Lukas Stampfer 

Multi-terminal Josephson Junctions in InAsSb/Al Nanocrosses

The work in this thesis was motivated by the theoretical prediction, that few-channel multi-terminal Josephson junctions can host topological states. We explored different platforms and material combinations to this end. The Al proximitized 2DEGs proved to be a platform too complicated to fabricate and tune into the few channel regime. More success was had by using VLS nanocrosses. We first confirmed the excellent junction quality and showed individual gatetuning of the leads in InSb nanocrosses.  The need to have a patterned Al shell however motivated the use of the chemically more stable InAsSb. This material allowed us to create and characterize a few channel four terminal Josephson junction, and prove in particular the flux tuning and gateability of the critical currents. Yet no topological effects were observed. Free standing nanowires from the same growth batch, with an in-situ shadow in the Al hint that the wet-etching significantly damages the weak-link reducing the critical current and hampering coherence.  These individual nanowires with in-situ shadowed junctions were used to characterize the leads of the multi-terminal Josephson junction, in particular with the periodic critical-current modulation revealing the ring-like charge carrier distribution in the leads. Another studyconducted on those devices was focused around their response to RF radiation, not only resulting in the familiar quantized Shapiro steps, but also revealing the charge associated with the first three multiple Andreev resonances. The higher junction quality of the in-situ shadowed nanowire finally also motivates the significant effort undertaken to adapt existing shadowing techniques to multi-terminal Josephson junctions. In particular, we show prototypes for suspended SiO bridges and NW based shadowing. In summary a platform for multi-terminal Josephson junctions in nanocrosses is developed.

Zoom link: https://ucph-ku.zoom.us/j/67722654693