Dissertation Committee Chair: Prof. Jay D Sau
Committee:
Prof. Sankar Das Sarma
Prof. Victor Yakovenko
Prof. Maissam Barkeshli
Prof. Ichiro Takeuchi (Deanโs representative)
Abstract: The Fe-based chalcogenide compound, FeTe1-xSex (FTS), has recently attracted attention as a potential candidate for a readily available platform for topological superconducting (TSC) phase on its surface. The co-existence of the strong topological insulating (TI) phase and cylindrical Fermi sheets provide the two necessary ingredients for the TSC phase on its surface.
The strong TI phase in FTS depends on the relative Te/Se composition. The widely studied FTS sample, FeTe.55Se.45, interestingly, lies close to the topological-trivial phase boundary which is estimated to be close to x = .5 by a recent experiment. Additionally, a typical sample of FTS suffers from fluctuations in Te/Se composition at multiple length scales due to its alloy nature. Thus, in a topological FTS sample such fluctuations can give rise to regions where the phase is driven out of its topological nature. Such trivial domains would be scattered throughout the sample. After carefully exploring the effects of such topological domain disordered phase in an effective model of FTS, we conclude that its resultant signatures can be distinguished in a scanning tunneling spectroscopy (STS) measurement of the topological surface states (TSS) density.
Another possibility of an exotic phase arises on the TSC surface when a linear defect forms a Josephson junction (JJ) characterized by a phase shift of ๐ for the SC order parameter (OP). Such a ๐-JJ induces exotic helical Majorana modes - signatures of which has been observed by a recent tunneling experiment on the surface of FTS in the density of states (DOS) within the SC gap at a crystalline domain wall (DW) associated with half-unit-cell-shift (HUCS). Observation of such signatures naturally poses question about the origin of such ๐-JJ which is yet to be fully understood. We propose a mechanism that stabilizes a ๐-junction at the HUCS DW when the intrinsic superconducting pairing is of sยฑ character which is the case for bulk FTS. We argue that if the DW induced inter-pocket transmission between the ฮ and M pockets in FTS is strong enough, the resultant enhancement of the two OPs of opposite sign stabilizes the ๐-junction.