Department of Physics • University of Cologne

Topological quantum chemistry (TQC) framework has provided a complete description of the universal properties of all possible atomic band insulators in all space groups considering the crystalline unitary symmetries. It links the chemical and symmetry structure of a given material with its topological properties. While this formalism filled the gap between the mathematical classification and the practical diagnosis of topological materials, an obvious limitation is that it only applies to weakly interacting systems. It is an open question to which extent this formalism can be generalized to correlated systems that can exhibit symmetry protected topological Mott insulators. In this talk I will first introduce TQC and its application and then I will address this question by combining cluster perturbation theory and topological Hamiltonians within TQC. This simple formalism will be applied to calculate to the phase diagram of a representative model. The results are compared to numerically exact calculations from density matrix renormalization group and variational Monte Carlo simulations together with many-body topological invariants. References 1. M.G. Vergniory, L. Elcoro, C. Felser, N. Regnault, B.A. Bernevig, Z.Wang, “A complete catalogue of HighQuality Topological Materials“, Nature 566, 480-485 (2019) 2. B. Bradlyn, L. Elcoro, J. Cano, M.G. Vergniory, Z. Wang, C. Felser, M.I. Aroyo, B.A. Bernevig, “Topological quantum chemistry”, Nature 547 (7663), 298-305 (2017). 3. M. Iraola et al, in preparation

In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonics”. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a single-magnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity optomagnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field of Quantum Magnonics with emphasis on Cavity Optomagnonics, and present some theoretical results from our group which aim to push the boundaries of the current state of the art.

In this talk, I will show that the defects of the valence plaquette solid(VPS) order parameter, in addition to possessing non-trivial quantum numbers, have fracton mobility constraints in the VPS phase. The spinon inside a single vortex cannot move freely in any direction, while a dipolar pair of vortices with spinon pairs can only move perpendicular to its dipole moment. These mobility constraints, while they persist near QCP, can potentially inhibit the condensation of spinons and preclude a continuous transition from the VPS to the Néel antiferromagnet. Instead, the VPS melting transition can be driven by the proliferation of spinon dipoles. In particular, we argue that a 2d VPS can melt into a stable gapless phase in the form of an algebraic bond liquid with algebraic correlations and long-range entanglement. Such a bond liquid phase yields a concrete example of the elusive 2d Bose metal with symmetry fractionalization.

The talk will cover spectroscopic investigations on the catalytic properties of iron nanoparticle arrays and details on the formation of nanographene in nano-heterostructures. The increased complexity of real-life catalysts as compared to surface science leads to the “materials gap”. To close this gap, studies of adsorption and reaction experiments on nanoparticles supported on 2D materials were conducted under in situ conditions with the particular aim to elucidate the reactivity of such more “realistic” nanoparticles. It is shown that the use of a quantitative spectroscopy allows for gaining a detailed insight even to these complex systems, thereby facilitating a further step into bridging the materials gap from fundamental science to applied sciences. The use of the 2D materials as a chemically “innocent” substrate gives the intriguing possibility to study the system without the additional influence of side processes such as spill over and reverse spill over. Such studies allow for characterization of novel cluster systems towards their reactivity and adsorption behavior, e.g. for iron nanoclusters with the probe molecule CO. Such reactivity studies are also possible for the more complex chemistry of ethylene on Pt nanoclusters where detailed insights to the reaction pathway become available. Thereby the changes when going from single crystals to nanoclusters become evident. As a second topic, the formation of van-der-Waals nano-heterostructures is discussed. Insights from spectroscopic and microscopic studies allowed us to characterize the individual preparation steps and the final nano-heterostructure consisting of graphene / metal / h-BN stack. Experiments with different cluster sizes are shown to obtain insights to the pore filling of the h-BN substrate. This gives a direct measure of the agglomeration of the nanoclusters during preparation. Literature F. Düll, et al. ACS Applied Nano Materials 2 (2019) 7019. F. Düll, et al. Journal of Chemical Physics 152 (2020) 224710. F. Düll, et al. Physical Chemistry Chemical Physics 21 (2019) 21287.

Superconducting nanowire single-photon detectors (SNSPDs) provide sufficient sensitivity to enable laser induced fluorescence (LIF) experiments in the mid-infrared, an exciting technical development for physical chemistry given the importance of vibrational spectroscopy to molecular science. In this talk, I will describe how an SNSPD works and how to use it. I will also present results of experiments on the vibrational dynamics of monolayers and multilayers of solid CO adsorbed at the surface of a NaCl crystal, to show the capabilities of time-resolved infrared LIF spectroscopy and provide observations of astonishing phenomena arising from dipole-dipole coupling between molecules. In these experiments, a laser light pulse excites about half of the CO to its