Daniel Steffensen

Nematicity, Superconductivity and Topological Effects in Multi-Orbital Systems

The study of phases of matter has never ceased to fascinate and puzzle the condensed matter community, and the discovery of high temperature superconductors in 1986, and topological phases in 1980, are no exceptions. Especially the iron-based superconductors have in recent years drawn a considerable amount of attention, due to their complex phase diagrams, and potential for harboring the exotic Majorana zero modes. Such quasiparticle excitations are governed by unconventional exchange statistics, and are believed to be a key component in fault-tolerant quantum computations.

In this talk we thus set out to study the class of multi-orbital systems, which can describe both specific iron-based superconductors, like FeSe, as well as a broader class of materials, such as magnetic materials that either coexist or are in proximity to a superconductor. We perform a thorough theoretical study of one of the electronic phases emerging prior to the superconducting phase transition in FeSe, the so-called nematic phase, in order to elucidate its role in the resulting gap structure, and to deepen our understanding of these materials. Furthermore, we study in what ways the neamtic phase is affected by disorder.

 Lastly, we study topological superconducting phases induced by magnetic textures in multi-orbital systems. We specifically propose an alternative route to engineering Majorana zero modes, by trapping these excitations in magnetic texture defects, which are assumed to coexist or be in proximity to a nodal superconductor. Moreover, we perform a topological symmetry classification of topological superconductors induced by magnetic textures in general, and find a plethora of Majorana edge modes.

Zoom link: https://ucph-ku.zoom.us/j/63860868604?pwd=d0UrOFBDOGVHeTgyeWk1U0lXeTR4UT09