Kirsten von Bergmann

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Topology is a useful concept to describe material properties. In magnetism topology can arise when the spins in a material are pointing in different real-space directions, leading to non-coplanar magnetic order. Such complex magnetic states feature a scalar spin chirality, which can manifest in an additional, topological, contribution to the Hall effect and an orbital magnetization, even in the absence of spin-orbit coupling. In superconductors the topological properties are governed by the band structure in reciprocal space, and edge modes arise at boundaries to sample regions with different topology.
In this colloquium I will present spin-polarized scanning tunneling microscopy and spectroscopy experiments [1] of different topological systems. I will discuss examples of complex topological magnetic domain walls, such as meron-antimeron domain walls which transition into chains of skyrmions in applied magnetic fields [2], or non-coplanar superposition domain wall junctions in an antiferromagnet [3]. When magnetic films are in contact with superconductors topological superconductivity can emerge. Such magnet-superconductor-hybrid systems can show low-energy edge modes at the boundary to the pristine superconductor [4]. Also at boundaries between two different magnet-superconductor-hybrids with the same magnetic state chiral edge modes can arise, and experiments demonstrate that they are spin-polarized [5].
[1] K. von Bergmann et al., J. Phys.: Condens. Matter 26, 394002 (2014).
[2] R. Brüning et al., Phys. Rev. X 15, 021041 (2025).
[3] V. Saxena et al., Nature Commun. 16, 10808 (2025).
[4] R. Brüning et al., ACS Nano 19, 36215 (2025).
[5] F. Zahner et al., Nature Commun. 17, 3457 (2026).

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Universität Hamburg
PH2
Contact: Thomas Michely / Matteo Cacco