For the 1D systems, we were able to demonstrate the emergence of Majorana bound states in artificially fabricated atomic Fe and Mn chains on elemental superconducting substrates (Re, Nb). In particular, we have applied scanning tunneling microscopy (STM) based single-atom manipulation techniques to create perfect, disorder-free atomic spin chains on atomically clean elemental superconducting substrates and demonstrated the non-local nature of the Majorana bound states at both ends of such chains. Moreover, we have successfully applied Bogoliubov quasiparticle interference analysis of the 1D atomic chains, thereby revealing their low-energy in-gap band structure from which the topological nature of the hybrid magnet-superconductor system could be proven.
In Fe-chains on Re(0001) the observed zero-energy states are compatible with Majorana states. Also ab-initio based theory simulations supported this interpretation. However, excited bulk states overlap with these zero-energy states due to the limited energy resolution of the STS measurements at 300 mK. To observe isolated Majorana states inside a hard gap would require a reduction of temperature by a factor of 10, or the use of a superconducting substrate with a significantly higher transition temperature like niobium.
Indeed, Mn[1-10]-chains on Nb(110) feature clear zero-energy states for certain chain lengths. The experimental measurement of the low-energy states as a function of chain length revealed an oscillatory behavior, which can be understood by assuming hybridized Majorana states. Longer chains would be required to obtain non-interacting, isolated Majorana states. Importantly, the observed length dependent energy splitting oscillations can be regarded as a "smoking gun" for the existence of Majorana states since trivial states would not show such behaviour.
Mn[001]-chains on Nb(110) were shown to possess a gap within the Shiba bands which is reminiscent of a gapped Dirac Hamiltonian, thereby proving that the gap is of topological origin. The associated zero-energy states do not possess significant spectral weight at the chain ends. Instead, they feature a standing-wave pattern with spectral weight pushed to the sides of the chain. Extensive (ab-initio based) theoretical modeling reproduced such zero-energy states, and identified them as Majorana states characteristic for a multi-orbital magnet-superconductor hybrid system.
Based on the atomic-resolution spin mapping capabilities, we have also discovered novel types of complex non-collinear spin textures, e.g. a magnetic triple-q structure, in the framework of the project. If this triple-q structure is brought in contact with an elemental superconductor, topological superconductivity as well as Majorana modes can emerge. Moreover, we could demonstrate the emergence of chiral Majorana edge modes in 2D magnet - superconductor hybrid systems with atomically engineered interfacial layers.
