Observation of a.c. Josephson effect in gate tunable Josephson junction on topological insulator (Bi0.2Sb0.8)2 Te3 films

In the past decade, Majorana Fermions (MFs) has been getting much interest because of their non-Abelian statistics and their application for topological quantum computation. One of the promising system hosting MFs is the hybrid structures between topological insulators (TIs) and s-wave superconductors. As the probe of MFs, measurements of a.c. Josephson effect, i.e. the Shapiro steps and Josephson radiation, is a useful tool. We fabricated Josephson junction on the 3D TI (Bi
0.2
Sb
0.8
)
2
Te
3
grown on SrTiO
3
(111) substrate. We found that the supercurrent and the normal resistance of Josephson junction is gate-tunable and that the dominant carriers in the sample are from the topological surface states. Next, we measured the a.c. Josephson effects. In the Shapiro step measurement, we observed the both even and odd integer multiple of hf/2e steps. This contradicts with the existence of MFs in which odd multiple steps are expected to disappear. Furthermore, we also observed Josephson radiation with frequency of 2eV/h but no emission with frequency eV/h, a signature of MFs, is observed. We speculate that these results are due to huge dielectric constant of the substrate. Our experiment is the first report of a.c. Josephson effect on the gate-tunable Josephson junction of 3D TI film.