Roland Wiesendanger

Majorana zero modes, which obey non-Abelian exchange statistics, are promising candidates for topological quantum computation due to their robustness against environmental perturbations. The iron-based superconductor Fe(Te,Se) has been identified as an intrinsic topological superconductor, possibly hosting Majorana zero modes. In this paper, we report the observation of near-zero-energy localized states at multiple structural defects on the Fe(Te,Se) surface, which could be misidentified as Majorana zero modes without additional verification. By using spin-polarized scanning tunneling spectroscopy, we demonstrate that the near-zero-energy localized states on step edges and line defects originate from topologically trivial Yu-Shiba-Rusinov states. In addition, zero-energy bound states are also observed for regions without surface defects. A combined spatial and magnetic field dependent analysis of the spin-resolved tunneling spectra in these regions reveals that this type of zero-energy states cannot be attributed to the presence of Majorana bound states. These findings emphasize the importance of spin-dependent studies of low-energy states for pursuing Majorana zero modes.

Majorana modes offer great potential for fault-tolerant quantum computation due to their topological protection. However, intrinsic disorder makes the unambiguous detection of Majorana modes difficult in the commonly used hybrid superconductor–semiconductor nanowire platform. By contrast, for magnet–superconductor hybrid systems, Majorana modes were theoretically predicted to be very robust against disorder, but no experimental confirmation of this has been reported so far. Here we demonstrate that these modes are indeed robust in one-dimensional spin chains constructed from individual iron atoms on a Rashba surface alloy with proximity-induced superconductivity. Although the chains exhibit perfect crystalline order, we observe nanoscale potential disorder of the BiAg ₂ /Ag(111)/Nb(110) heterostructure by scanning tunnelling microscopy. However, this does not prevent the emergence of zero-energy modes at both ends of the atomic chains, in agreement with tight-binding calculations showing that these modes are only found in the topologically non-trivial regime of the phase diagram. Our results explain the earlier observation of zero-energy Majorana modes in disordered Fe chains on other superconducting substrates, and may provide an avenue for the realization of Majorana qubits.

Magnetic racetrack architectures that use topological magnetic particles to store information are one of the most promising concepts for future storage applications. Antiferromagnetic racetracks are particularly appealing as they are not susceptible to external magnetic fields. State-of-the-art racetracks use magnetic fields, spin-transfer, and spin–orbit torques caused by electric currents to move the bits across the entire circuit. However, the application of currents in many antiferromagnetic racetracks is limited because many of them are insulating. Recently, however, a concept for ferromagnetic racetrack memories that are free of global driving forces has been proposed. It has been demonstrated that various topological entities can be generated and transported over long distances solely through local magnetization rotation at the sample boundaries, independent of global driving forces. Here, we demonstrate that the local rotation of magnetization at the boundary of an antiferromagnetic sample can be exploited in racetracks to efficiently generate and transmit antiferromagnetic skyrmions. Additionally, we demonstrate that local switching of staggered magnetization at the edge of an antiferromagnetic racetrack can be even more successful than the rotational procedure. A comparison of ferromagnetic and antiferromagnetic processing of skyrmionic bits, together with energy considerations, shows that this procedure is fairly efficient in antiferromagnets.

The interplay of magnetism and superconductivity can lead to intriguing emergent phenomena. Here we combine two different two-dimensional antiferromagnetic magnet-superconductor hybrids (MSH) and study their properties using spin-polarized scanning tunneling microscopy. Both MSHs show the characteristics of a topological nodal point superconducting phase with edge modes to the trivial substrate superconductor. At the boundary between the two MSHs we find low-energy modes which are spin-polarized. Based on a tight-binding model we can explain the experimental observations by considering two different topological nodal point superconductors. At their boundary spin-polarized chiral edge modes emerge that connect topological nodal points of the two different MSH. We demonstrate via the complex band structure that due to an asymmetric lateral decay these edge modes are spin-polarized, regardless of the details of the spin structure at the boundary. This work shows how interfaces between two distinct topological nodal point superconductors can serve as a platform to engineer spin-polarized edge modes.

