relativity. Furthermore, there are provable results about such alternatives, and I will describe some from recent years, including some of my own.

Nuclear collisions allow the study of the Quark Gluon Plasma - a medium of deconfined quarks and gluons. At particle colliders, its presence is signaled by modifications of the measured particle spectra. The recent discovery of anisotropically distributed particles („collective flow“) in smaller collision systems while modifications to high-momentum spectra are absent is puzzling since both are associated with the presence of the QCD medium. In this work, no-medium baseline predictions for high-momentum observables are presented and theoretical limits to observing medium effects are discussed.

A major research effort in the area of condensed matter physics aims to optically manipulate the macroscopic properties of quantum materials [1]. Remarkable critical phenomena have been reported, such as the laser-induced metal-insulator transition [2], metamagnetic phase transitions [3], and even superconductivity [4]. The dynamical control of the order parameter has been achieved in magnetic materials [5], even via coherent processes, which rely on the optical generation of coherent collective eigenmodes [6,7]. In my talk, I will introduce the main light- matter interaction mechanisms enabling the optical activation of coherent quasiparticles, with a particular focus on magnetic excitations. I will discuss in details two aspects. First, the generation of collective spin excitations, namely spin waves or magnons, both at the center and at the edges of the Brillouin zone, while avoiding the absorption of light by the lattice and the electrons [8-9]. The regime of quantum spin dynamics triggered by the activation of high-energy magnons will be discussed as well [9-10]. Second, I will address the coupling between spins and electrons at the ultrashort timescale. Mechanisms allowing to drive spin waves both at the center and at the edges of the Brillouin zone, via mixed electronic and magnonic transitions, will be the topic of discussion [11-12]. I will then report on our very recent successful attempts to couple coherent THz magnons to charges at the picosecond timescale and our ongoing efforts to establish a manipulation of the spin-orbit coupling, via the excitation of coherent THz phonons [13]. In the final part of my talk, I will then present novel concepts to manipulate quantum materials, i.e. the dynamical modification of the spectrum and dispersion relation. I will show an unprecedented scheme to couple zone-edge and zone-centre modes [14], which results in the renormalization of the frequency and amplitude of the zone-centre magnons [15]. The observed regime of strongly nonlinear coherent spin dynamics is elusive of the conventional picture provided by linear spin wave theory. The development of a quantitative theoretical understanding of the observations, in collaboration with theorists, will be presented as well. In conclusion, I will briefly show extremely novel results concerning an alternative route to modify the magnon spectrum, which does not involve zone-edge magnons at all. The concepts discussed in my talk provide basic tools for a dynamical engineering of macroscopic coherent states of quantum materials, operating at the intrinsic characteristic time scales of the collective eigenmodes. The approaches presented here can be applied to a wide variety of systems and can be generalized to lattice excitations. [1] Advances in Physics 65, 58–238 (2016). [2] Nature 449, 72–74 (2007). [3] Physical Review Letters 116, 097401 (2016). [4] Science 331, 189–191 (2011). [5] Physical Review Letters 99, 047601 (2007). [6] Nature 435, 655 (2005). [7] Nature Photonics 5, 31–34 (2010). [8] Physical Review B 89, 060405(R) (2014). [9] Nature Communications 7, 10645 (2016). [10] Physical Review B 100, 024428 (2019). [11] Physical Review Letters 127, 077202 (2021). [12] Nature Physics 14, 370–374 (2018). [13] ArXiv (2023) doi:10.48550/arxiv.2310.08411. [14] Laser Photonics Rev. 2301152, (2024). [15] ArXiv (2023) doi: 10.48550/arXiv:2310.19667v1

Antiferromagnetic spintronics has been a very active research area of condensed matter in recent years. As we have learned how to manipulate collinear antiferromagnets actively and their emergent topology by means of new types of spin-orbit torques, a key problem remained: the inefficiency of relativistic mechanism. The necessity of relativistic effects to manipulate and detect Néel order arises from the spin degeneracy of collinear antiferromagnets in the non-relativistic limit– or at least it was thought. The discovery of d-wave magnetic order in momentum space motivated a closer look at the symmetry classification of collinear magnetic systems. This has emerged as the third basic collinear magnetic ordered phase of altermagnetism, which goes beyond ferromagnets and antiferromagnets. Altermagnets exhibit an unconventional spin-polarized d/g/i-wave band structure in reciprocal space, originating from the local sublattice anisotropies in direct space. This gives properties unique to altermagnets (e.g., the spin-splitter effect), while also having ferromagnetic (e.g., polarized currents) and antiferromagnetic (e.g., THz spin dynamics and zero net magnetization) characteristics useful for spintronics device functionalities. I will cover the basic introductory view to altermagnetism and its consequences to spintronics as well as new emerging exchange driven phenomena akin to spin-orbit coupling effects, such as p-wave magnetism, emerging from the basic concepts that gave rise to the discovery of altermagnetism.

