Research Activities

We explore novel materials for future applications in nano- and spinelectronic devices, quantum computation, or neuromorphic computing using powerful methods of theoretical solid state physics involving computations on large computer clusters and/or supercomputers. We can calculate the properties of real nanostructures such as their electronic and magnetic structure from first-principles, i.e. free of any empirical input parameters. This enables us not only to interpret and explain unexpected experiments but to predict the properties of new materials and help to guide experimental efforts at the institute and of our national and international collaborators. In particular the interpretation of experiments using scanning probe methods can be non-trivial and the combination with first-principles calculations can be very valuable.

For many technological applications the transport properties of nanostructures are crucial. However, due to their reduced dimensionality nanoscale devices such as carbon nanotube transistors behave quite differently from conventional silicon transistors. We explore the transport in nanostructures using semiclassical and quantum-mechanical models. Due to the huge number of atoms involved in a real device the electronic structure is treated with semi-empirical methods. In the future, we strive for a combination of the transport calculations with a first-principles description of the electronic properties.

Magnetism of nanostructures

Today, accurate first-principles (parameter-free) calculations of the electronic structure are feasible for a wide class of materials and structures. Using large-scale computer clusters or supercomputers we can study the structural, electronic, chemical, and magnetic properties of real nanostructures e.g. molecules or atomic chains on surfaces. The properties of novel materials can be explained and even be predicted. In the past this has often triggered experimental work. We are also concerned with the theory of scanning probe methods such as spin-polarized scanning tunneling microscopy or magnetic exchange force microscopy which allow to reveal magnetic structures down to the atomic scale. These tools have boosted nanoscience, however, the interpretation of experiments is often difficult. In combination with first-principles calculations many unexpected effects have been clarified.

Topological spin structures

Since the discovery of magnetic skyrmions - stable, localized magnetic whirls - the research on topological spin structures has rapidly increased and has become a major topic in condensed-matter physics with many potential applications (cf. „The 2020 Skyrmionics Roadmap“, C. Back et al, J. Phys. D: Appl. Phys. (2020)). In collaboration with the experimental research group of Prof. Wiesendanger (University of Hamburg) and colleagues from the Forschungszentrum Jülich we revealed skyrmions in ultrathin magnetic films at surfaces (Nature Phys. 7, 713 (2011)). This discovery opened a new class of materials for skyrmions: transition-metal interfaces. Such interfaces occur in all magnetic multilayers which are already being used today in spintronic devices such as readheads of hard disks or magnetic sensors and thus provide a link to current technology. Besides the application of skyrmions in race-track memory devices or logic gates these topological spin structures hold great promise in neuromorphic computing. At hybrid magnetic-superconductor interfaces skyrmions can induce Majorana fermions which may be used in topological quantum computers.

Quantum Transport

In the past four decades, down-scaling the size of silicon devices to characteristic lengths below 100 nanometers has led to an unprecedented enhancement of computer performance allowing today's information technology. However, further shrinking of the structures may be limited by effects such as leakage currents and increasing power consumption. In addition, quantum effects become important on the nanometer scale. New materials such as nanotubes and nanowires are being intensively explored today by both industrial and academic research. For example, carbon nanotubes have raised high hopes due to their unique combination of structural and electronic properties such as ballistic transport. We explore the transport in nanoelectronic devices using semiclassical and quantum-mechanical models. Another field of interest is the transport of single-atom or molecule junctions. Novel effects such as the ballistic or the tunneling anisotropic magnetoresistance (AMR) have been discovered and are explored in collaboration with experimental partners.