Morphology, magnetism and superconductivity of iron layers on niobium

Topological quantum systems are currently being discussed as promising candidates for the realization of qubits in the development of quantum computers. In particular, these quantum systems are based on Majorana zero modes which are predicted in hybrids of solids that combine BCS-like superconductivity with magnetism and strong spin-orbit interaction or chiral spin structures. In this work, such physical systems are investigated to evaluate whether they can serve as a basis for topological quantum computing and in particular, whether they are suitable for the realization of topologically nontrivial electronic states, including Majorana zero modes. In the first part of this work, niobium(111) was investigated, as its surface provides a promising fundamental platform for the realization of 2D magnet-superconductor hybrid structures. After the deposition of magnetic material, the threefold surface symmetry of Nb(111) could lead to magnetic frustration, which could favor the formation of chiral spin structures such as skyrmions. Such systems could theoretically host Majorana zero modes. Since the surface should be free of impurities for the deposition of magnetic material on Nb(111) and, unlike Nb(110), there is still no established method for cleaning the surface, a cleaning method for the (111) surface was first developed in this work. Subsequently, using scanning tunneling microscopy (STM), the atomic structure of the clean Nb(111) surface could be visualized and studied for the first time. This revealed a reconstruction whose occurrence can be theoretically explained by a so-called premelting of the surface. Parallel to the investigation of Nb(111), the preparation and investigation of Nb(110) was also started. It was found that the Nb(110) surface is much more suitable for depositing thin films of magnetic material such as iron on it, which would be the next step in the construction of a topological superconductor. Therefore, in the second part of this work the approach was to deposit two-dimensional islands consisting of one to two atomic magnetic layers on the surface of Nb(110) (Shiba lattice). After the experimental realization of such a system by iron layers on niobium(110), the atomic structure of the three reconstructions that were found could be resolved. Moreover, the electronic and magnetic structure of the different monolayers (ML) and the double layer (DL) was determined by conventional and spin-polarized scanning tunneling microscopy. Ferromagnetic monodomain structures with out-of-plane magnetization and different coercive field strengths were found for the three types of reconstructions, with the DL exhibiting the same coercive field strength as the underlying ML. In addition, the so-called Shiba bands, which are essential for the artificial engineering of topological superconductors, were investigated by spectroscopic measurements at low energy. Disorder in the spacial and energetic locations of the Shiba bands were found, which are energetically of the order of the substrate gap energy. The influence of this disorder on the formation of Majorana modes has been investigated in this work, with the result that most likely the reconstructions in this system prevent the formation of topological superconductivity and Majorana modes. Investigations of the Shiba bands of DL iron on niobium(110), on the other hand, revealed evidence of edge states near the Fermi energy, although this work did not fully clarify whether or not they may be regarded as an indication for topological superconductivity. In summary, this work reveals essential measures for the development of an artificial two-dimensional topological superconductor. In particular, it also addresses the difficulties that can be encountered in this development process. However, the results also reveal a number of promising possibilities for further developments that could eventually form the basis for two-dimensional topological superconductivity with Majorana zero modes.