Cancer cells starts as self, yet by the time they are detected they have often evolved mechanisms to evade the immune system. Which specific features immune selections acts on, and whether they are fundamental to cancer evolution is the subject of our research. We discuss two aspects where the production of nonself features is fundamental to cancer evolution: neoantigen creation through mutational processes and de-repression of nucleic acids derived from repetitive elements. We show how both species arise due to fundamental evolutionary tradeoffs which tell us about the essential role of the immune system in ensuring genome fidelity. Moreover, their understanding can help us better design emerging therapies such as anticancer vaccines.

Superconductivity has attracted much attention in the field, not only due to its intriguing fundamental mechanism, but also its practical applications such as superconducting wires and detectors, as well as the potential applications including superconducting computing and logics. Though it’s been discovered more than a century ago, the novel phenomena based on superconductivity are still emerging. In this talk, I will introduce my research on the intriguing phenomena of superconductivity. First, I will discuss the superconducting diode effect, which describes the different positive and negative critical currents in a superconductor. We observed this effect with and without an external magnetic field in heterostructures based on van der Waals materials. The second phenomenon is the “global critical current effect”, which means that the critical current of a superconductor is not only dependent on its own properties but can be influenced or even determined by adjacent superconductors. Inspired by this observation, we designed a superconductor with a special geometry, in which we transferred the superconducting diode effect to another part of the superconductor that cannot manifest it on its own.

Spin foam quantum gravity is a non-perturbative, background independent approach to quantizing gravity. It formulates a path integral as a sum over quantum geometries encoded in group theoretic data as in loop quantum gravity; thus it is frequently seen as a way of defining a covariant dynamics for this theory. In this talk, I want to motivate quantum gravity as a manifestly background independent theory and define the gravitational path integral à la spin foams. As a regularisation, I introduce a triangulation and discuss how gravity can be formulated in the discrete, via Regge calculus in a piece-wise flat way. Then I discuss how quantum geometry is encoded in a spin foam and how it connects to Regge calculus in a putative semi-classical limit. I will review outstanding research challenges, in particular for the Lorentzian theory, and discuss concretely the construction of a cosmological model coupled to a scalar field.

Interfacing van der Waals materials allows manipulating the fundamental symmetries of electron states in solids with unprecedented precision using the individual layers as atomically precise, bottom-up building blocks. The resulting heterostructures have emerged as a versatile, electronically tunable solid-state platform for studying quantum phases with tailored electronic, optical, and spin properties. Here, I will discuss recent examples of intentionally and unintentionally broken symmetries in van der Waals heterointerfaces and their interrogation by non-linear optoelectronic transport [1]. In graphene-based heterostructures, we address gate-tunable, non-linear transport arising from proximity-induced spin-orbit coupling and breaking of the relevant crystal symmetries, which may be used to efficiently manipulate spin-polarized carriers by both optical and electrical means. In epitaxial graphene/Bi2Te2Se heterointerfaces with a commensurate lattice alignment, we find an enhanced helicity-dependent photocurrent, which originates from the peculiar spin-orbit proximity of the commensurate interface [1]. In graphene/WTe2 heterostructures, we demonstrate a current- induced Kerr rotation related to non-linear polarisation of the Fermi surface. We provide a theoretical model based on the non-linear anomalous Hall effect in the heterostructure, whose detection is enabled by the reduced symmetry of the incommensurate heterointerface [2]. Finally, I will outline the use of nanolithography to control the symmetry of van der Waals materials in a top-down approach. In particular, nanoscale superlattices may allow to enhance electronic correlations by flat band formation in a broad class of materials. Such correlated flat bands have non-trivial quantum geometric and topological properties that are addressable in suitable optoelectronic experiments with a resonant optical excitation. References [1] J. Kiemle, P. Zimmermann, A.W. Holleitner, C. Kastl, Light-field and spin-orbit-driven currents in van der Waals materials, Nanophotonics 2020, 9, 2693 - 2708. [2] J. Kiemle, L. Powalla, K. Polyudov, L. Gulati, M. Singh, A. W. Holleitner, M. Burghard, C. Kastl, Gate-Tunable Helical Currents in Commensurate Topological Insulator/Graphene Heterostructures, ACS Nano 2022, 16, 12338-12344. [3] L. Powalla, J. Kiemle, E. J. König, A. P. Schnyder, J. Knolle, K. Kern, A. W. Holleitner, C. Kastl, M. Burghard, Berry curvature-induced local spin polarisation in gated graphene/WTe2 heterostructures, Nat. Commun. 2022, 13, 3152.

tba

Van der Waals Epitaxy of Transition Metal Disulphide Nanostructures: Tailoring Electronic Structure by Growth Variation