We thank Y. S. Eo for discussions on the properties of SmB 6 , F. C. Wellstood for discussions on the possible applications of superconducting Klein tunnelling devices, and H. M. Iftekhar Jaim for assistance with X-ray measurements. This project was funded by ONR N00014-13-1-0635; ONR N00014-15-1-2222; AFOSR number FA9550-14-10332; NSF (DMR-1410665); and C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation (SRC) programme sponsored by MARCO and DARPA. We acknowledge support from the Maryland NanoCenter. J.P. acknowledges support from the Gordon and Betty Moore Foundation's EPiQS Initiative through grant number GBMF4419. V.G. was supported by DOE-BES (DESC0001911) and the Simons Foundation. This work was also supported in part by the Center for Spintronic Materials in Advanced infoRmation Technologies (SMART), one of the centers in nCORE, an SRC programme sponsored by NSF and NIST. The work at University of California, Irvine, was carried out using the electron microscopy facilities of the Irvine Materials Research Institute (IMRI) and was supported by the National Science Foundation through grant DMR-1506535 and by DOE-BES under grant DE-SC0014430. We acknowledge support from the National Institute of Standards and Technology Cooperative Agreement 70NANB17H301.

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Nature thanks Ewelina Hankiewicz, David Goldhaber-Gordon and Jinfeng Jia for their contribution to the peer review of this work.