PhD Defence Thies Jansen | Topology and correlations in oxides and tellurides

Topology and correlations in oxides and tellurides

Thies Jansen is a PhD student in the Department Interfaces and Correlated Electron Systems. (Co)Promotors are prof.dr.ir. A. Brinkman and dr. C. Li from the Faculty of Science & Technology.

Information processing technology hinges on the electronic properties of materials and our ability to tune these properties. With rising demand for processing power, interest in new materials with novel electronic properties is growing. The combination of band topology and electronic correlations can lead to such new electronic states. In this thesis we explore oxides and tellurides as material systems to potentially study the combination of topology with electronic correlations.

We consider three types of correlations: Coulomb repulsion, magnetism and superconductivity. In the oxides studied, the Coulomb repulsion and magnetism are intrinsically present and we try to induce topological properties by reducing the symmetry of the crystal structure by embedding the oxide in an heterostructure. We show that, although the heterostructure is successfully fabricated, no topological properties are observed due to structural distortions. Furthermore, we image the suppression of magnetism and theoretically investigate the influence of the Coulomb repulsion on the crystal structure in these types of oxides.

In the tellurides, specifically MnBi₂Te₄ (MBT), we study the combination of topology and correlations by enforcing superconducting correlations in the antiferromagnetic topological insulator MBT in a Nb-MBT-Nb junction. We observe clear Josephson coupling, indicated by the onset of a supercurrent, Shapiro steps, and quantum interference. The SQUID-like quantum interference pattern suggests supercurrent through a one dimensional edge channel. While inducing supercurrent through the chiral edge channel is difficult, quasi-helical edge states provide the necessary channels to induce superconducting correlations.

We conclude that among the material systems studied, no new emerging electronic states arise from the combination of topology with either Coulomb interaction, magnetism, or superconductivity. Nevertheless, based on the insights developed, we propose routes for future research and identify the combination of superconductivity in MBT as the most promising material system in this thesis for studying correlations and topology.