Josephson Current through Topological Trilayer YB₆/SmB₆/YB₆ Junctions

Samarium hexaboride (SmB₆) is a topological Kondo insulator (TKI) characterized by robust topological surface states and true bulk insulation. Previously, we had observed perfect Andreev reflection in epitaxial heterostructures consisting of s-wave superconductor YB₆ and TKI SmB₆ as a signature of topologically protected superconducting Klein tunneling [1]. Recently, we have succeeded in fabricating topological Josephson junctions based on YB₆/SmB₆/YB₆ trilayers grown epitaxially on 3” Si wafers by in-situ co-sputtering and patterned into vertical junction arrays using standard photolithography. The junctions measured at temperatures down to 0.8 K display resistively-shunted junction characteristics and well-behaved Fraunhofer diffraction patterns. Clear Shapiro steps are observed upon microwave irradiation. We are studying the details of the current distribution in the junctions of different SmB₆ thicknesses through analysis of the Fraunhofer patterns at various temperatures. The ratio between the critical current at zero magnetic field and that at the first side lobe is indicative of an unconventional supercurrent density distribution, while the Josephson penetration depth places the devices in the small junction limit. The critical current ratio increases as the SmB₆ layer thickness is increased through (double) the approximate thickness of the surface state layer of SmB₆, indicating the suppression of tunneling current as the bulk insulation becomes more pronounced.