Antiferromagnetic materials have recently emerged as promising candidates in spintronics. At the same time, more complex localized non-coplanar magnetic states such as skyrmions are in the research focus due to their intriguing dynamical and transport properties. Recently, a conceptual shift has occurred to envision the use of such magnetic defects not only in one-dimensional race track devices but also to exploit their unique properties in two-dimensional networks. Here we use local strain in a collinear antiferromagnetic film to induce a complex domain wall network. Using spin-polarized scanning tunneling microscopy we characterize the different building blocks of the network – ranging from collinear magnetic domains, over non-collinear domain walls, to non-coplanar localized domain wall junctions – on the atomic scale. We find that the triple domain wall junctions exhibit a structural handedness. The origin is an exchange-driven lateral relaxation as explained using first-principles calculations. We predict that the domain wall junctions exhibit topological orbital magnetization generated by their non-coplanar spin structure, implying topological transport properties due to the network.

Majorana modes offer great potential for fault-tolerant quantum computation due to their topological protection. However, for superconductor-semiconductor nanowire hybrids, intrinsic disorder makes the unambiguous detection of Majorana modes difficult. Here, we construct 1D spin chains from individual Fe atoms on the Rashba surface alloy BiAg2/Ag(111) with proximity-induced superconductivity from a Nb(110) substrate. While the Fe chains exhibit perfect crystalline order, we observe nano-scale potential disorder of the BiAg2/Ag(111)/Nb(110) heterostructure by scanning tunneling microscopy. However, this does not prevent the emergence of zero-energy modes at both ends of the Fe chains, in agreement with tight-binding calculations showing that they are only found in the topologically non-trivial regime of the phase diagram. These Majorana modes are indeed robust against potential disorder.

The generation of topologically nontrivial magnetic configurations has been a pivotal topic in both basic and applied nanomagnetism research. Localized noncoplanar magnetic defects such as skyrmions or merons were found to interact strongly with currents, making them interesting candidates for future spintronics applications. So far, mostly systems with a high rotational symmetry have been investigated where skyrmions were axially symmetric. Here, we study a low-symmetry system by spin-polarized scanning tunneling microscopy and an atomistic spin model using parameters based on first-principles calculations. We demonstrate how a delicate balance between energy terms generates both topologically trivial and nontrivial domain walls, depending on their nonequivalent crystallographic direction. The topological walls consist of alternating merons and antimerons, and the topological charge is 1 for every 6-nm length of the wall. The incorporation of holes in the films facilitates the transition from an in-plane ferromagnetic ground state to a spin-spiral state. Both domain walls and spirals transition into isolated elongated magnetic skyrmions in applied magnetic fields. These findings establish low-symmetry systems as a versatile platform for spin-texture engineering.

We determine the Dzyaloshinskii-Moriya interaction within and between two magnetic cobalt layers separated by a nonmagnetic spacer through calculations. We investigate different materials for the nonmagnetic layer, focusing on the experimentally realized Co/Ag/Co system. We laterally shift the atoms in the nonmagnetic layer to achieve the symmetry breaking required for the interlayer Dzyaloshinskii-Moriya interaction. We compare the resulting interactions with the Levy-Fert model and observe a good overall agreement between the model and the calculations for the dependence on the atomic positions. Additionally, we derive a formula for the strength of the interlayer isotropic exchange interaction depending on the position of the atoms in the nonmagnetic layer and compare it to the first-principles results. We investigate the limitations of the Levy-Fert model by turning off the spin-orbit coupling separately on the nonmagnetic and magnetic atoms and by studying the effect of band filling. Our work advances the understanding of the microscopic mechanisms of the interlayer Dzyaloshinskii-Moriya interaction and gives insight into possible new material combinations with strong interlayer Dzyaloshinskii-Moriya interaction. Published by the American Physical Society 2024