Group Leader Profiles

The research in our group is focused on the electronic and magnetic properties of few level systems looking for new quantum limits at the atomic scale. We are exploiting the interplay of magnetism, superconductivity, and correlation effects to isolate few level systems and understand their dynamics. Using scanning tunneling microscopy at lowest temperatures (between 10mK and 500mK), we study individual magnetic impurities coupled to superconducting substrates. We are interested in the resulting phenomena, such as Yu-Shiba-Rusinov states, and their suitability for quantum sensing or information processing. In addition, we combine electron spin resonance spectroscopy with scanning tunneling microscopy to understand and manipulate single spin systems isolated from their enviornment.

The theory group has a long standing experience in the study of quantum phenomena in the field of atomic, molecular and optical physics. A special focus is on the man-body properties of strongly interacting quantum systems, as naturally realized with dipolar gases, cold atoms in optical lattices, polar molecules, and photons in a Rydberg media. The main research goals are the creation of exotic states of matter with ultra-cold gases, the design of quantum simulators for topological ordered phases and the study of their application for quantum information, as well as the understanding of strongly correlated states.

Our group investigates correlated electron systems, i.e., materials where interactions between electrons are crucial if we want to understand their properties. We have a certain focus on numerical investigations of model systems: While models are of course a severe simplification of a material, this abstraction implies at the same time that we can use them to test our understanding of the dominant processes and to identify the most important aspects. Current focuses of our research are multi-orbital systems, e.g. iron-based superconductors or iridates, and topological states of matter that arise through electron-electron and electron-spin interactions.

We are extending the coupled cluster theory, one of the most successful theories for ab-initio simulations of molecules, to study strongly correlated, extended and periodic molecular systems. We are developing novel coupled cluster approaches and embedding methodologies, and use automatic coding techniques to implement the new methods. These methods can be applied to various molecular or model systems, with strongly and weakly correlated electrons, to calculate ground and excited state properties and to predict or explain experimental findings.

Our interdisciplinary research group explores how to teach crystals tricks of living matter by investigating dynamic features of molecular framework materials such as metal-organic and covalent organic frameworks (MOFs and COFs). By specifically tuning the structural topology of the framework, we create soft porous crystals which exhibit pore contraction and/or expansion as a response to the adsorption of gases and fluids or external triggers such as light irradiation. Such materials can act as responsive cargo-release systems, nanoscopic sensors or feature counterintuitive phenomena such as negative gas adsorption. We furthermore construct frameworks which contain molecular machines such as light-driven molecular motors and switches as responsive and intrinsically dynamic building blocks. We aim towards collective operating molecular machines in the solid state which are able to actively transport molecules in the pore space and facilitate dynamic conversion and storage of energy carriers and other small molecules. Our diverse team uses a wide range of synthetic and experimental tools and collaborates in national and international research projects to push the boundaries of dynamic features in crystalline solids.

Our group works at the interface between nanophotonics and DNA nanotechnology. We aim for the realization of artificial nanofactories, in which DNA-based cellular mimics can be built and work in concert, in direct resemblance to the biological systems in living cells. Using bottom-up (DNA-origami) and top-down (electron beam lithography) nanotechniques we develop nanophotonic building blocks for biology, chemistry, and materials science with both tailored optical response and active functionality. The dynamic building blocks allow us quantitative and kinetic understanding of structural changes, biological processes and phase changes materials down to the single particle level.

In our interdisciplinary and international research team of polymer chemists, physical chemists and materials scientists we are developing functional and intelligent polymer materials and devices.

One of the main aims is to control and manipulate structure-property relationships of hierarchical architectures from the molecular via the nanoscopic to the macroscopic device level.

Functionalities include:

• redox-activity for electrochemical sensors and electrocatalysis
• optical and electronic properties for polymer (opto)electronics
• stimuli-responsive behavior applicable as intelligent skin for controlled drug release in pharmaceutical applications and as actuators and sensors for soft robotics applications

In the Quantum Many-Body Theory department, electronic properties of solids are analyzed and computed with a main emphasis on systems where electronic correlations play a crucial role, such as high temperature superconductor and other transition metal oxides. Besides bulk properties of one-, two- and three-dimensional systems also surface states of topological phases, as well as problems with a mesoscopic length scale such as quantum dots, quantum wires, and quantum Hall systems are being studied. The correlation problem is treated by various modern numerical and field-theoretical techniques.

Quantum Control with hundreds of qubits is possible if neutral single atoms are trapped in arrays of optical tweezers or in optical lattices. Controlled interaction either via Rydberg excitation or magnetic interactions in optical lattices allow to setup strongly correlated quantums states. Singe atom optical readout via fluorescence allows to connect the qubits to a measurement apparatus without any electircal wiring. We are operating both quantum simulators and computers on these ideas. key words: Quantum Computing, Quantum Simulation, Atomic physics).

Our research group studies electronic and magnetic structures of quantum materials. A special focus is on topological materials, which exhibit unusual properties, such as exotic surface states and anomalous transport phenomena, that are unaffected by continuous deformations, e.g., stretching, compressing, or twisting. Our aim is to develop a theoretical framework to describe these topological properties, and to find new ways how to use them in the laboratory and for device applications. We seek to classify topological materials in terms of symmetries and to discover new remarkable examples. Current research priorities focus on the topological properties of nodal-line semimetals, topological metals with nodal planes, quantum magnets, and unconventional superconductors, which we study using both analytical and numerical techniques.

Research in the Solid State Nanophysics Group focuses on the study of the many unusual ways in which electrons organize themselves as a result of interactions and correlations among their charge and spin degrees of freedom, when these electrons are confined in one or more dimensions on the nanometer scale. Transport and optical properties are investigated with local probe methods, at low temperatures, in high magnetic fields, under high frequency radiation or any combination thereof. The electrons are confined in strictly two-dimensional crystals such as graphene or other single layers of the large class of layered materials with weak interlayer forces. Also hybrid stacks of these two-dimensional crystals are fabricated and explored in a quest for novel functionalities and interaction physics.

Entanglement of electrons (electron correlations) in solids, in combination with details of the crystal lattice structure, produce a surprisingly rich variety of electronic phases, that are liquid, liquid-crystal and crystalline states of the charge and spin degrees of freedom. These complex electronic phases and the subtle competition among them very often give rise to novel functionality. The department will be studying these interesting novel phases in transition metal oxides and related compounds where the narrow d-bands, which give rise to strong electron correlations, in combination with the rich chemistry of such materials provides excellent opportunities for new discoveries. The goal of this research will be to hunt for new materials exhibiting exotic electronic states of matter, showing phenomena such as superconductivity or high thermoelectricity, and to explore them with advanced measurement techniques to unveil the physical mechanisms that could be drivers of potentially highly desirable functionality.

We focus on the spectroscopic and magnetic study of molecular nanomagnets. These materials have been proposed for applications in fields ranging from quantum computing to magnetic data storage. We are especially interested in understanding the transition from the microscopic quantum mechanical world of small particles to the macroscopic classical world that we live in. We specialize in advanced spectroscopic studies, including those based on electron spin resonance-related techniques, to investigate the magnetic anisotropy and quantum coherence and their origins.