In recent years, Majorana physics has attracted considerable attention because of exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here, we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a 2D topological insulator that exhibits the quantum spin Hall (QSH) effect. The a.c. Josephson effect demonstrates that the supercurrent has a 4π periodicity in the superconducting phase difference, as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, this response like that of a superconducting quantum interference device to a perpendicular magnetic field shows that the 4π-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest towards the QSH regime, and thus provide evidence for induced topological superconductivity in the QSH edge states.
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We thank V. Hock and L. Maier for technical assistance and acknowledge S. Tarucha, L. Glazman, Y. Peng, F. von Oppen, E.M. Hankiewicz, G. Tkachov and B. Trauzettel for enlightening discussions. This work is supported by the German Research Foundation (Leibniz Program, DFG-Sonderforschungsbereich 1170 ‘Tocotronics’ and DFG-Schwerpunktprogramme 1666), the Elitenetzwerk Bayern program Topologische Isolatoren. R.S.D. acknowledges support from Grants-in-Aid for Scientific Research A (No. 16H02204) and Young Scientists B (No. 26790008). T.M.K. is financially supported by the European Research Council Advanced Grant No.339306 (METIQUM). E.B., T.M.K. and L.W.M. thank the Alexander von Humboldt foundation for its support.
The authors declare no competing financial interests.
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Bocquillon, E., Deacon, R., Wiedenmann, J. et al. Gapless Andreev bound states in the quantum spin Hall insulator HgTe. Nature Nanotech 12, 137–143 (2017). https://doi.org/10.1038/nnano.2016.159
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